Python error fix

Misusing the Assignment Operator (=)

There are several cases in Python where you’re not able to make assignments to objects. Some examples are assigning to literals and function calls. In the code block below, you can see a few examples that attempt to do this and the resulting SyntaxError tracebacks:

>>> len('hello') = 5
  File "<stdin>", line 1
SyntaxError: can't assign to function call

>>> 'foo' = 1
  File "<stdin>", line 1
SyntaxError: can't assign to literal

>>> 1 = 'foo'
  File "<stdin>", line 1
SyntaxError: can't assign to literal

The first example tries to assign the value 5 to the len() call. The SyntaxError message is very helpful in this case. It tells you that you can’t assign a value to a function call.

The second and third examples try to assign a string and an integer to literals. The same rule is true for other literal values. Once again, the traceback messages indicate that the problem occurs when you attempt to assign a value to a literal.

It’s likely that your intent isn’t to assign a value to a literal or a function call. For instance, this can occur if you accidentally leave off the extra equals sign (=), which would turn the assignment into a comparison. A comparison, as you can see below, would be valid:

>>> len('hello') == 5
True

Most of the time, when Python tells you that you’re making an assignment to something that can’t be assigned to, you first might want to check to make sure that the statement shouldn’t be a Boolean expression instead. You may also run into this issue when you’re trying to assign a value to a Python keyword, which you’ll cover in the next section.

Misspelling, Missing, or Misusing Python Keywords

Python keywords are a set of protected words that have special meaning in Python. These are words you can’t use as identifiers, variables, or function names in your code. They’re a part of the language and can only be used in the context that Python allows.

There are three common ways that you can mistakenly use keywords:

1. Misspelling a keyword
2. Missing a keyword
3. Misusing a keyword

If you misspell a keyword in your Python code, then you’ll get a SyntaxError. For example, here’s what happens if you spell the keyword for incorrectly:

>>> fro i in range(10):
  File "<stdin>", line 1
    fro i in range(10):
        ^
SyntaxError: invalid syntax

The message reads SyntaxError: invalid syntax, but that’s not very helpful. The traceback points to the first place where Python could detect that something was wrong. To fix this sort of error, make sure that all of your Python keywords are spelled correctly.

Another common issue with keywords is when you miss them altogether:

>>> for i range(10):
  File "<stdin>", line 1
    for i range(10):
              ^
SyntaxError: invalid syntax

Once again, the exception message isn’t that helpful, but the traceback does attempt to point you in the right direction. If you move back from the caret, then you can see that the in keyword is missing from the for loop syntax.

You can also misuse a protected Python keyword. Remember, keywords are only allowed to be used in specific situations. If you use them incorrectly, then you’ll have invalid syntax in your Python code. A common example of this is the use of continue or break outside of a loop. This can easily happen during development when you’re implementing things and happen to move logic outside of a loop:

>>> names = ['pam', 'jim', 'michael']
>>> if 'jim' in names:
...     print('jim found')
...     break
...
  File "<stdin>", line 3
SyntaxError: 'break' outside loop

>>> if 'jim' in names:
...     print('jim found')
...     continue
...
  File "<stdin>", line 3
SyntaxError: 'continue' not properly in loop

Here, Python does a great job of telling you exactly what’s wrong. The messages "'break' outside loop" and "'continue' not properly in loop" help you figure out exactly what to do. If this code were in a file, then Python would also have the caret pointing right to the misused keyword.

Another example is if you attempt to assign a Python keyword to a variable or use a keyword to define a function:

>>> pass = True
  File "<stdin>", line 1
    pass = True
         ^
SyntaxError: invalid syntax

>>> def pass():
  File "<stdin>", line 1
    def pass():
           ^
SyntaxError: invalid syntax

When you attempt to assign a value to pass, or when you attempt to define a new function called pass, you’ll get a SyntaxError and see the "invalid syntax" message again.

It might be a little harder to solve this type of invalid syntax in Python code because the code looks fine from the outside. If your code looks good, but you’re still getting a SyntaxError, then you might consider checking the variable name or function name you want to use against the keyword list for the version of Python that you’re using.

The list of protected keywords has changed with each new version of Python. For example, in Python 3.6 you could use await as a variable name or function name, but as of Python 3.7, that word has been added to the keyword list. Now, if you try to use await as a variable or function name, this will cause a SyntaxError if your code is for Python 3.7 or later.

Another example of this is print, which differs in Python 2 vs Python 3:

print is a keyword in Python 2, so you can’t assign a value to it. In Python 3, however, it’s a built-in function that can be assigned values.

You can run the following code to see the list of keywords in whatever version of Python you’re running:

import keyword
print(keyword.kwlist)

keyword also provides the useful keyword.iskeyword(). If you just need a quick way to check the pass variable, then you can use the following one-liner:

>>> import keyword; keyword.iskeyword('pass')
True

This code will tell you quickly if the identifier that you’re trying to use is a keyword or not.

Missing Parentheses, Brackets, and Quotes

Often, the cause of invalid syntax in Python code is a missed or mismatched closing parenthesis, bracket, or quote. These can be hard to spot in very long lines of nested parentheses or longer multi-line blocks. You can spot mismatched or missing quotes with the help of Python’s tracebacks:

>>> message = 'don't'
  File "<stdin>", line 1
    message = 'don't'
                   ^
SyntaxError: invalid syntax

Here, the traceback points to the invalid code where there’s a t' after a closing single quote. To fix this, you can make one of two changes:

1. Escape the single quote with a backslash ('don't')
2. Surround the entire string in double-quotes instead ("don't")

Another common mistake is to forget to close string. With both double-quoted and single-quoted strings, the situation and traceback are the same:

>>> message = "This is an unclosed string
  File "<stdin>", line 1
    message = "This is an unclosed string
                                        ^
SyntaxError: EOL while scanning string literal

This time, the caret in the traceback points right to the problem code. The SyntaxError message, "EOL while scanning string literal", is a little more specific and helpful in determining the problem. This means that the Python interpreter got to the end of a line (EOL) before an open string was closed. To fix this, close the string with a quote that matches the one you used to start it. In this case, that would be a double quote (").

Quotes missing from statements inside an f-string can also lead to invalid syntax in Python:

 1# theofficefacts.py
 2ages = {
 3    'pam': 24,
 4    'jim': 24,
 5    'michael': 43
 6}
 7print(f'Michael is {ages["michael]} years old.')

Here, the reference to the ages dictionary inside the printed f-string is missing the closing double quote from the key reference. The resulting traceback is as follows:

$ python theofficefacts.py
  File "theofficefacts.py", line 7
    print(f'Michael is {ages["michael]} years old.')
         ^
SyntaxError: f-string: unterminated string

Python identifies the problem and tells you that it exists inside the f-string. The message "unterminated string" also indicates what the problem is. The caret in this case only points to the beginning of the f-string.

This might not be as helpful as when the caret points to the problem area of the f-string, but it does narrow down where you need to look. There’s an unterminated string somewhere inside that f-string. You just have to find out where. To fix this problem, make sure that all internal f-string quotes and brackets are present.

The situation is mostly the same for missing parentheses and brackets. If you leave out the closing square bracket from a list, for example, then Python will spot that and point it out. There are a few variations of this, however. The first is to leave the closing bracket off of the list:

# missing.py
def foo():
    return [1, 2, 3

print(foo())

When you run this code, you’ll be told that there’s a problem with the call to print():

$ python missing.py
  File "missing.py", line 5
    print(foo())
        ^
SyntaxError: invalid syntax

What’s happening here is that Python thinks the list contains three elements: 1, 2, and 3 print(foo()). Python uses whitespace to group things logically, and because there’s no comma or bracket separating 3 from print(foo()), Python lumps them together as the third element of the list.

Another variation is to add a trailing comma after the last element in the list while still leaving off the closing square bracket:

# missing.py
def foo():
    return [1, 2, 3,

print(foo())

Now you get a different traceback:

$ python missing.py
  File "missing.py", line 6

                ^
SyntaxError: unexpected EOF while parsing

In the previous example, 3 and print(foo()) were lumped together as one element, but here you see a comma separating the two. Now, the call to print(foo()) gets added as the fourth element of the list, and Python reaches the end of the file without the closing bracket. The traceback tells you that Python got to the end of the file (EOF), but it was expecting something else.

In this example, Python was expecting a closing bracket (]), but the repeated line and caret are not very helpful. Missing parentheses and brackets are tough for Python to identify. Sometimes the only thing you can do is start from the caret and move backward until you can identify what’s missing or wrong.

Mistaking Dictionary Syntax

You saw earlier that you could get a SyntaxError if you leave the comma off of a dictionary element. Another form of invalid syntax with Python dictionaries is the use of the equals sign (=) to separate keys and values, instead of the colon:

>>> ages = {'pam'=24}
  File "<stdin>", line 1
    ages = {'pam'=24}
                 ^
SyntaxError: invalid syntax

Once again, this error message is not very helpful. The repeated line and caret, however, are very helpful! They’re pointing right to the problem character.

This type of issue is common if you confuse Python syntax with that of other programming languages. You’ll also see this if you confuse the act of defining a dictionary with a dict() call. To fix this, you could replace the equals sign with a colon. You can also switch to using dict():

>>> ages = dict(pam=24)
>>> ages
{'pam': 24}

You can use dict() to define the dictionary if that syntax is more helpful.

Using the Wrong Indentation

There are two sub-classes of SyntaxError that deal with indentation issues specifically:

1. IndentationError
2. TabError

While other programming languages use curly braces to denote blocks of code, Python uses whitespace. That means that Python expects the whitespace in your code to behave predictably. It will raise an IndentationError if there’s a line in a code block that has the wrong number of spaces:

 1# indentation.py
 2def foo():
 3    for i in range(10):
 4        print(i)
 5  print('done')
 6
 7foo()

This might be tough to see, but line 5 is only indented 2 spaces. It should be in line with the for loop statement, which is 4 spaces over. Thankfully, Python can spot this easily and will quickly tell you what the issue is.

There’s also a bit of ambiguity here, though. Is the print('done') line intended to be after the for loop or inside the for loop block? When you run the above code, you’ll see the following error:

$ python indentation.py
  File "indentation.py", line 5
    print('done')
                ^
IndentationError: unindent does not match any outer indentation level

Even though the traceback looks a lot like the SyntaxError traceback, it’s actually an IndentationError. The error message is also very helpful. It tells you that the indentation level of the line doesn’t match any other indentation level. In other words, print('done') is indented 2 spaces, but Python can’t find any other line of code that matches this level of indentation. You can fix this quickly by making sure the code lines up with the expected indentation level.

The other type of SyntaxError is the TabError, which you’ll see whenever there’s a line that contains either tabs or spaces for its indentation, while the rest of the file contains the other. This might go hidden until Python points it out to you!

If your tab size is the same width as the number of spaces in each indentation level, then it might look like all the lines are at the same level. However, if one line is indented using spaces and the other is indented with tabs, then Python will point this out as a problem:

 1# indentation.py
 2def foo():
 3    for i in range(10):
 4        print(i)
 5    print('done')
 6
 7foo()

Here, line 5 is indented with a tab instead of 4 spaces. This code block could look perfectly fine to you, or it could look completely wrong, depending on your system settings.

Python, however, will notice the issue immediately. But before you run the code to see what Python will tell you is wrong, it might be helpful for you to see an example of what the code looks like under different tab width settings:

$ tabs 4 # Sets the shell tab width to 4 spaces
$ cat -n indentation.py
     1   # indentation.py
     2   def foo():
     3       for i in range(10)
     4           print(i)
     5       print('done')
     6   
     7   foo()

$ tabs 8 # Sets the shell tab width to 8 spaces (standard)
$ cat -n indentation.py
     1   # indentation.py
     2   def foo():
     3       for i in range(10)
     4           print(i)
     5           print('done')
     6   
     7   foo()

$ tabs 3 # Sets the shell tab width to 3 spaces
$ cat -n indentation.py
     1   # indentation.py
     2   def foo():
     3       for i in range(10)
     4           print(i)
     5      print('done')
     6   
     7   foo()

Notice the difference in display between the three examples above. Most of the code uses 4 spaces for each indentation level, but line 5 uses a single tab in all three examples. The width of the tab changes, based on the tab width setting:

• If the tab width is 4, then the print statement will look like it’s outside the for loop. The console will print 'done' at the end of the loop.
• If the tab width is 8, which is standard for a lot of systems, then the print statement will look like it’s inside the for loop. The console will print 'done' after each number.
• If the tab width is 3, then the print statement looks out of place. In this case, line 5 doesn’t match up with any indentation level.

When you run the code, you’ll get the following error and traceback:

$ python indentation.py
  File "indentation.py", line 5
    print('done')
                ^
TabError: inconsistent use of tabs and spaces in indentation

Notice the TabError instead of the usual SyntaxError. Python points out the problem line and gives you a helpful error message. It tells you clearly that there’s a mixture of tabs and spaces used for indentation in the same file.

The solution to this is to make all lines in the same Python code file use either tabs or spaces, but not both. For the code blocks above, the fix would be to remove the tab and replace it with 4 spaces, which will print 'done' after the for loop has finished.

Defining and Calling Functions

You might run into invalid syntax in Python when you’re defining or calling functions. For example, you’ll see a SyntaxError if you use a semicolon instead of a colon at the end of a function definition:

>>> def fun();
  File "<stdin>", line 1
    def fun();
             ^
SyntaxError: invalid syntax

The traceback here is very helpful, with the caret pointing right to the problem character. You can clear up this invalid syntax in Python by switching out the semicolon for a colon.

In addition, keyword arguments in both function definitions and function calls need to be in the right order. Keyword arguments always come after positional arguments. Failure to use this ordering will lead to a SyntaxError:

>>> def fun(a, b):
...     print(a, b)
...
>>> fun(a=1, 2)
  File "<stdin>", line 1
SyntaxError: positional argument follows keyword argument

Here, once again, the error message is very helpful in telling you exactly what is wrong with the line.

Changing Python Versions

Sometimes, code that works perfectly fine in one version of Python breaks in a newer version. This is due to official changes in language syntax. The most well-known example of this is the print statement, which went from a keyword in Python 2 to a built-in function in Python 3:

>>> # Valid Python 2 syntax that fails in Python 3
>>> print 'hello'
  File "<stdin>", line 1
    print 'hello'
                ^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print('hello')?

This is one of the examples where the error message provided with the SyntaxError shines! Not only does it tell you that you’re missing parenthesis in the print call, but it also provides the correct code to help you fix the statement.

Another problem you might encounter is when you’re reading or learning about syntax that’s valid syntax in a newer version of Python, but isn’t valid in the version you’re writing in. An example of this is the f-string syntax, which doesn’t exist in Python versions before 3.6:

>>> # Any version of python before 3.6 including 2.7
>>> w ='world'
>>> print(f'hello, {w}')
  File "<stdin>", line 1
    print(f'hello, {w}')
                      ^
SyntaxError: invalid syntax

In versions of Python before 3.6, the interpreter doesn’t know anything about the f-string syntax and will just provide a generic "invalid syntax" message. The problem, in this case, is that the code looks perfectly fine, but it was run with an older version of Python. When in doubt, double-check which version of Python you’re running!

Python syntax is continuing to evolve, and there are some cool new features introduced in Python 3.8:

• Walrus operator (assignment expressions)
• F-string syntax for debugging
• Positional-only arguments

If you want to try out some of these new features, then you need to make sure you’re working in a Python 3.8 environment. Otherwise, you’ll get a SyntaxError.

Python 3.8 also provides the new SyntaxWarning. You’ll see this warning in situations where the syntax is valid but still looks suspicious. An example of this would be if you were missing a comma between two tuples in a list. This would be valid syntax in Python versions before 3.8, but the code would raise a TypeError because a tuple is not callable:

>>> [(1,2)(2,3)]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object is not callable

This TypeError means that you can’t call a tuple like a function, which is what the Python interpreter thinks you’re doing.

In Python 3.8, this code still raises the TypeError, but now you’ll also see a SyntaxWarning that indicates how you can go about fixing the problem:

>>> [(1,2)(2,3)]
<stdin>:1: SyntaxWarning: 'tuple' object is not callable; perhaps you missed a comma?
Traceback (most recent call last):   
  File "<stdin>", line 1, in <module>    
TypeError: 'tuple' object is not callable

The helpful message accompanying the new SyntaxWarning even provides a hint ("perhaps you missed a comma?") to point you in the right direction!