Get validation error django

Form and field validation¶

Form validation happens when the data is cleaned. If you want to customize
this process, there are various places to make changes, each one serving a
different purpose. Three types of cleaning methods are run during form
processing. These are normally executed when you call the is_valid()
method on a form. There are other things that can also trigger cleaning and
validation (accessing the errors attribute or calling full_clean()
directly), but normally they won’t be needed.

In general, any cleaning method can raise ValidationError if there is a
problem with the data it is processing, passing the relevant information to
the ValidationError constructor. See below
for the best practice in raising ValidationError. If no ValidationError
is raised, the method should return the cleaned (normalized) data as a Python
object.

Most validation can be done using validators — helpers that can be reused.
Validators are functions (or callables) that take a single argument and raise
ValidationError on invalid input. Validators are run after the field’s
to_python and validate methods have been called.

Validation of a form is split into several steps, which can be customized or
overridden:

• The to_python() method on a Field is the first step in every
  validation. It coerces the value to a correct datatype and raises
  ValidationError if that is not possible. This method accepts the raw
  value from the widget and returns the converted value. For example, a
  FloatField will turn the data into a Python float or raise a
  ValidationError.

• The validate() method on a Field handles field-specific validation
  that is not suitable for a validator. It takes a value that has been
  coerced to a correct datatype and raises ValidationError on any error.
  This method does not return anything and shouldn’t alter the value. You
  should override it to handle validation logic that you can’t or don’t
  want to put in a validator.

• The run_validators() method on a Field runs all of the field’s
  validators and aggregates all the errors into a single
  ValidationError. You shouldn’t need to override this method.

• The clean() method on a Field subclass is responsible for running
  to_python(), validate(), and run_validators() in the correct
  order and propagating their errors. If, at any time, any of the methods
  raise ValidationError, the validation stops and that error is raised.
  This method returns the clean data, which is then inserted into the
  cleaned_data dictionary of the form.

• The clean_<fieldname>() method is called on a form subclass – where
  <fieldname> is replaced with the name of the form field attribute.
  This method does any cleaning that is specific to that particular
  attribute, unrelated to the type of field that it is. This method is not
  passed any parameters. You will need to look up the value of the field
  in self.cleaned_data and remember that it will be a Python object
  at this point, not the original string submitted in the form (it will be
  in cleaned_data because the general field clean() method, above,
  has already cleaned the data once).

  For example, if you wanted to validate that the contents of a
  CharField called serialnumber was unique,
  clean_serialnumber() would be the right place to do this. You don’t
  need a specific field (it’s a CharField), but you want a
  formfield-specific piece of validation and, possibly, cleaning/normalizing
  the data.

  The return value of this method replaces the existing value in
  cleaned_data, so it must be the field’s value from cleaned_data (even
  if this method didn’t change it) or a new cleaned value.

• The form subclass’s clean() method can perform validation that requires
  access to multiple form fields. This is where you might put in checks such as
  “if field A is supplied, field B must contain a valid email address”.
  This method can return a completely different dictionary if it wishes, which
  will be used as the cleaned_data.

  Since the field validation methods have been run by the time clean() is
  called, you also have access to the form’s errors attribute which
  contains all the errors raised by cleaning of individual fields.

  Note that any errors raised by your Form.clean() override will not
  be associated with any field in particular. They go into a special
  “field” (called __all__), which you can access via the
  non_field_errors() method if you need to. If you
  want to attach errors to a specific field in the form, you need to call
  add_error().

  Also note that there are special considerations when overriding
  the clean() method of a ModelForm subclass. (see the
  ModelForm documentation for more information)

These methods are run in the order given above, one field at a time. That is,
for each field in the form (in the order they are declared in the form
definition), the Field.clean() method (or its override) is run, then
clean_<fieldname>(). Finally, once those two methods are run for every
field, the Form.clean() method, or its override, is executed whether
or not the previous methods have raised errors.

Examples of each of these methods are provided below.

As mentioned, any of these methods can raise a ValidationError. For any
field, if the Field.clean() method raises a ValidationError, any
field-specific cleaning method is not called. However, the cleaning methods
for all remaining fields are still executed.

raise ValidationError(
    _('Invalid value: %(value)s'),
    code='invalid',
    params={'value': '42'},
)

ValidationError(_('Invalid value: %s') % value)

# Good
raise ValidationError([
    ValidationError(_('Error 1'), code='error1'),
    ValidationError(_('Error 2'), code='error2'),
])

# Bad
raise ValidationError([
    _('Error 1'),
    _('Error 2'),
])

from django.core import validators
from django.forms import CharField

class SlugField(CharField):
    default_validators = [validators.validate_slug]

slug = forms.CharField(validators=[validators.validate_slug])

from django import forms
from django.core.validators import validate_email

class MultiEmailField(forms.Field):
    def to_python(self, value):
        """Normalize data to a list of strings."""
        # Return an empty list if no input was given.
        if not value:
            return []
        return value.split(',')

    def validate(self, value):
        """Check if value consists only of valid emails."""
        # Use the parent's handling of required fields, etc.
        super().validate(value)
        for email in value:
            validate_email(email)

class ContactForm(forms.Form):
    subject = forms.CharField(max_length=100)
    message = forms.CharField()
    sender = forms.EmailField()
    recipients = MultiEmailField()
    cc_myself = forms.BooleanField(required=False)

from django import forms
from django.core.exceptions import ValidationError

class ContactForm(forms.Form):
    # Everything as before.
    ...

    def clean_recipients(self):
        data = self.cleaned_data['recipients']
        if "fred@example.com" not in data:
            raise ValidationError("You have forgotten about Fred!")

        # Always return a value to use as the new cleaned data, even if
        # this method didn't change it.
        return data

from django import forms
from django.core.exceptions import ValidationError

class ContactForm(forms.Form):
    # Everything as before.
    ...

    def clean(self):
        cleaned_data = super().clean()
        cc_myself = cleaned_data.get("cc_myself")
        subject = cleaned_data.get("subject")

        if cc_myself and subject:
            # Only do something if both fields are valid so far.
            if "help" not in subject:
                raise ValidationError(
                    "Did not send for 'help' in the subject despite "
                    "CC'ing yourself."
                )

def clean(self):
    super().clean()
    cc_myself = self.cleaned_data.get("cc_myself")
    ...

from django import forms

class ContactForm(forms.Form):
    # Everything as before.
    ...

    def clean(self):
        cleaned_data = super().clean()
        cc_myself = cleaned_data.get("cc_myself")
        subject = cleaned_data.get("subject")

        if cc_myself and subject and "help" not in subject:
            msg = "Must put 'help' in subject when cc'ing yourself."
            self.add_error('cc_myself', msg)
            self.add_error('subject', msg)

The Forms API¶

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': 'foo@example.com',
...         'cc_myself': True}
>>> f = ContactForm(data)

>>> f = ContactForm()
>>> f.is_bound
False
>>> f = ContactForm({'subject': 'hello'})
>>> f.is_bound
True

>>> f = ContactForm({})
>>> f.is_bound
True

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': 'foo@example.com',
...         'cc_myself': True}
>>> f = ContactForm(data)
>>> f.is_valid()
True

>>> data = {'subject': '',
...         'message': 'Hi there',
...         'sender': 'invalid email address',
...         'cc_myself': True}
>>> f = ContactForm(data)
>>> f.is_valid()
False

>>> f.errors
{'sender': ['Enter a valid email address.'], 'subject': ['This field is required.']}

>>> f.errors.as_data()
{'sender': [ValidationError(['Enter a valid email address.'])],
'subject': [ValidationError(['This field is required.'])]}

>>> f.errors.as_json()
{"sender": [{"message": "Enter a valid email address.", "code": "invalid"}],
"subject": [{"message": "This field is required.", "code": "required"}]}

>>> f = ContactForm()
>>> f.is_valid()
False
>>> f.errors
{}

>>> f = ContactForm(initial={'subject': 'Hi there!'})

>>> from django import forms
>>> class CommentForm(forms.Form):
...     name = forms.CharField(initial='class')
...     url = forms.URLField()
...     comment = forms.CharField()
>>> f = CommentForm(initial={'name': 'instance'}, auto_id=False)
>>> print(f)
<tr><th>Name:</th><td><input type="text" name="name" value="instance" required></td></tr>
<tr><th>Url:</th><td><input type="url" name="url" required></td></tr>
<tr><th>Comment:</th><td><input type="text" name="comment" required></td></tr>

>>> import uuid
>>> class UUIDCommentForm(CommentForm):
...     identifier = forms.UUIDField(initial=uuid.uuid4)
>>> f = UUIDCommentForm()
>>> f.get_initial_for_field(f.fields['identifier'], 'identifier')
UUID('972ca9e4-7bfe-4f5b-af7d-07b3aa306334')
>>> f.get_initial_for_field(f.fields['identifier'], 'identifier')
UUID('1b411fab-844e-4dec-bd4f-e9b0495f04d0')
>>> # Using BoundField.initial, for comparison
>>> f['identifier'].initial
UUID('28a09c59-5f00-4ed9-9179-a3b074fa9c30')
>>> f['identifier'].initial
UUID('28a09c59-5f00-4ed9-9179-a3b074fa9c30')

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': 'foo@example.com',
...         'cc_myself': True}
>>> f = ContactForm(data, initial=data)
>>> f.has_changed()
False

>>> f = ContactForm(request.POST, initial=data)
>>> f.has_changed()

>>> f = ContactForm(request.POST, initial=data)
>>> if f.has_changed():
...     print("The following fields changed: %s" % ", ".join(f.changed_data))
>>> f.changed_data
['subject', 'message']

>>> for row in f.fields.values(): print(row)
...
<django.forms.fields.CharField object at 0x7ffaac632510>
<django.forms.fields.URLField object at 0x7ffaac632f90>
<django.forms.fields.CharField object at 0x7ffaac3aa050>
>>> f.fields['name']
<django.forms.fields.CharField object at 0x7ffaac6324d0>

>>> f.as_div().split("</div>")[0]
'<div><label for="id_subject">Subject:</label><input type="text" name="subject" maxlength="100" required id="id_subject">'
>>> f["subject"].label = "Topic"
>>> f.as_div().split("</div>")[0]
'<div><label for="id_subject">Topic:</label><input type="text" name="subject" maxlength="100" required id="id_subject">'

>>> f.base_fields["subject"].label_suffix = "?"
>>> another_f = CommentForm(auto_id=False)
>>> f.as_div().split("</div>")[0]
'<div><label for="id_subject">Subject?</label><input type="text" name="subject" maxlength="100" required id="id_subject">'

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': 'foo@example.com',
...         'cc_myself': True}
>>> f = ContactForm(data)
>>> f.is_valid()
True
>>> f.cleaned_data
{'cc_myself': True, 'message': 'Hi there', 'sender': 'foo@example.com', 'subject': 'hello'}

>>> data = {'subject': '',
...         'message': 'Hi there',
...         'sender': 'invalid email address',
...         'cc_myself': True}
>>> f = ContactForm(data)
>>> f.is_valid()
False
>>> f.cleaned_data
{'cc_myself': True, 'message': 'Hi there'}

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': 'foo@example.com',
...         'cc_myself': True,
...         'extra_field_1': 'foo',
...         'extra_field_2': 'bar',
...         'extra_field_3': 'baz'}
>>> f = ContactForm(data)
>>> f.is_valid()
True
>>> f.cleaned_data # Doesn't contain extra_field_1, etc.
{'cc_myself': True, 'message': 'Hi there', 'sender': 'foo@example.com', 'subject': 'hello'}

>>> from django import forms
>>> class OptionalPersonForm(forms.Form):
...     first_name = forms.CharField()
...     last_name = forms.CharField()
...     nick_name = forms.CharField(required=False)
>>> data = {'first_name': 'John', 'last_name': 'Lennon'}
>>> f = OptionalPersonForm(data)
>>> f.is_valid()
True
>>> f.cleaned_data
{'nick_name': '', 'first_name': 'John', 'last_name': 'Lennon'}

>>> f = ContactForm()
>>> print(f)
<tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" required></td></tr>
<tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" required></td></tr>
<tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" required></td></tr>
<tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself"></td></tr>

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': 'foo@example.com',
...         'cc_myself': True}
>>> f = ContactForm(data)
>>> print(f)
<tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" value="hello" required></td></tr>
<tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" value="Hi there" required></td></tr>
<tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" value="foo@example.com" required></td></tr>
<tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself" checked></td></tr>

>>> f = ContactForm()
>>> f.as_div()

<div>
<label for="id_subject">Subject:</label>
<input type="text" name="subject" maxlength="100" required id="id_subject">
</div>
<div>
<label for="id_message">Message:</label>
<input type="text" name="message" required id="id_message">
</div>
<div>
<label for="id_sender">Sender:</label>
<input type="email" name="sender" required id="id_sender">
</div>
<div>
<label for="id_cc_myself">Cc myself:</label>
<input type="checkbox" name="cc_myself" id="id_cc_myself">
</div>

>>> f = ContactForm()
>>> f.as_p()
'<p><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required></p>n<p><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required></p>n<p><label for="id_sender">Sender:</label> <input type="text" name="sender" id="id_sender" required></p>n<p><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself"></p>'
>>> print(f.as_p())
<p><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required></p>
<p><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required></p>
<p><label for="id_sender">Sender:</label> <input type="email" name="sender" id="id_sender" required></p>
<p><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself"></p>

>>> f = ContactForm()
>>> f.as_ul()
'<li><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required></li>n<li><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required></li>n<li><label for="id_sender">Sender:</label> <input type="email" name="sender" id="id_sender" required></li>n<li><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself"></li>'
>>> print(f.as_ul())
<li><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required></li>
<li><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required></li>
<li><label for="id_sender">Sender:</label> <input type="email" name="sender" id="id_sender" required></li>
<li><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself"></li>

>>> f = ContactForm()
>>> f.as_table()
'<tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" required></td></tr>n<tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" required></td></tr>n<tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" required></td></tr>n<tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself"></td></tr>'
>>> print(f)
<tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" required></td></tr>
<tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" required></td></tr>
<tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" required></td></tr>
<tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself"></td></tr>

from django import forms

class ContactForm(forms.Form):
    error_css_class = 'error'
    required_css_class = 'required'

    # ... and the rest of your fields here

>>> f = ContactForm(data)
>>> print(f.as_table())
<tr class="required"><th><label class="required" for="id_subject">Subject:</label>    ...
<tr class="required"><th><label class="required" for="id_message">Message:</label>    ...
<tr class="required error"><th><label class="required" for="id_sender">Sender:</label>      ...
<tr><th><label for="id_cc_myself">Cc myself:<label> ...
>>> f['subject'].label_tag()
<label class="required" for="id_subject">Subject:</label>
>>> f['subject'].legend_tag()
<legend class="required" for="id_subject">Subject:</legend>
>>> f['subject'].label_tag(attrs={'class': 'foo'})
<label for="id_subject" class="foo required">Subject:</label>
>>> f['subject'].legend_tag(attrs={'class': 'foo'})
<legend for="id_subject" class="foo required">Subject:</legend>

>>> data = {'subject': '',
...         'message': 'Hi there',
...         'sender': 'invalid email address',
...         'cc_myself': True}
>>> f = ContactForm(data, auto_id=False)
>>> print(f.as_div())
<div>Subject:<ul class="errorlist"><li>This field is required.</li></ul><input type="text" name="subject" maxlength="100" required></div>
<div>Message:<textarea name="message" cols="40" rows="10" required>Hi there</textarea></div>
<div>Sender:<ul class="errorlist"><li>Enter a valid email address.</li></ul><input type="email" name="sender" value="invalid email address" required></div>
<div>Cc myself:<input type="checkbox" name="cc_myself" checked></div>
>>> print(f.as_table())
<tr><th>Subject:</th><td><ul class="errorlist"><li>This field is required.</li></ul><input type="text" name="subject" maxlength="100" required></td></tr>
<tr><th>Message:</th><td><textarea name="message" cols="40" rows="10" required></textarea></td></tr>
<tr><th>Sender:</th><td><ul class="errorlist"><li>Enter a valid email address.</li></ul><input type="email" name="sender" value="invalid email address" required></td></tr>
<tr><th>Cc myself:</th><td><input checked type="checkbox" name="cc_myself"></td></tr>
>>> print(f.as_ul())
<li><ul class="errorlist"><li>This field is required.</li></ul>Subject: <input type="text" name="subject" maxlength="100" required></li>
<li>Message: <textarea name="message" cols="40" rows="10" required></textarea></li>
<li><ul class="errorlist"><li>Enter a valid email address.</li></ul>Sender: <input type="email" name="sender" value="invalid email address" required></li>
<li>Cc myself: <input checked type="checkbox" name="cc_myself"></li>
>>> print(f.as_p())
<p><ul class="errorlist"><li>This field is required.</li></ul></p>
<p>Subject: <input type="text" name="subject" maxlength="100" required></p>
<p>Message: <textarea name="message" cols="40" rows="10" required></textarea></p>
<p><ul class="errorlist"><li>Enter a valid email address.</li></ul></p>
<p>Sender: <input type="email" name="sender" value="invalid email address" required></p>
<p>Cc myself: <input checked type="checkbox" name="cc_myself"></p>

>>> form = ContactForm()
>>> print(form['subject'])
<input id="id_subject" type="text" name="subject" maxlength="100" required>

>>> form = ContactForm()
>>> for boundfield in form: print(boundfield)
<input id="id_subject" type="text" name="subject" maxlength="100" required>
<input type="text" name="message" id="id_message" required>
<input type="email" name="sender" id="id_sender" required>
<input type="checkbox" name="cc_myself" id="id_cc_myself">

>>> f = ContactForm(auto_id=False)
>>> print(f['message'])
<input type="text" name="message" required>
>>> f = ContactForm(auto_id='id_%s')
>>> print(f['message'])
<input type="text" name="message" id="id_message" required>

class GPSCoordinatesBoundField(BoundField):
    @property
    def country(self):
        """
        Return the country the coordinates lie in or None if it can't be
        determined.
        """
        value = self.value()
        if value:
            return get_country_from_coordinates(value)
        else:
            return None

class GPSCoordinatesField(Field):
    def get_bound_field(self, form, field_name):
        return GPSCoordinatesBoundField(form, self, field_name)

<form enctype="multipart/form-data" method="post" action="/foo/">

# Bound form with an image field
>>> from django.core.files.uploadedfile import SimpleUploadedFile
>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': 'foo@example.com',
...         'cc_myself': True}
>>> file_data = {'mugshot': SimpleUploadedFile('face.jpg', <file data>)}
>>> f = ContactFormWithMugshot(data, file_data)

# Bound form with an image field, data from the request
>>> f = ContactFormWithMugshot(request.POST, request.FILES)

# Unbound form with an image field
>>> f = ContactFormWithMugshot()

>>> f = ContactFormWithMugshot()
>>> f.is_multipart()
True

{% if form.is_multipart %}
    <form enctype="multipart/form-data" method="post" action="/foo/">
{% else %}
    <form method="post" action="/foo/">
{% endif %}
{{ form }}
</form>

>>> class ContactFormWithPriority(ContactForm):
...     priority = forms.CharField()
>>> f = ContactFormWithPriority(auto_id=False)
>>> print(f.as_div())
<div>Subject:<input type="text" name="subject" maxlength="100" required></div>
<div>Message:<textarea name="message" cols="40" rows="10" required></textarea></div>
<div>Sender:<input type="email" name="sender" required></div>
<div>Cc myself:<input type="checkbox" name="cc_myself"></div>
<div>Priority:<input type="text" name="priority" required></div>

>>> from django import forms
>>> class PersonForm(forms.Form):
...     first_name = forms.CharField()
...     last_name = forms.CharField()
>>> class InstrumentForm(forms.Form):
...     instrument = forms.CharField()
>>> class BeatleForm(InstrumentForm, PersonForm):
...     haircut_type = forms.CharField()
>>> b = BeatleForm(auto_id=False)
>>> print(b.as_div())
<div>First name:<input type="text" name="first_name" required></div>
<div>Last name:<input type="text" name="last_name" required></div>
<div>Instrument:<input type="text" name="instrument" required></div>
<div>Haircut type:<input type="text" name="haircut_type" required></div>

>>> from django import forms

>>> class ParentForm(forms.Form):
...     name = forms.CharField()
...     age = forms.IntegerField()

>>> class ChildForm(ParentForm):
...     name = None

>>> list(ChildForm().fields)
['age']

>>> mother = PersonForm(prefix="mother")
>>> father = PersonForm(prefix="father")
>>> print(mother.as_div())
<div><label for="id_mother-first_name">First name:</label><input type="text" name="mother-first_name" required id="id_mother-first_name"></div>
<div><label for="id_mother-last_name">Last name:</label><input type="text" name="mother-last_name" required id="id_mother-last_name"></div>
>>> print(father.as_div())
<div><label for="id_father-first_name">First name:</label><input type="text" name="father-first_name" required id="id_father-first_name"></div>
<div><label for="id_father-last_name">Last name:</label><input type="text" name="father-last_name" required id="id_father-last_name"></div>

>>> class PersonForm(forms.Form):
...     ...
...     prefix = 'person'

Built-in Form Field Validations in Django Forms are the default validations that come predefined to all fields. Every field comes in with some built-in validations from Django validators. Each Field class constructor takes some fixed arguments.

The error_messages argument lets you specify manual error messages for attributes of the field. The error_messages argument lets you override the default messages that the field will raise. Pass in a dictionary with keys matching the error messages you want to override. For example, here is the default error message:

>>> from django import forms
>>> generic = forms.CharField()
>>> generic.clean('')
Traceback (most recent call last):
  ...
ValidationError: ['This field is required.']

And here is a custom error message:

>>> name = forms.CharField(
                error_messages={
               'required': 'Please enter your name'
                })
>>> name.clean('')
Traceback (most recent call last):
  ...
ValidationError: ['Please enter your name']

Syntax

field_name = models.Field(option = value)

Django Form Field Validation error_messages Explanation

Illustration of error_messages using an Example. Consider a project named geeksforgeeks having an app named geeks.

Refer to the following articles to check how to create a project and an app in Django.

• How to Create a Basic Project using MVT in Django?
• How to Create an App in Django ?

Enter the following code into forms.py file of geeks app. We will be using CharField for experimenting for all field options.

Python manage.py runserver

Валидация форм и полей¶

Валидация формы происходит при очистке данных. Если вы хотите настроить этот процесс, есть различные места для внесения изменений, каждое из которых служит для разных целей. В процессе обработки формы выполняются три типа методов очистки. Обычно они выполняются, когда вы вызываете метод is_valid() на форме. Есть и другие вещи, которые также могут вызвать очистку и проверку (обращение к атрибуту errors или прямой вызов full_clean()), но обычно они не нужны.

В общем, любой метод очистки может поднять ValidationError, если есть проблема с данными, которые он обрабатывает, передавая соответствующую информацию конструктору ValidationError. See below для лучшей практики поднятия ValidationError. Если не поднимается ValidationError, метод должен вернуть очищенные (нормализованные) данные в виде объекта Python.

Большинство валидаций можно выполнить с помощью validators — помощников, которые можно использовать повторно. Валидаторы — это функции (или callables), которые принимают один аргумент и вызывают ValidationError при недопустимом вводе. Валидаторы запускаются после вызова методов to_python и validate поля.

Валидация формы разбита на несколько этапов, которые можно настроить или отменить:

• Метод to_python() на Field является первым шагом в каждой валидации. Он преобразует значение к правильному типу данных и выдает сообщение ValidationError, если это невозможно. Этот метод принимает необработанное значение от виджета и возвращает преобразованное значение. Например, FloatField превратит данные в Python float или выдаст ValidationError.

• Метод validate() на Field обрабатывает специфическую для поля валидацию, которая не подходит для валидатора. Он принимает значение, которое было приведено к правильному типу данных, и при любой ошибке выдает сообщение ValidationError. Этот метод ничего не возвращает и не должен изменять значение. Вы должны переопределить его для обработки логики валидации, которую вы не можете или не хотите поместить в валидатор.

• Метод run_validators() на поле Field запускает все валидаторы поля и объединяет все ошибки в один ValidationError. Вам не нужно переопределять этот метод.

• Метод clean() в подклассе Field отвечает за выполнение to_python(), validate() и run_validators() в правильном порядке и распространение их ошибок. Если в любой момент времени какой-либо из методов вызывает ошибку ValidationError, валидация останавливается, и эта ошибка выдается. Этот метод возвращает чистые данные, которые затем вставляются в словарь cleaned_data формы.

• Метод clean_<fieldname>() вызывается на подклассе формы – где <fieldname> заменяется на имя атрибута поля формы. Этот метод выполняет любую очистку, специфичную для данного атрибута, не связанную с типом поля, которым он является. Этому методу не передаются никакие параметры. Вам нужно будет найти значение поля в self.cleaned_data и помнить, что в этот момент это будет объект Python, а не исходная строка, представленная в форме (она будет в cleaned_data, потому что метод general field clean(), описанный выше, уже однажды очистил данные).

  Например, если вы хотите проверить, что содержимое CharField под названием serialnumber является уникальным, clean_serialnumber() будет подходящим местом для этого. Вам не нужно конкретное поле (это CharField), но вам нужен специфический для поля формы фрагмент проверки и, возможно, очистки/нормализации данных.

  Возвращаемое значение этого метода заменяет существующее значение в cleaned_data, поэтому это должно быть значение поля из cleaned_data (даже если этот метод не изменил его) или новое очищенное значение.

• Метод clean() подкласса формы может выполнять валидацию, требующую доступа к нескольким полям формы. Сюда можно отнести такие проверки, как «если поле A предоставлено, то поле B должно содержать действительный адрес электронной почты». При желании этот метод может вернуть совершенно другой словарь, который будет использован в качестве cleaned_data.

  Поскольку методы валидации полей были запущены к моменту вызова clean(), у вас также есть доступ к атрибуту errors формы, который содержит все ошибки, возникшие при очистке отдельных полей.

  Обратите внимание, что любые ошибки, возникающие при переопределении Form.clean(), не будут связаны с каким-либо конкретным полем. Они попадают в специальное «поле» (называемое __all__), к которому вы можете получить доступ через метод non_field_errors(), если вам это необходимо. Если вы хотите прикрепить ошибки к определенному полю формы, вам нужно вызвать add_error().

  Также обратите внимание, что существуют особые соображения при переопределении метода clean() подкласса ModelForm. (см. ModelForm documentation для получения дополнительной информации)

Эти методы выполняются в указанном выше порядке, по одному полю за раз. То есть, для каждого поля формы (в порядке их объявления в определении формы) выполняется метод Field.clean() (или его переопределение), затем clean_<fieldname>(). Наконец, когда эти два метода выполнены для каждого поля, выполняется метод Form.clean(), или его переопределение, независимо от того, вызвали ли предыдущие методы ошибки.

Примеры каждого из этих методов приведены ниже.

Как уже упоминалось, любой из этих методов может вызвать ошибку ValidationError. Для любого поля, если метод Field.clean() вызывает ValidationError, любой метод очистки, специфичный для данного поля, не вызывается. Однако методы очистки для всех оставшихся полей все равно выполняются.

raise ValidationError(
    _('Invalid value: %(value)s'),
    code='invalid',
    params={'value': '42'},
)

ValidationError(_('Invalid value: %s') % value)

# Good
raise ValidationError([
    ValidationError(_('Error 1'), code='error1'),
    ValidationError(_('Error 2'), code='error2'),
])

# Bad
raise ValidationError([
    _('Error 1'),
    _('Error 2'),
])

from django.core import validators
from django.forms import CharField

class SlugField(CharField):
    default_validators = [validators.validate_slug]

slug = forms.CharField(validators=[validators.validate_slug])

from django import forms
from django.core.validators import validate_email

class MultiEmailField(forms.Field):
    def to_python(self, value):
        """Normalize data to a list of strings."""
        # Return an empty list if no input was given.
        if not value:
            return []
        return value.split(',')

    def validate(self, value):
        """Check if value consists only of valid emails."""
        # Use the parent's handling of required fields, etc.
        super().validate(value)
        for email in value:
            validate_email(email)

class ContactForm(forms.Form):
    subject = forms.CharField(max_length=100)
    message = forms.CharField()
    sender = forms.EmailField()
    recipients = MultiEmailField()
    cc_myself = forms.BooleanField(required=False)

from django import forms
from django.core.exceptions import ValidationError

class ContactForm(forms.Form):
    # Everything as before.
    ...

    def clean_recipients(self):
        data = self.cleaned_data['recipients']
        if "fred@example.com" not in data:
            raise ValidationError("You have forgotten about Fred!")

        # Always return a value to use as the new cleaned data, even if
        # this method didn't change it.
        return data

from django import forms
from django.core.exceptions import ValidationError

class ContactForm(forms.Form):
    # Everything as before.
    ...

    def clean(self):
        cleaned_data = super().clean()
        cc_myself = cleaned_data.get("cc_myself")
        subject = cleaned_data.get("subject")

        if cc_myself and subject:
            # Only do something if both fields are valid so far.
            if "help" not in subject:
                raise ValidationError(
                    "Did not send for 'help' in the subject despite "
                    "CC'ing yourself."
                )

def clean(self):
    super().clean()
    cc_myself = self.cleaned_data.get("cc_myself")
    ...

from django import forms

class ContactForm(forms.Form):
    # Everything as before.
    ...

    def clean(self):
        cleaned_data = super().clean()
        cc_myself = cleaned_data.get("cc_myself")
        subject = cleaned_data.get("subject")

        if cc_myself and subject and "help" not in subject:
            msg = "Must put 'help' in subject when cc'ing yourself."
            self.add_error('cc_myself', msg)
            self.add_error('subject', msg)

In this post we’ll learn to create user-defined functions, displaying validation errors in the template for Django Form Validations.

Table Of Contents

• Introduction
• Creating Form
• Rendering Form
• Saving Form
• Form Validation User-Defined Functions
• Conclusion

Introduction

The forms are a Django Module which deals with all form-related functions from binding POST data to form, Validating form and rendering HTML field in the template.
We’ll be using below models.py file as an example to demonstrate form validations.

from django.db import models
from django.contrib.auth.models import User
from datetime import datetime

class AuthUserProfile(models.Model):
    user_profile_id = models.AutoField(primary_key=True)
    user = models.OneToOneField(User, on_delete=models.CASCADE, related_name='auth_user_profile')
    dob =  models.DateField(blank=True, null=True)
    is_deleted = models.PositiveSmallIntegerField(default=0)
    created_at = models.DateTimeField(auto_now=datetime.now(), null=True)
    updated_at = models.DateTimeField(auto_now=datetime.now(), null=True)

    class Meta():
        db_table = 'auth_user_profile'
        verbose_name = 'User Profile'
        verbose_name_plural = 'User Profiles'

    def __str__(self):
        return self.user

Create a form which has these fields (first_name, last_name, username, password, email) from User models and field (dob) in AuthUserProfile Model and also will add custom field and non-field level validation.

Creating Form

In forms.py file import forms from Django. Inherit forms.Form to UserForm and add attributes to the field.

from django import forms
from datetime import datetime
from django.contrib.auth.models import User

class UserForm(forms.Form):
    first_name = forms.CharField(label="First Name*",widget=forms.TextInput(attrs={'required':True,'class':"form-control"}))
    last_name = forms.CharField(label="Last Name*",widget=forms.TextInput(attrs={'required':True,'class':"form-control"}))
    username = forms.CharField(label="User Name*",widget=forms.TextInput(attrs={'required':True,'class':"form-control"}))
    email = forms.CharField(label="Email",widget=forms.TextInput(attrs={'type':'email','required':False,'class':"form-control"}))
    date_of_birth = forms.CharField(label="Date of Birth",widget=forms.TextInput(attrs={'type':'date','required':True,'class':"form-control"}))
    password = forms.CharField(label="Password*",widget=forms.TextInput(attrs={'required':True,'class':"form-control", 'type' : "password"}))
    confirm_password = forms.CharField(label="Confirm Password*",widget=forms.TextInput(attrs={'required':True,'class':"form-control", 'type' : "password"}))
    
    def clean(self):
        # user age must be above 18 to register
        if self.cleaned_data.get('date_of_birth'):
            dob = datetime.strptime(self.cleaned_data.get('date_of_birth'),"%Y-%m-%d")
            now = datetime.now()
            diff = now.year-dob.year

            if diff < 18: msg="User must be atleast 18 years old" self.add_error(None, msg) #check if user name is unique username_count = User.objects.filter(username=self.cleaned_data.get('username')).count() if username_count>0:
            msg="Username '{}' has already been used.".format(self.cleaned_data.get('username'))
            self.add_error(None, msg)
        
    def clean_confirm_password(self):
        password = self.cleaned_data.get('password')
        confirm_password = self.cleaned_data.get('confirm_password')
        if confirm_password!=password:
            msg = "Password and Confirm Passwords must match."
            self.add_error('confirm_password', msg)

You may notice clean() and clean_confirm_password() methods in UserForm the form they are validation methods.
The clean() the method is form level validation this can also be used to perform field-level validation.

And the clean_confirm_password() is a field-level validation for confirm_password the field it checks if confirm_password!=password then adds error to a then particular field.

Rendering Form

Rendering of forms is an easy part we must pass the form object as an argument to render function.

In views.py create a function user_profile_create which will display rendered form.

from django.contrib.auth.models import User
from users.models import AuthUserProfile
from forms.forms import UserForm
from django.contrib.auth.hashers import make_password
from django.contrib import messages

def user_profile_create(request):
    form = UserForm()
    template="forms/user_profile_create_form.html"
    return render(request,template,{"form":form})

form = UserForm() creates form object of UserForm and is passed as an argument to the render() function.

In urls.py file add routes to view.

urlpatterns = [
    path('user/profile/create', views.user_profile_create, name='user-profile-create'),
]

Create an HTML file in your apps template folder naming user_profile_create_form.html.

<!DOCTYPE html>
<html lang="en">            
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Django Form Validation</title>
        <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.0/css/bootstrap.min.css">
        <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
        <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.0/js/bootstrap.min.js"></script>
        <style>
            .error{
                color:red;
            }
        </style>
    </head>            
    <body>
        <div class="container">
        
            {% if messages %}
                {% for message in messages %}
                    {% if message.level == DEFAULT_MESSAGE_LEVELS.SUCCESS %}
                    <div class="alert alert-success"
                    role="alert">
                        <div id="primary-notification-div">
                            {{ message }}
                        </div>
                    </div>
                    {% endif %}
                {% endfor %}
            {% endif %}
                
            <h1>User Profile</h1>
            <form action="{% url 'forms:user-profile-save' %}" method="post">
                {% csrf_token %}
    
                {% if form.errors %}
                    {% for error in form.non_field_errors %}
                        <div class="alert alert-danger">
                            <strong>{{ error|escape }}</strong>
                        </div>
                    {% endfor %}
    
                {% endif %}
    
    
                <div class="row">
                    <div class="col-md-3">
                        {{form.first_name.label}}
                        {{form.first_name}}
    
                        {% if form.errors.first_name %}
                            <label for="" class="error">{{form.errors.first_name|striptags}}</label>
                        {% endif %}
                    </div>
    
                    <div class="col-md-3">
                        {{form.last_name.label}}
                        {{form.last_name}}
                        {% if form.errors.last_name %}
                            <label for="" class="error">{{form.errors.last_name|striptags}}</label>
                        {% endif %}
                    </div>
    
                    <div class="col-md-3">
                        {{form.username.label}}
                        {{form.username}}
                        {% if form.errors.username %}
                            <label for="" class="error">{{form.errors.username|striptags}}</label>
                        {% endif %}
                    </div>
    
                    <div class="col-md-3">
                        {{form.email.label}}
                        {{form.email}}
                        {% if form.errors.email %}
                            <label for="" class="error">{{form.errors.email|striptags}}</label>
                        {% endif %}
                    </div>
    
                    <div class="col-md-3">
                        {{form.date_of_birth.label}}
                        {{form.date_of_birth}}
                        {% if form.errors.date_of_birth %}
                            <label for="" class="error">{{form.errors.date_of_birth|striptags}}</label>
                        {% endif %}
                    </div>
                </div>
    
                <div class="row" style="margin-top: 25px;">
                    <div class="col-md-3">
                        {{form.password.label}}
                        {{form.password}}
                        {% if form.errors.password %}
                            <label for="" class="error">{{form.errors.password|striptags}}</label>
                        {% endif %}
                    </div>
                </div>
    
                <div class="row" style="margin-top: 25px;">
    
                    <div class="col-md-3">
                        {{form.confirm_password.label}}
                        {{form.confirm_password}}
                        {% if form.errors.confirm_password %}
                        <label for="" class="error">{{form.errors.confirm_password|striptags}}</label>
                        {% endif %}
                    </div>
    
                    <div class="col-md-12" style="margin-top: 25px;">
                        <input type="submit" class="btn btn-sm btn-primary" value="submit">
                    </div>
                </div>
            </form>
        </div>
    </body>                
</html>

This is how our form will look when we go to route user/profile/create.

• The message displays success message once messages.add_message(request, messages.SUCCESS, ".....") is called it is just like the flash message.
• The form.errors is called on validation has failed.
• The form.non_field_errors display errors on top of form this errors are not associated to a particular field.
• The form.errors. displays the error of the particular field.

This is how errors are displayed in the form.

Saving Form

In urls.py file add routes to save the form.

urlpatterns = [
    path('user/profile/save', views.user_profile_create, name='user-profile-save'),
]

In views.py file add function user_profile_save() to save form data.

def user_profile_save(request):

    form = UserForm(request.POST)

    if form.is_valid():

        query = {
            "first_name" : form.cleaned_data.get('first_name'),
            "last_name" : form.cleaned_data.get('last_name'),
            "username" : form.cleaned_data.get('username'),
            "password" : make_password(form.cleaned_data.get('password')),
            "email" : form.cleaned_data.get('email'),
            "is_superuser" : 0,
            "is_staff" : 1,
            "is_active" : 1,
        }

        user = User.objects.create(**query)
        
        query={
            "user_id" : user.id,
            "dob" : form.cleaned_data.get('dob'),
        }
        
        AuthUserProfile.objects.create(**query)
        
        messages.add_message(request, messages.SUCCESS, "User Profile created successfully.")
        
        return HttpResponseRedirect(reverse('forms:user-profile-create'))
    
    template="forms/user_profile_create_form.html"
    
    return render(request,template,{"form":form})

The request.POST is passed to UserForm(request.POST) this binds the submitted data to Form Class.
The form.is_valid() returns a Boolean value if True then the form is clean and if False then there may be validation error.
To view validation errors after .is_valid() method we can print form.errors to view validation errors.

Calling form.cleaned_data.get('') gives use of the sanitized value to that field. Inside .is_valid() we have called model methods to save form data.

Form Validation User-Defined functions

To defined a custom validation function in Form Class name function with prefix clean followed by underscore and field name which must be validated.

Example

def clean_first_name(self):
    pass #this validates field first_name

def clean_username(self):
    pass #this validates field username

If the value of the field is not as expected that you can raise validation error or add error by mentioning field name self.add_error('field_name', "Error Message").
If you want to raise non-field error than set the first argument of add_error() method None followed by the message you want to be displayed.

self.add_error(None, msg) #this creates a non-field error

Conclusion

We have come to the end of our post on Django Form Validation.
If you have any doubts or suggestions please mention in the comments section and we’ll reach you soon and we would also love to hear requests and your recommendations for new tutorials/posts.

Related Posts

• Python Django Forms | Creating, Rendering, Validating and Saving Forms
• Django – Multiple Files Validation and Uploads

Bound and unbound forms

A Form instance is either bound to a set of data, or unbound.

• If it’s bound to a set of data, it’s capable of validating that data and rendering the form as HTML with the data displayed in the HTML.
• If it’s unbound, it cannot do validation (because there’s no data to validate!), but it can still render the blank form as HTML.

class Form

To create an unbound Form instance, instantiate the class:

>>> f = ContactForm()

To bind data to a form, pass the data as a dictionary as the first parameter to your Form class constructor:

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': '[email protected]',
...         'cc_myself': True}
>>> f = ContactForm(data)

In this dictionary, the keys are the field names, which correspond to the attributes in your Form class. The values are the data you’re trying to validate. These will usually be strings, but there’s no requirement that they be strings; the type of data you pass depends on the Field, as we’ll see in a moment.

Form.is_bound

If you need to distinguish between bound and unbound form instances at runtime, check the value of the form’s is_bound attribute:

>>> f = ContactForm()
>>> f.is_bound
False
>>> f = ContactForm({'subject': 'hello'})
>>> f.is_bound
True

Note that passing an empty dictionary creates a bound form with empty data:

>>> f = ContactForm({})
>>> f.is_bound
True

If you have a bound Form instance and want to change the data somehow, or if you want to bind an unbound Form instance to some data, create another Form instance. There is no way to change data in a Form instance. Once a Form instance has been created, you should consider its data immutable, whether it has data or not.

Using forms to validate data

Form.clean()

Implement a clean() method on your Form when you must add custom validation for fields that are interdependent. See Cleaning and validating fields that depend on each other for example usage.

Form.is_valid()

The primary task of a Form object is to validate data. With a bound Form instance, call the is_valid() method to run validation and return a boolean designating whether the data was valid:

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': '[email protected]',
...         'cc_myself': True}
>>> f = ContactForm(data)
>>> f.is_valid()
True

Let’s try with some invalid data. In this case, subject is blank (an error, because all fields are required by default) and sender is not a valid email address:

>>> data = {'subject': '',
...         'message': 'Hi there',
...         'sender': 'invalid email address',
...         'cc_myself': True}
>>> f = ContactForm(data)
>>> f.is_valid()
False

Form.errors

Access the errors attribute to get a dictionary of error messages:

>>> f.errors
{'sender': ['Enter a valid email address.'], 'subject': ['This field is required.']}

In this dictionary, the keys are the field names, and the values are lists of strings representing the error messages. The error messages are stored in lists because a field can have multiple error messages.

You can access errors without having to call is_valid() first. The form’s data will be validated the first time either you call is_valid() or access errors.

The validation routines will only get called once, regardless of how many times you access errors or call is_valid(). This means that if validation has side effects, those side effects will only be triggered once.

Form.errors.as_data()

Returns a dict that maps fields to their original ValidationError instances.

>>> f.errors.as_data()
{'sender': [ValidationError(['Enter a valid email address.'])],
'subject': [ValidationError(['This field is required.'])]}

Use this method anytime you need to identify an error by its code. This enables things like rewriting the error’s message or writing custom logic in a view when a given error is present. It can also be used to serialize the errors in a custom format (e.g. XML); for instance, as_json() relies on as_data().

The need for the as_data() method is due to backwards compatibility. Previously ValidationError instances were lost as soon as their rendered error messages were added to the Form.errors dictionary. Ideally Form.errors would have stored ValidationError instances and methods with an as_ prefix could render them, but it had to be done the other way around in order not to break code that expects rendered error messages in Form.errors.

Form.errors.as_json(escape_html=False)

Returns the errors serialized as JSON.

>>> f.errors.as_json()
{"sender": [{"message": "Enter a valid email address.", "code": "invalid"}],
"subject": [{"message": "This field is required.", "code": "required"}]}

By default, as_json() does not escape its output. If you are using it for something like AJAX requests to a form view where the client interprets the response and inserts errors into the page, you’ll want to be sure to escape the results on the client-side to avoid the possibility of a cross-site scripting attack. You can do this in JavaScript with element.textContent = errorText or with jQuery’s $(el).text(errorText) (rather than its .html() function).

If for some reason you don’t want to use client-side escaping, you can also set escape_html=True and error messages will be escaped so you can use them directly in HTML.

Form.errors.get_json_data(escape_html=False)

Returns the errors as a dictionary suitable for serializing to JSON. Form.errors.as_json() returns serialized JSON, while this returns the error data before it’s serialized.

The escape_html parameter behaves as described in Form.errors.as_json().

Form.add_error(field, error)

This method allows adding errors to specific fields from within the Form.clean() method, or from outside the form altogether; for instance from a view.

The field argument is the name of the field to which the errors should be added. If its value is None the error will be treated as a non-field error as returned by Form.non_field_errors().

The error argument can be a string, or preferably an instance of ValidationError. See Raising ValidationError for best practices when defining form errors.

Note that Form.add_error() automatically removes the relevant field from cleaned_data.

Form.has_error(field, code=None)

This method returns a boolean designating whether a field has an error with a specific error code. If code is None, it will return True if the field contains any errors at all.

To check for non-field errors use NON_FIELD_ERRORS as the field parameter.

Form.non_field_errors()

This method returns the list of errors from Form.errors that aren’t associated with a particular field. This includes ValidationErrors that are raised in Form.clean() and errors added using Form.add_error(None,
"...").

Behavior of unbound forms

It’s meaningless to validate a form with no data, but, for the record, here’s what happens with unbound forms:

>>> f = ContactForm()
>>> f.is_valid()
False
>>> f.errors
{}

Dynamic initial values

Form.initial

Use initial to declare the initial value of form fields at runtime. For example, you might want to fill in a username field with the username of the current session.

To accomplish this, use the initial argument to a Form. This argument, if given, should be a dictionary mapping field names to initial values. Only include the fields for which you’re specifying an initial value; it’s not necessary to include every field in your form. For example:

>>> f = ContactForm(initial={'subject': 'Hi there!'})

These values are only displayed for unbound forms, and they’re not used as fallback values if a particular value isn’t provided.

If a Field defines initial and you include initial when instantiating the Form, then the latter initial will have precedence. In this example, initial is provided both at the field level and at the form instance level, and the latter gets precedence:

>>> from django import forms
>>> class CommentForm(forms.Form):
...     name = forms.CharField(initial='class')
...     url = forms.URLField()
...     comment = forms.CharField()
>>> f = CommentForm(initial={'name': 'instance'}, auto_id=False)
>>> print(f)
<tr><th>Name:</th><td><input type="text" name="name" value="instance" required></td></tr>
<tr><th>Url:</th><td><input type="url" name="url" required></td></tr>
<tr><th>Comment:</th><td><input type="text" name="comment" required></td></tr>

Form.get_initial_for_field(field, field_name)

Use get_initial_for_field() to retrieve initial data for a form field. It retrieves data from Form.initial and Field.initial, in that order, and evaluates any callable initial values.

Checking which form data has changed

Form.has_changed()

Use the has_changed() method on your Form when you need to check if the form data has been changed from the initial data.

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': '[email protected]',
...         'cc_myself': True}
>>> f = ContactForm(data, initial=data)
>>> f.has_changed()
False

When the form is submitted, we reconstruct it and provide the original data so that the comparison can be done:

>>> f = ContactForm(request.POST, initial=data)
>>> f.has_changed()

has_changed() will be True if the data from request.POST differs from what was provided in initial or False otherwise. The result is computed by calling Field.has_changed() for each field in the form.

Form.changed_data

The changed_data attribute returns a list of the names of the fields whose values in the form’s bound data (usually request.POST) differ from what was provided in initial. It returns an empty list if no data differs.

>>> f = ContactForm(request.POST, initial=data)
>>> if f.has_changed():
...     print("The following fields changed: %s" % ", ".join(f.changed_data))
>>> f.changed_data
['subject', 'message']

Accessing the fields from the form

Form.fields

You can access the fields of Form instance from its fields attribute:

>>> for row in f.fields.values(): print(row)
...
<django.forms.fields.CharField object at 0x7ffaac632510>
<django.forms.fields.URLField object at 0x7ffaac632f90>
<django.forms.fields.CharField object at 0x7ffaac3aa050>
>>> f.fields['name']
<django.forms.fields.CharField object at 0x7ffaac6324d0>

You can alter the field of Form instance to change the way it is presented in the form:

>>> f.as_table().split('n')[0]
'<tr><th>Name:</th><td><input name="name" type="text" value="instance" required></td></tr>'
>>> f.fields['name'].label = "Username"
>>> f.as_table().split('n')[0]
'<tr><th>Username:</th><td><input name="name" type="text" value="instance" required></td></tr>'

Beware not to alter the base_fields attribute because this modification will influence all subsequent ContactForm instances within the same Python process:

>>> f.base_fields['name'].label = "Username"
>>> another_f = CommentForm(auto_id=False)
>>> another_f.as_table().split('n')[0]
'<tr><th>Username:</th><td><input name="name" type="text" value="class" required></td></tr>'

Accessing “clean” data

Form.cleaned_data

Each field in a Form class is responsible not only for validating data, but also for “cleaning” it – normalizing it to a consistent format. This is a nice feature, because it allows data for a particular field to be input in a variety of ways, always resulting in consistent output.

For example, DateField normalizes input into a Python datetime.date object. Regardless of whether you pass it a string in the format '1994-07-15', a datetime.date object, or a number of other formats, DateField will always normalize it to a datetime.date object as long as it’s valid.

Once you’ve created a Form instance with a set of data and validated it, you can access the clean data via its cleaned_data attribute:

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': '[email protected]',
...         'cc_myself': True}
>>> f = ContactForm(data)
>>> f.is_valid()
True
>>> f.cleaned_data
{'cc_myself': True, 'message': 'Hi there', 'sender': '[email protected]', 'subject': 'hello'}

Note that any text-based field – such as CharField or EmailField – always cleans the input into a string. We’ll cover the encoding implications later in this document.

If your data does not validate, the cleaned_data dictionary contains only the valid fields:

>>> data = {'subject': '',
...         'message': 'Hi there',
...         'sender': 'invalid email address',
...         'cc_myself': True}
>>> f = ContactForm(data)
>>> f.is_valid()
False
>>> f.cleaned_data
{'cc_myself': True, 'message': 'Hi there'}

cleaned_data will always only contain a key for fields defined in the Form, even if you pass extra data when you define the Form. In this example, we pass a bunch of extra fields to the ContactForm constructor, but cleaned_data contains only the form’s fields:

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': '[email protected]',
...         'cc_myself': True,
...         'extra_field_1': 'foo',
...         'extra_field_2': 'bar',
...         'extra_field_3': 'baz'}
>>> f = ContactForm(data)
>>> f.is_valid()
True
>>> f.cleaned_data # Doesn't contain extra_field_1, etc.
{'cc_myself': True, 'message': 'Hi there', 'sender': '[email protected]', 'subject': 'hello'}

When the Form is valid, cleaned_data will include a key and value for all its fields, even if the data didn’t include a value for some optional fields. In this example, the data dictionary doesn’t include a value for the nick_name field, but cleaned_data includes it, with an empty value:

>>> from django import forms
>>> class OptionalPersonForm(forms.Form):
...     first_name = forms.CharField()
...     last_name = forms.CharField()
...     nick_name = forms.CharField(required=False)
>>> data = {'first_name': 'John', 'last_name': 'Lennon'}
>>> f = OptionalPersonForm(data)
>>> f.is_valid()
True
>>> f.cleaned_data
{'nick_name': '', 'first_name': 'John', 'last_name': 'Lennon'}

In this above example, the cleaned_data value for nick_name is set to an empty string, because nick_name is CharField, and CharFields treat empty values as an empty string. Each field type knows what its “blank” value is – e.g., for DateField, it’s None instead of the empty string. For full details on each field’s behavior in this case, see the “Empty value” note for each field in the “Built-in Field classes” section below.

You can write code to perform validation for particular form fields (based on their name) or for the form as a whole (considering combinations of various fields). More information about this is in Form and field validation.

Outputting forms as HTML

The second task of a Form object is to render itself as HTML. To do so, print it:

>>> f = ContactForm()
>>> print(f)
<tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" required></td></tr>
<tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" required></td></tr>
<tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" required></td></tr>
<tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself"></td></tr>

If the form is bound to data, the HTML output will include that data appropriately. For example, if a field is represented by an <input type="text">, the data will be in the value attribute. If a field is represented by an <input type="checkbox">, then that HTML will include checked if appropriate:

>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': '[email protected]',
...         'cc_myself': True}
>>> f = ContactForm(data)
>>> print(f)
<tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" value="hello" required></td></tr>
<tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" value="Hi there" required></td></tr>
<tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" value="[email protected]" required></td></tr>
<tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself" checked></td></tr>

This default output is a two-column HTML table, with a <tr> for each field. Notice the following:

• For flexibility, the output does not include the <table> and </table> tags, nor does it include the <form> and </form> tags or an <input type="submit"> tag. It’s your job to do that.
• Each field type has a default HTML representation. CharField is represented by an <input type="text"> and EmailField by an <input type="email">. BooleanField(null=False) is represented by an <input type="checkbox">. Note these are merely sensible defaults; you can specify which HTML to use for a given field by using widgets, which we’ll explain shortly.
• The HTML name for each tag is taken directly from its attribute name in the ContactForm class.
• The text label for each field – e.g. 'Subject:', 'Message:' and 'Cc myself:' is generated from the field name by converting all underscores to spaces and upper-casing the first letter. Again, note these are merely sensible defaults; you can also specify labels manually.
• Each text label is surrounded in an HTML <label> tag, which points to the appropriate form field via its id. Its id, in turn, is generated by prepending 'id_' to the field name. The id attributes and <label> tags are included in the output by default, to follow best practices, but you can change that behavior.
• The output uses HTML5 syntax, targeting <!DOCTYPE html>. For example, it uses boolean attributes such as checked rather than the XHTML style of checked='checked'.

Although <table> output is the default output style when you print a form, other output styles are available. Each style is available as a method on a form object, and each rendering method returns a string.

as_p()

Form.as_p()

as_p() renders the form as a series of <p> tags, with each <p> containing one field:

>>> f = ContactForm()
>>> f.as_p()
'<p><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required></p>n<p><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required></p>n<p><label for="id_sender">Sender:</label> <input type="text" name="sender" id="id_sender" required></p>n<p><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself"></p>'
>>> print(f.as_p())
<p><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required></p>
<p><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required></p>
<p><label for="id_sender">Sender:</label> <input type="email" name="sender" id="id_sender" required></p>
<p><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself"></p>

as_ul()

Form.as_ul()

as_ul() renders the form as a series of <li> tags, with each <li> containing one field. It does not include the <ul> or </ul>, so that you can specify any HTML attributes on the <ul> for flexibility:

>>> f = ContactForm()
>>> f.as_ul()
'<li><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required></li>n<li><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required></li>n<li><label for="id_sender">Sender:</label> <input type="email" name="sender" id="id_sender" required></li>n<li><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself"></li>'
>>> print(f.as_ul())
<li><label for="id_subject">Subject:</label> <input id="id_subject" type="text" name="subject" maxlength="100" required></li>
<li><label for="id_message">Message:</label> <input type="text" name="message" id="id_message" required></li>
<li><label for="id_sender">Sender:</label> <input type="email" name="sender" id="id_sender" required></li>
<li><label for="id_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_cc_myself"></li>

as_table()

Form.as_table()

Finally, as_table() outputs the form as an HTML <table>. This is exactly the same as print. In fact, when you print a form object, it calls its as_table() method behind the scenes:

>>> f = ContactForm()
>>> f.as_table()
'<tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" required></td></tr>n<tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" required></td></tr>n<tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" required></td></tr>n<tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself"></td></tr>'
>>> print(f)
<tr><th><label for="id_subject">Subject:</label></th><td><input id="id_subject" type="text" name="subject" maxlength="100" required></td></tr>
<tr><th><label for="id_message">Message:</label></th><td><input type="text" name="message" id="id_message" required></td></tr>
<tr><th><label for="id_sender">Sender:</label></th><td><input type="email" name="sender" id="id_sender" required></td></tr>
<tr><th><label for="id_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_cc_myself"></td></tr>

Styling required or erroneous form rows

Form.error_css_class

Form.required_css_class

It’s pretty common to style form rows and fields that are required or have errors. For example, you might want to present required form rows in bold and highlight errors in red.

The Form class has a couple of hooks you can use to add class attributes to required rows or to rows with errors: set the Form.error_css_class and/or Form.required_css_class attributes:

from django import forms

class ContactForm(forms.Form):
    error_css_class = 'error'
    required_css_class = 'required'

    # ... and the rest of your fields here

Once you’ve done that, rows will be given "error" and/or "required" classes, as needed. The HTML will look something like:

>>> f = ContactForm(data)
>>> print(f.as_table())
<tr class="required"><th><label class="required" for="id_subject">Subject:</label>    ...
<tr class="required"><th><label class="required" for="id_message">Message:</label>    ...
<tr class="required error"><th><label class="required" for="id_sender">Sender:</label>      ...
<tr><th><label for="id_cc_myself">Cc myself:<label> ...
>>> f['subject'].label_tag()
<label class="required" for="id_subject">Subject:</label>
>>> f['subject'].label_tag(attrs={'class': 'foo'})
<label for="id_subject" class="foo required">Subject:</label>

Configuring form elements’ HTML id attributes and <label> tags

Form.auto_id

By default, the form rendering methods include:

• HTML id attributes on the form elements.
• The corresponding <label> tags around the labels. An HTML <label> tag designates which label text is associated with which form element. This small enhancement makes forms more usable and more accessible to assistive devices. It’s always a good idea to use <label> tags.

The id attribute values are generated by prepending id_ to the form field names. This behavior is configurable, though, if you want to change the id convention or remove HTML id attributes and <label> tags entirely.

Use the auto_id argument to the Form constructor to control the id and label behavior. This argument must be True, False or a string.

If auto_id is False, then the form output will not include <label> tags nor id attributes:

>>> f = ContactForm(auto_id=False)
>>> print(f.as_table())
<tr><th>Subject:</th><td><input type="text" name="subject" maxlength="100" required></td></tr>
<tr><th>Message:</th><td><input type="text" name="message" required></td></tr>
<tr><th>Sender:</th><td><input type="email" name="sender" required></td></tr>
<tr><th>Cc myself:</th><td><input type="checkbox" name="cc_myself"></td></tr>
>>> print(f.as_ul())
<li>Subject: <input type="text" name="subject" maxlength="100" required></li>
<li>Message: <input type="text" name="message" required></li>
<li>Sender: <input type="email" name="sender" required></li>
<li>Cc myself: <input type="checkbox" name="cc_myself"></li>
>>> print(f.as_p())
<p>Subject: <input type="text" name="subject" maxlength="100" required></p>
<p>Message: <input type="text" name="message" required></p>
<p>Sender: <input type="email" name="sender" required></p>
<p>Cc myself: <input type="checkbox" name="cc_myself"></p>

If auto_id is set to True, then the form output will include <label> tags and will use the field name as its id for each form field:

>>> f = ContactForm(auto_id=True)
>>> print(f.as_table())
<tr><th><label for="subject">Subject:</label></th><td><input id="subject" type="text" name="subject" maxlength="100" required></td></tr>
<tr><th><label for="message">Message:</label></th><td><input type="text" name="message" id="message" required></td></tr>
<tr><th><label for="sender">Sender:</label></th><td><input type="email" name="sender" id="sender" required></td></tr>
<tr><th><label for="cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="cc_myself"></td></tr>
>>> print(f.as_ul())
<li><label for="subject">Subject:</label> <input id="subject" type="text" name="subject" maxlength="100" required></li>
<li><label for="message">Message:</label> <input type="text" name="message" id="message" required></li>
<li><label for="sender">Sender:</label> <input type="email" name="sender" id="sender" required></li>
<li><label for="cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="cc_myself"></li>
>>> print(f.as_p())
<p><label for="subject">Subject:</label> <input id="subject" type="text" name="subject" maxlength="100" required></p>
<p><label for="message">Message:</label> <input type="text" name="message" id="message" required></p>
<p><label for="sender">Sender:</label> <input type="email" name="sender" id="sender" required></p>
<p><label for="cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="cc_myself"></p>

If auto_id is set to a string containing the format character '%s', then the form output will include <label> tags, and will generate id attributes based on the format string. For example, for a format string 'field_%s', a field named subject will get the id value 'field_subject'. Continuing our example:

>>> f = ContactForm(auto_id='id_for_%s')
>>> print(f.as_table())
<tr><th><label for="id_for_subject">Subject:</label></th><td><input id="id_for_subject" type="text" name="subject" maxlength="100" required></td></tr>
<tr><th><label for="id_for_message">Message:</label></th><td><input type="text" name="message" id="id_for_message" required></td></tr>
<tr><th><label for="id_for_sender">Sender:</label></th><td><input type="email" name="sender" id="id_for_sender" required></td></tr>
<tr><th><label for="id_for_cc_myself">Cc myself:</label></th><td><input type="checkbox" name="cc_myself" id="id_for_cc_myself"></td></tr>
>>> print(f.as_ul())
<li><label for="id_for_subject">Subject:</label> <input id="id_for_subject" type="text" name="subject" maxlength="100" required></li>
<li><label for="id_for_message">Message:</label> <input type="text" name="message" id="id_for_message" required></li>
<li><label for="id_for_sender">Sender:</label> <input type="email" name="sender" id="id_for_sender" required></li>
<li><label for="id_for_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_for_cc_myself"></li>
>>> print(f.as_p())
<p><label for="id_for_subject">Subject:</label> <input id="id_for_subject" type="text" name="subject" maxlength="100" required></p>
<p><label for="id_for_message">Message:</label> <input type="text" name="message" id="id_for_message" required></p>
<p><label for="id_for_sender">Sender:</label> <input type="email" name="sender" id="id_for_sender" required></p>
<p><label for="id_for_cc_myself">Cc myself:</label> <input type="checkbox" name="cc_myself" id="id_for_cc_myself"></p>

If auto_id is set to any other true value – such as a string that doesn’t include %s – then the library will act as if auto_id is True.

By default, auto_id is set to the string 'id_%s'.

Form.label_suffix

A translatable string (defaults to a colon (:) in English) that will be appended after any label name when a form is rendered.

It’s possible to customize that character, or omit it entirely, using the label_suffix parameter:

>>> f = ContactForm(auto_id='id_for_%s', label_suffix='')
>>> print(f.as_ul())
<li><label for="id_for_subject">Subject</label> <input id="id_for_subject" type="text" name="subject" maxlength="100" required></li>
<li><label for="id_for_message">Message</label> <input type="text" name="message" id="id_for_message" required></li>
<li><label for="id_for_sender">Sender</label> <input type="email" name="sender" id="id_for_sender" required></li>
<li><label for="id_for_cc_myself">Cc myself</label> <input type="checkbox" name="cc_myself" id="id_for_cc_myself"></li>
>>> f = ContactForm(auto_id='id_for_%s', label_suffix=' ->')
>>> print(f.as_ul())
<li><label for="id_for_subject">Subject -></label> <input id="id_for_subject" type="text" name="subject" maxlength="100" required></li>
<li><label for="id_for_message">Message -></label> <input type="text" name="message" id="id_for_message" required></li>
<li><label for="id_for_sender">Sender -></label> <input type="email" name="sender" id="id_for_sender" required></li>
<li><label for="id_for_cc_myself">Cc myself -></label> <input type="checkbox" name="cc_myself" id="id_for_cc_myself"></li>

Note that the label suffix is added only if the last character of the label isn’t a punctuation character (in English, those are ., !, ? or :).

Fields can also define their own label_suffix. This will take precedence over Form.label_suffix. The suffix can also be overridden at runtime using the label_suffix parameter to label_tag().

Form.use_required_attribute

When set to True (the default), required form fields will have the required HTML attribute.

Formsets instantiate forms with use_required_attribute=False to avoid incorrect browser validation when adding and deleting forms from a formset.

Configuring the rendering of a form’s widgets

Form.default_renderer

Specifies the renderer to use for the form. Defaults to None which means to use the default renderer specified by the FORM_RENDERER setting.

You can set this as a class attribute when declaring your form or use the renderer argument to Form.__init__(). For example:

from django import forms

class MyForm(forms.Form):
    default_renderer = MyRenderer()

or:

form = MyForm(renderer=MyRenderer())

Notes on field ordering

In the as_p(), as_ul() and as_table() shortcuts, the fields are displayed in the order in which you define them in your form class. For example, in the ContactForm example, the fields are defined in the order subject, message, sender, cc_myself. To reorder the HTML output, change the order in which those fields are listed in the class.

There are several other ways to customize the order:

Form.field_order

By default Form.field_order=None, which retains the order in which you define the fields in your form class. If field_order is a list of field names, the fields are ordered as specified by the list and remaining fields are appended according to the default order. Unknown field names in the list are ignored. This makes it possible to disable a field in a subclass by setting it to None without having to redefine ordering.

You can also use the Form.field_order argument to a Form to override the field order. If a Form defines field_order and you include field_order when instantiating the Form, then the latter field_order will have precedence.

Form.order_fields(field_order)

You may rearrange the fields any time using order_fields() with a list of field names as in field_order.

How errors are displayed

If you render a bound Form object, the act of rendering will automatically run the form’s validation if it hasn’t already happened, and the HTML output will include the validation errors as a <ul class="errorlist"> near the field. The particular positioning of the error messages depends on the output method you’re using:

>>> data = {'subject': '',
...         'message': 'Hi there',
...         'sender': 'invalid email address',
...         'cc_myself': True}
>>> f = ContactForm(data, auto_id=False)
>>> print(f.as_table())
<tr><th>Subject:</th><td><ul class="errorlist"><li>This field is required.</li></ul><input type="text" name="subject" maxlength="100" required></td></tr>
<tr><th>Message:</th><td><input type="text" name="message" value="Hi there" required></td></tr>
<tr><th>Sender:</th><td><ul class="errorlist"><li>Enter a valid email address.</li></ul><input type="email" name="sender" value="invalid email address" required></td></tr>
<tr><th>Cc myself:</th><td><input checked type="checkbox" name="cc_myself"></td></tr>
>>> print(f.as_ul())
<li><ul class="errorlist"><li>This field is required.</li></ul>Subject: <input type="text" name="subject" maxlength="100" required></li>
<li>Message: <input type="text" name="message" value="Hi there" required></li>
<li><ul class="errorlist"><li>Enter a valid email address.</li></ul>Sender: <input type="email" name="sender" value="invalid email address" required></li>
<li>Cc myself: <input checked type="checkbox" name="cc_myself"></li>
>>> print(f.as_p())
<p><ul class="errorlist"><li>This field is required.</li></ul></p>
<p>Subject: <input type="text" name="subject" maxlength="100" required></p>
<p>Message: <input type="text" name="message" value="Hi there" required></p>
<p><ul class="errorlist"><li>Enter a valid email address.</li></ul></p>
<p>Sender: <input type="email" name="sender" value="invalid email address" required></p>
<p>Cc myself: <input checked type="checkbox" name="cc_myself"></p>

Customizing the error list format

By default, forms use django.forms.utils.ErrorList to format validation errors. If you’d like to use an alternate class for displaying errors, you can pass that in at construction time:

>>> from django.forms.utils import ErrorList
>>> class DivErrorList(ErrorList):
...     def __str__(self):
...         return self.as_divs()
...     def as_divs(self):
...         if not self: return ''
...         return '<div class="errorlist">%s</div>' % ''.join(['<div class="error">%s</div>' % e for e in self])
>>> f = ContactForm(data, auto_id=False, error_class=DivErrorList)
>>> f.as_p()
<div class="errorlist"><div class="error">This field is required.</div></div>
<p>Subject: <input type="text" name="subject" maxlength="100" required></p>
<p>Message: <input type="text" name="message" value="Hi there" required></p>
<div class="errorlist"><div class="error">Enter a valid email address.</div></div>
<p>Sender: <input type="email" name="sender" value="invalid email address" required></p>
<p>Cc myself: <input checked type="checkbox" name="cc_myself"></p>

More granular output

The as_p(), as_ul(), and as_table() methods are shortcuts – they’re not the only way a form object can be displayed.

class BoundField

Used to display HTML or access attributes for a single field of a Form instance.

The __str__() method of this object displays the HTML for this field.

To retrieve a single BoundField, use dictionary lookup syntax on your form using the field’s name as the key:

>>> form = ContactForm()
>>> print(form['subject'])
<input id="id_subject" type="text" name="subject" maxlength="100" required>

To retrieve all BoundField objects, iterate the form:

>>> form = ContactForm()
>>> for boundfield in form: print(boundfield)
<input id="id_subject" type="text" name="subject" maxlength="100" required>
<input type="text" name="message" id="id_message" required>
<input type="email" name="sender" id="id_sender" required>
<input type="checkbox" name="cc_myself" id="id_cc_myself">

The field-specific output honors the form object’s auto_id setting:

>>> f = ContactForm(auto_id=False)
>>> print(f['message'])
<input type="text" name="message" required>
>>> f = ContactForm(auto_id='id_%s')
>>> print(f['message'])
<input type="text" name="message" id="id_message" required>

Attributes of BoundField

BoundField.auto_id

The HTML ID attribute for this BoundField. Returns an empty string if Form.auto_id is False.

BoundField.data

This property returns the data for this BoundField extracted by the widget’s value_from_datadict() method, or None if it wasn’t given:

>>> unbound_form = ContactForm()
>>> print(unbound_form['subject'].data)
None
>>> bound_form = ContactForm(data={'subject': 'My Subject'})
>>> print(bound_form['subject'].data)
My Subject

BoundField.errors

A list-like object that is displayed as an HTML <ul class="errorlist"> when printed:

>>> data = {'subject': 'hi', 'message': '', 'sender': '', 'cc_myself': ''}
>>> f = ContactForm(data, auto_id=False)
>>> print(f['message'])
<input type="text" name="message" required>
>>> f['message'].errors
['This field is required.']
>>> print(f['message'].errors)
<ul class="errorlist"><li>This field is required.</li></ul>
>>> f['subject'].errors
[]
>>> print(f['subject'].errors)

>>> str(f['subject'].errors)
''

BoundField.field

The form Field instance from the form class that this BoundField wraps.

BoundField.form

The Form instance this BoundField is bound to.

BoundField.help_text

The help_text of the field.

BoundField.html_name

The name that will be used in the widget’s HTML name attribute. It takes the form prefix into account.

BoundField.id_for_label

Use this property to render the ID of this field. For example, if you are manually constructing a <label> in your template (despite the fact that label_tag() will do this for you):

<label for="{{ form.my_field.id_for_label }}">...</label>{{ my_field }}

By default, this will be the field’s name prefixed by id_ (“id_my_field” for the example above). You may modify the ID by setting attrs on the field’s widget. For example, declaring a field like this:

my_field = forms.CharField(widget=forms.TextInput(attrs={'id': 'myFIELD'}))

and using the template above, would render something like:

<label for="myFIELD">...</label><input id="myFIELD" type="text" name="my_field" required>

BoundField.is_hidden

Returns True if this BoundField’s widget is hidden.

BoundField.label

The label of the field. This is used in label_tag().

BoundField.name

The name of this field in the form:

>>> f = ContactForm()
>>> print(f['subject'].name)
subject
>>> print(f['message'].name)
message

BoundField.widget_type

New in Django 3.1.

Returns the lowercased class name of the wrapped field’s widget, with any trailing input or widget removed. This may be used when building forms where the layout is dependent upon the widget type. For example:

{% for field in form %}
    {% if field.widget_type == 'checkbox' %}
        # render one way
    {% else %}
        # render another way
    {% endif %}
{% endfor %}

Methods of BoundField

BoundField.as_hidden(attrs=None, **kwargs)

Returns a string of HTML for representing this as an <input type="hidden">.

**kwargs are passed to as_widget().

This method is primarily used internally. You should use a widget instead.

BoundField.as_widget(widget=None, attrs=None, only_initial=False)

Renders the field by rendering the passed widget, adding any HTML attributes passed as attrs. If no widget is specified, then the field’s default widget will be used.

only_initial is used by Django internals and should not be set explicitly.

BoundField.css_classes(extra_classes=None)

When you use Django’s rendering shortcuts, CSS classes are used to indicate required form fields or fields that contain errors. If you’re manually rendering a form, you can access these CSS classes using the css_classes method:

>>> f = ContactForm(data={'message': ''})
>>> f['message'].css_classes()
'required'

If you want to provide some additional classes in addition to the error and required classes that may be required, you can provide those classes as an argument:

>>> f = ContactForm(data={'message': ''})
>>> f['message'].css_classes('foo bar')
'foo bar required'

BoundField.label_tag(contents=None, attrs=None, label_suffix=None)

To separately render the label tag of a form field, you can call its label_tag() method:

>>> f = ContactForm(data={'message': ''})
>>> print(f['message'].label_tag())
<label for="id_message">Message:</label>

You can provide the contents parameter which will replace the auto-generated label tag. An attrs dictionary may contain additional attributes for the <label> tag.

The HTML that’s generated includes the form’s label_suffix (a colon, by default) or, if set, the current field’s label_suffix. The optional label_suffix parameter allows you to override any previously set suffix. For example, you can use an empty string to hide the label on selected fields. If you need to do this in a template, you could write a custom filter to allow passing parameters to label_tag.

BoundField.value()

Use this method to render the raw value of this field as it would be rendered by a Widget:

>>> initial = {'subject': 'welcome'}
>>> unbound_form = ContactForm(initial=initial)
>>> bound_form = ContactForm(data={'subject': 'hi'}, initial=initial)
>>> print(unbound_form['subject'].value())
welcome
>>> print(bound_form['subject'].value())
hi

Customizing BoundField

If you need to access some additional information about a form field in a template and using a subclass of Field isn’t sufficient, consider also customizing BoundField.

A custom form field can override get_bound_field():

Field.get_bound_field(form, field_name)

Takes an instance of Form and the name of the field. The return value will be used when accessing the field in a template. Most likely it will be an instance of a subclass of BoundField.

If you have a GPSCoordinatesField, for example, and want to be able to access additional information about the coordinates in a template, this could be implemented as follows:

class GPSCoordinatesBoundField(BoundField):
    @property
    def country(self):
        """
        Return the country the coordinates lie in or None if it can't be
        determined.
        """
        value = self.value()
        if value:
            return get_country_from_coordinates(value)
        else:
            return None

class GPSCoordinatesField(Field):
    def get_bound_field(self, form, field_name):
        return GPSCoordinatesBoundField(form, self, field_name)

Now you can access the country in a template with {{ form.coordinates.country }}.

Binding uploaded files to a form

Dealing with forms that have FileField and ImageField fields is a little more complicated than a normal form.

Firstly, in order to upload files, you’ll need to make sure that your <form> element correctly defines the enctype as "multipart/form-data":

<form enctype="multipart/form-data" method="post" action="/foo/">

Secondly, when you use the form, you need to bind the file data. File data is handled separately to normal form data, so when your form contains a FileField and ImageField, you will need to specify a second argument when you bind your form. So if we extend our ContactForm to include an ImageField called mugshot, we need to bind the file data containing the mugshot image:

# Bound form with an image field
>>> from django.core.files.uploadedfile import SimpleUploadedFile
>>> data = {'subject': 'hello',
...         'message': 'Hi there',
...         'sender': '[email protected]',
...         'cc_myself': True}
>>> file_data = {'mugshot': SimpleUploadedFile('face.jpg', <file data>)}
>>> f = ContactFormWithMugshot(data, file_data)

In practice, you will usually specify request.FILES as the source of file data (just like you use request.POST as the source of form data):

# Bound form with an image field, data from the request
>>> f = ContactFormWithMugshot(request.POST, request.FILES)

Constructing an unbound form is the same as always – omit both form data and file data:

# Unbound form with an image field
>>> f = ContactFormWithMugshot()

Testing for multipart forms

Form.is_multipart()

If you’re writing reusable views or templates, you may not know ahead of time whether your form is a multipart form or not. The is_multipart() method tells you whether the form requires multipart encoding for submission:

>>> f = ContactFormWithMugshot()
>>> f.is_multipart()
True

Here’s an example of how you might use this in a template:

{% if form.is_multipart %}
    <form enctype="multipart/form-data" method="post" action="/foo/">
{% else %}
    <form method="post" action="/foo/">
{% endif %}
{{ form }}
</form>

Subclassing forms

If you have multiple Form classes that share fields, you can use subclassing to remove redundancy.

When you subclass a custom Form class, the resulting subclass will include all fields of the parent class(es), followed by the fields you define in the subclass.

In this example, ContactFormWithPriority contains all the fields from ContactForm, plus an additional field, priority. The ContactForm fields are ordered first:

>>> class ContactFormWithPriority(ContactForm):
...     priority = forms.CharField()
>>> f = ContactFormWithPriority(auto_id=False)
>>> print(f.as_ul())
<li>Subject: <input type="text" name="subject" maxlength="100" required></li>
<li>Message: <input type="text" name="message" required></li>
<li>Sender: <input type="email" name="sender" required></li>
<li>Cc myself: <input type="checkbox" name="cc_myself"></li>
<li>Priority: <input type="text" name="priority" required></li>

It’s possible to subclass multiple forms, treating forms as mixins. In this example, BeatleForm subclasses both PersonForm and InstrumentForm (in that order), and its field list includes the fields from the parent classes:

>>> from django import forms
>>> class PersonForm(forms.Form):
...     first_name = forms.CharField()
...     last_name = forms.CharField()
>>> class InstrumentForm(forms.Form):
...     instrument = forms.CharField()
>>> class BeatleForm(InstrumentForm, PersonForm):
...     haircut_type = forms.CharField()
>>> b = BeatleForm(auto_id=False)
>>> print(b.as_ul())
<li>First name: <input type="text" name="first_name" required></li>
<li>Last name: <input type="text" name="last_name" required></li>
<li>Instrument: <input type="text" name="instrument" required></li>
<li>Haircut type: <input type="text" name="haircut_type" required></li>

It’s possible to declaratively remove a Field inherited from a parent class by setting the name of the field to None on the subclass. For example:

>>> from django import forms

>>> class ParentForm(forms.Form):
...     name = forms.CharField()
...     age = forms.IntegerField()

>>> class ChildForm(ParentForm):
...     name = None

>>> list(ChildForm().fields)
['age']

Prefixes for forms

Form.prefix

You can put several Django forms inside one <form> tag. To give each Form its own namespace, use the prefix keyword argument:

>>> mother = PersonForm(prefix="mother")
>>> father = PersonForm(prefix="father")
>>> print(mother.as_ul())
<li><label for="id_mother-first_name">First name:</label> <input type="text" name="mother-first_name" id="id_mother-first_name" required></li>
<li><label for="id_mother-last_name">Last name:</label> <input type="text" name="mother-last_name" id="id_mother-last_name" required></li>
>>> print(father.as_ul())
<li><label for="id_father-first_name">First name:</label> <input type="text" name="father-first_name" id="id_father-first_name" required></li>
<li><label for="id_father-last_name">Last name:</label> <input type="text" name="father-last_name" id="id_father-last_name" required></li>

The prefix can also be specified on the form class:

>>> class PersonForm(forms.Form):
...     ...
...     prefix = 'person'

In this tutorial, we will learn how to show custom validation exceptions on Django admin.

Understanding The Problem

Django admin site will raise exceptions when we try to save objects with non-acceptable values. Often we need to create our custom validators and raise custom exceptions depending on the needs of the project. Like if you want the phone number to be in a particular format or you want to make sure the first name starts with a capital letter.

There are a lot of ways to achieve this but if your goal is to ensure proper validation limited from the admin interface only, then this tutorial is for you.

Note: If creating validators that remain constant throughout the app is your goal, then you should check out this article — Creating Custom Model Validation In Django

Displaying Custom Validation Exception in Django Admin

We will use the popular blog project for this tutorial. The goal is to ensure that the title of the post is in the proper title case.

Let’s have a look at our existing models.py and admin.py file.

models.py

class Post(models.Model):
    title = models.CharField(max_length=200, unique=True)
    slug = models.SlugField(max_length=200, unique=True)
    author = models.ForeignKey(
        User, on_delete=models.CASCADE, related_name="blog_posts")
    updated_on = models.DateTimeField(auto_now=True)
    content = models.TextField()
    created_on = models.DateTimeField(auto_now_add=True)
    status = models.IntegerField(choices=STATUS, default=0)
    summary = models.CharField(max_length=500, null=True, blank=True)

    class Meta:
        ordering = ["-created_on"]

    def __str__(self):
        return self.title

admin.py

@admin.register(Post)
class PostAdmin(SummernoteModelAdmin):
    list_display = ('title', 'slug', 'status', 'created_on')
    list_filter = ('status', 'created_on')
    search_fields = ['title', 'content']
    prepopulated_fields = {'slug': ('title',)}

Now to make custom validators for the admin site we need to create a custom form for our model. So in the same file or in a separate file create a form like this.

from django import forms

class PostForm(forms.ModelForm):
    def clean(self):
        title = self.cleaned_data['title']
        if not title.istitle():
            raise forms.ValidationError({'title': "Not a proper titlecased string"})

The model form comes with the clean() method that is responsible for performing validations.

Take the title value from the cleaned_data dictionary and run the validation through it, if it fails the raise the ValidationError with the exception message.

Finally. we need to link our form with the PostAdmin.

@admin.register(Post)
class PostAdmin(SummernoteModelAdmin):
    form = PostForm
    list_display = ('title', 'slug', 'status', 'created_on')
    list_filter = ('status', 'created_on')
    search_fields = ['title', 'content']
    prepopulated_fields = {'slug': ('title',)}

That’s it! Save the files run the server navigate to the admin and test the flow.

DJANGO