Try catch error python

Introduction to try catch exception in Python

A program in python terminates as it encounters an error. Mainly there are two common errors; syntax error and exceptions.

Python exceptions are errors that happen during execution of the program. Python try and except statements are one of the important statements that can catch exceptions. I

n this tutorial we will cover Python exceptions in more details  and will see how python try except statements  help us to try catch these exceptions along with examples. Moreover, we will also learn about some of the built-in exceptions in python and how to use them.

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Difference between Python Exception and Error

Python exceptions and errors are two different things. There are some common differences between them which distinguish both from each other. These are the following differences between them.

• Errors cannot be handled, while Python exceptions can be catchd at the run time.
• The error indicates a problem that mainly occurs due to the lack of system resources while Exceptions are the problems which can occur at runtime and compile time. It mainly occurs in the code written by the developers.
• An error can be a syntax (parsing) error, while there can be many types of exceptions that could occur during the execution.
• An error might indicate critical problems that a reasonable application should not try to catch, while an exception might indicate conditions that an application should try to catch.

Different type of errors in Python

Errors are the problems in a program due to which the program will stop the execution. There can be different types of errors which might occur while coding. Some of which are Syntax error, recursion error and logical error. In this section we will closely look at each of these errors in more detail.

Syntax error in Python

When we run our python code, the first thing that interpreter will do is convert the code into python bytecode which then will be executed. If the interpreter finds any error/ invalid syntax during this state, it will not be able to convert our code into python code which means we have used invalid syntax somewhere in our code. The good thing is that most of the interpreters show us/give hints about the error which helps us to resolve it.  If you are a beginner to Python language, you will see syntaxError a lot while running your code.

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Here is an example showing the syntax error.

# defining variables
a = 10 b = 20

Output:

You can see the python is indicating the error and pointing to the error.

Recursion error in Python

This recursion error occurs when we call a function. As the name suggests, recursion error when too many methods, one inside another is executed with one an infinite recursion.

See the following simple example of recursion error:

# defining a function:
def main():
   return main()

<i># calling the function</i>
main()

Output:

See the last line, which shows the error. We get a recursionError because the function calls itself infinitely times without any break or other return statement.

Python exceptions

Sometimes, even if the syntax of the expression is correct, it may still cause an error when it executes. Such errors are called logical errors or exceptions. Logical errors are the errors that are detected during execution time and are not unconditionally fatal. Later in the tutorial we will cover how to catch these errors using python try except method.

Again there can be many different kinds of Python exceptions you may face. Some of them are typeError, ZeroDivisionError, importError and many more. We will see some of the examples of exceptions here.

See the example which gives typeError when we try to add two different data types.

# string data type
name = "bashir"

<i># integer data type</i>
age = 21

# printing by adding int and string
print(age + name)

Output:

ALSO READ: 4 ways to add row to existing DataFrame in Pandas

Here is one more example which gives zeroDivisionError when we try to divide some number by zero.

# zero division
print(23/0)

Output:

    print(23/0)
ZeroDivisionError: division by zero

Now see the last example of import module error. This error occurs when we import a module that does not exist.

# importing module
import abce

Output:

    import abce
ModuleNotFoundError: No module named 'abce'

Python try catch exception

In Python, it is possible to write programs that catch selected exceptions. Python try except statements are the most common way to catch Python exceptions. In this section we will cover some of the common ways using python try except methods to catch different exceptions that might occur during the execution time of code.

Before going into the built-in python expedition handling statements, let us first learn how we can raise an exception if a condition is not fulfilled. In Python we use keyword raise to throw an exception. Let us say we want to take integer input from the user, and if the user enters a string value, then the program should throw an exception.

See the following example.

# taking input from user
number = "b"

# checkting type of variable
if type(number)==int:
   pass

# execute else part if the variable if not int type
else:
   raise TypeError("Only numeric values are required!!")

Output:

  raise TypeError("Only numeric values are required!!")
TypeError: Only numeric values are required!!

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Python try except method to catch exception

As we already know that when syntactically correct code runs into an errors, then Python will throw an exception error. This exception will crash the program if not handled. The except clause determines how our program responds to exceptions.

Python try and except statements are used to catch and handle such exceptions. Python will first execute the try statement as a normal part of the program. If it successfully executes the try block, then it will not go to the except block. If the program catches any error during execution of the try block, then the execution of the program will shift to the except block.

Python try catch syntax

Python syntax to perform a try and catch can be achieved using following  try except block looks like this:

try:
   ## do normal statement
except Exception:
   ## handle the error

Example to catch exception with python try except

Let us now take a real example and try to divide some numeric value by zero. We know that if we do not handle the zeroError our code will crash and will give us errors. See the example below how we managed to catch zeroError using Python try and except block.

# Python try and except method
try:
   print(23/0)

# Execute except block if zero division occurs
except ZeroDivisionError:
   # handle zero division error
   print("Cannot divide by zero!")

Output:

Cannot divide by zero!

Notice that our program executes successfully in spite of facing zeroDivisionError.

In a similar way we can catch other exceptions that occur during execution of a program. Let us catch the import error that occurred during the import of unknown or uninstalled modules.

# Python try and except method
try:
   import abcd

# execute except block if import error occurs
except ImportError:
   # handle import error
   print("Unable to import module!!")

Output:

Unable to import module!!

See the following example which catchs the TypeError that occurs in Python when we try to add an integer with string.

# Python try and except method
try:
   print(12 + "bashir")

# excute except block if type error occurs
except TypeError:
   # handle type error
   print("Strings cannot be added to integer")

Output:

Strings cannot be added to integer

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Python try except with ELSE to catch exception

The Python else clause is useful when we want to run a piece of code only when there will be no error or exception. Because The code inside the else block will be only executed  if the try clause does not raise an exception.

Python try and catch with else syntax

See the simple syntax of python try catch with else statement.

# try block
try:
    # statements run if no exception occurs
except (name_of_exception):
    # Handle exeption
# Else block only executes if no exception occurs
else:
    # else statements 

Python try and catch with else example

Now let us take a practical example to implement python try catch with else block. See the following example where the else statement will only be executed if there will be no exception.

#  Python try and except method
try:
   number = 24/6

# execute except block if zero division occurs
except ZeroDivisionError:
   # hanlde exception
   print("Cannot divide by")

# Run this else block if no exception occurs
else:
   print("the answer is {}".format(number))

Output:

the answer is 4.0

If we try to divide the number by 0 it will raise an exception and the else statement will not be executed. See the example below:

#  Python try and except method
try:
   number = 24/0

#execute except blcok if zero division occurs
except ZeroDivisionError:
   # handle exception
   print("Cannot divide by zero")

# Executes else block only if no exception occurs
else:
   print("the answer is {}".format(number))

Output:

Cannot divide by zero

Let us take one more example of the importing module as well using the python try except and else statement.

#  Python try and except method
try:
   import math

# Execute except block if module error accurs
except ModuleNotFoundError:
   print("no module found!!")

# Execute else block only if no exception occurs
else:
   print(math.pow(2, 3))

Output:

8.0

You can notice that the else statement in python try except only executes when the program does not produce any exceptions.

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Python try expect with FINALLY to catch exception

We might come across a situation in our program when we want to execute the program even if an exception occurs. We can achieve this either by writing the piece of code in try and except both blocks ( which is not  recommended when we have a large amount of code) or we can use the keyword finally. Finally, the statement is opposite of else , it always executes after try and except blocks.

Python try and catch with finally syntax

Here is simple syntax of python try catch with finally block.

# try block
try:
    # statements run if no exception occurs
except (name_of_exception):
    # Hanlde exception
# this block will be executed always
# independent of except status
finally:
    # final statements

Python try and catch with finally example

See the example below which uses finally block along with python try except.

#  Python try and except method
try:
   number = 24/0

# execute except block if zero division occur
except ZeroDivisionError:
   print("Cannot divide by zero")

# Always run finally block
finally:
   print("This block successfully was executed!")

Output:

Cannot divide by zero
This block successfully was executed!

Notice that even our code produces ZeroDivisionError but still the finally block was executed successfully.

Now let us take one more example of importing a module without any exception.

# Python try and except method
try:
   import math

# run except block if module error occurs
except ModuleNotFoundError:
   print("No module exist")

# Always run finally block
finally:
   print("Good job!")

Output:

Good job!

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Multiple EXCEPT clauses to catch exceptions in Python

So far in each example we have used only one except block. However, Python allows us to use multiple except blocks as well. While executing the try block if there is an exception, then the very first exception block will be considered, if the first except block catchs the exception, others will not be executed, else each except blocks will be checked respectively. Multiple except clauses are helpful when we are not sure about the type of exception that might occur.

Syntax to catch multiple exceptions

Here is the simple syntax of multiple except clauses.

# try and catch block
try:
    # run try statements if no exception occurs

# run this block if exception_one occurs
except exception_one:
    # handle exception_one

# run this block if exception_two occurs
except exception_two:
    # hanlde exception_two

.
.
.

# run this block if exception_n occurs
exception exception_n:
    # handle exception_n

Examples to catch multiple exceptions in Python

See the example below which uses multiple except blocks in Python.

#  Python try and except method
try:
   import math
   number = 24/6
   print(number)

# run except block if module error occur
except ModuleNotFoundError:
   print("No module exist")

# run except blcok if zero division occurs
except ZeroDivisionError:
   print("Cannot divide by zero!")

# Always run finally block
finally:
   print("Good job!")

Output:

4.0
Good job!

Now let us import an unknown module. See the example below:

#  Python try and except method
try:
   import abcd
   number = 24/6
   print(number)

#run except block if module error occurs
except ModuleNotFoundError:
   print("No module exist")

# run this block if zero division occurs
except ZeroDivisionError:
   print("Cannot divide by zero!")

# Always run finally block
finally:
   print("Good job!")

Output:

No module exist
Good job!

ALSO READ: How to declare global variables in Python

The assertError exception handling in Python

Instead of waiting for a program to crash midway, we can also start making precautions by making assertions in Python. We can assert a condition that if this condition is met to be true then execute the program. And if the condition turns out to be False, we can have a program to throw an assertion error exception.

See the following example which shows how assertion works in Python

try:
   denominator = 0
   assert denominator != 0, "Cannot divide by zero"
   print(24/ denominator)
 
# executes the assertionError if assert condition is false
except AssertionError as msg:
   print(msg)

Output:

Cannot divide by zero

Summary

Python exceptions are inherited from the class BaseException. Exceptions are throw by the interpreter while executing the programming and crashes when it come across any exception. In order to run our program successfully it is very important to catch such exceptions. In this tutorial we covered python exceptions, difference between and error and exceptions. Moreover, we learned about python try except statements to catch expectations and also come across some of the important python built-in exceptions.

Further Reading

Python exceptions
try and except statement
More about python exceptions

Содержание:развернуть

• Как устроен механизм исключений
• Как обрабатывать исключения в Python (try except)

• As — сохраняет ошибку в переменную

• Finally — выполняется всегда

• Else — выполняется когда исключение не было вызвано

• Несколько блоков except

• Несколько типов исключений в одном блоке except

• Raise — самостоятельный вызов исключений

• Как пропустить ошибку

• Исключения в lambda функциях
• 20 типов встроенных исключений в Python
• Как создать свой тип Exception

Программа, написанная на языке Python, останавливается сразу как обнаружит ошибку. Ошибки могут быть (как минимум) двух типов:

• Синтаксические ошибки — возникают, когда написанное выражение не соответствует правилам языка (например, написана лишняя скобка);
• Исключения — возникают во время выполнения программы (например, при делении на ноль).

Синтаксические ошибки исправить просто (если вы используете IDE, он их подсветит). А вот с исключениями всё немного сложнее — не всегда при написании программы можно сказать возникнет или нет в данном месте исключение. Чтобы приложение продолжило работу при возникновении проблем, такие ошибки нужно перехватывать и обрабатывать с помощью блока try/except.

Как устроен механизм исключений

В Python есть встроенные исключения, которые появляются после того как приложение находит ошибку. В этом случае текущий процесс временно приостанавливается и передает ошибку на уровень вверх до тех пор, пока она не будет обработано. Если ошибка не будет обработана, программа прекратит свою работу (а в консоли мы увидим Traceback с подробным описанием ошибки).

💁‍♂️ Пример: напишем скрипт, в котором функция ожидает число, а мы передаём сроку (это вызовет исключение «TypeError»):

def b(value):
print("-> b")
print(value + 1) # ошибка тут

> -> a
> -> b
> Traceback (most recent call last):
> File "test.py", line 11, in <module>
> a("10")
> File "test.py", line 8, in a
> b(value)
> File "test.py", line 3, in b
> print(value + 1)
> TypeError: can only concatenate str (not "int") to str

В данном примере мы запускаем файл «test.py» (через консоль). Вызывается функция «a«, внутри которой вызывается функция «b«. Все работает хорошо до сточки print(value + 1). Тут интерпретатор понимает, что нельзя конкатенировать строку с числом, останавливает выполнение программы и вызывает исключение «TypeError».

Далее ошибка передается по цепочке в обратном направлении: «b» → «a» → «test.py«. Так как в данном примере мы не позаботились обработать эту ошибку, вся информация по ошибке отобразится в консоли в виде Traceback.

Traceback (трассировка) — это отчёт, содержащий вызовы функций, выполненные в определенный момент. Трассировка помогает узнать, что пошло не так и в каком месте это произошло.

Traceback лучше читать снизу вверх ↑

В нашем примере Traceback содержится следующую информацию (читаем снизу вверх):

1. TypeError — тип ошибки (означает, что операция не может быть выполнена с переменной этого типа);
2. can only concatenate str (not "int") to str — подробное описание ошибки (конкатенировать можно только строку со строкой);
3. Стек вызова функций (1-я линия — место, 2-я линия — код). В нашем примере видно, что в файле «test.py» на 11-й линии был вызов функции «a» со строковым аргументом «10». Далее был вызов функции «b». print(value + 1) это последнее, что было выполнено — тут и произошла ошибка.
4. most recent call last — означает, что самый последний вызов будет отображаться последним в стеке (в нашем примере последним выполнился print(value + 1)).

В Python ошибку можно перехватить, обработать, и продолжить выполнение программы — для этого используется конструкция try ... except ....

Как обрабатывать исключения в Python (try except)

В Python исключения обрабатываются с помощью блоков try/except. Для этого операция, которая может вызвать исключение, помещается внутрь блока try. А код, который должен быть выполнен при возникновении ошибки, находится внутри except.

Например, вот как можно обработать ошибку деления на ноль:

try:
a = 7 / 0
except:
print('Ошибка! Деление на 0')

Здесь в блоке try находится код a = 7 / 0 — при попытке его выполнить возникнет исключение и выполнится код в блоке except (то есть будет выведено сообщение «Ошибка! Деление на 0»). После этого программа продолжит свое выполнение.

💭 PEP 8 рекомендует, по возможности, указывать конкретный тип исключения после ключевого слова except (чтобы перехватывать и обрабатывать конкретные исключения):

try:
a = 7 / 0
except ZeroDivisionError:
print('Ошибка! Деление на 0')

Однако если вы хотите перехватывать все исключения, которые сигнализируют об ошибках программы, используйте тип исключения Exception:

try:
a = 7 / 0
except Exception:
print('Любая ошибка!')

As — сохраняет ошибку в переменную

Перехваченная ошибка представляет собой объект класса, унаследованного от «BaseException». С помощью ключевого слова as можно записать этот объект в переменную, чтобы обратиться к нему внутри блока except:

try:
file = open('ok123.txt', 'r')
except FileNotFoundError as e:
print(e)

> [Errno 2] No such file or directory: 'ok123.txt'

В примере выше мы обращаемся к объекту класса «FileNotFoundError» (при выводе на экран через print отобразится строка с полным описанием ошибки).

У каждого объекта есть поля, к которым можно обращаться (например если нужно логировать ошибку в собственном формате):

import datetime

> 20-11-2021 18:42:01 [FileNotFoundError]: No such file or directory, filename: ok123.txt

Finally — выполняется всегда

При обработке исключений можно после блока try использовать блок finally. Он похож на блок except, но команды, написанные внутри него, выполняются обязательно. Если в блоке try не возникнет исключения, то блок finally выполнится так же, как и при наличии ошибки, и программа возобновит свою работу.

Обычно try/except используется для перехвата исключений и восстановления нормальной работы приложения, а try/finally для того, чтобы гарантировать выполнение определенных действий (например, для закрытия внешних ресурсов, таких как ранее открытые файлы).

В следующем примере откроем файл и обратимся к несуществующей строке:

file = open('ok.txt', 'r')

> файл закрыт!
> Traceback (most recent call last):
> File "test.py", line 5, in <module>
> print(lines[5])
> IndexError: list index out of range

Даже после исключения «IndexError», сработал код в секции finally, который закрыл файл.

p.s. данный пример создан для демонстрации, в реальном проекте для работы с файлами лучше использовать менеджер контекста with.

Также можно использовать одновременно три блока try/except/finally. В этом случае:

• в try — код, который может вызвать исключения;
• в except — код, который должен выполниться при возникновении исключения;
• в finally — код, который должен выполниться в любом случае.

def sum(a, b):
res = 0

> a = 1, b = 2, res = 3

Else — выполняется когда исключение не было вызвано

Иногда нужно выполнить определенные действия, когда код внутри блока try не вызвал исключения. Для этого используется блок else.

Допустим нужно вывести результат деления двух чисел и обработать исключения в случае попытки деления на ноль:

b = int(input('b = '))
c = int(input('c = '))
try:
a = b / c
except ZeroDivisionError:
print('Ошибка! Деление на 0')
else:
print(f"a = {a}")

> b = 10
> c = 1
> a = 10.0

В этом случае, если пользователь присвоит переменной «с» ноль, то появится исключение и будет выведено сообщение «‘Ошибка! Деление на 0′», а код внутри блока else выполняться не будет. Если ошибки не будет, то на экране появятся результаты деления.

Несколько блоков except

В программе может возникнуть несколько исключений, например:

1. Ошибка преобразования введенных значений к типу float («ValueError»);
2. Деление на ноль («ZeroDivisionError»).

В Python, чтобы по-разному обрабатывать разные типы ошибок, создают несколько блоков except:

try:
b = float(input('b = '))
c = float(input('c = '))
a = b / c
except ZeroDivisionError:
print('Ошибка! Деление на 0')
except ValueError:
print('Число введено неверно')
else:
print(f"a = {a}")

> b = 10
> c = питон
> Число введено неверно

Теперь для разных типов ошибок есть свой обработчик.

Несколько типов исключений в одном блоке except

Можно также обрабатывать в одном блоке except сразу несколько исключений. Для этого они записываются в круглых скобках, через запятую сразу после ключевого слова except. Чтобы обработать сообщения «ZeroDivisionError» и «ValueError» в одном блоке записываем их следующим образом:

try:
b = float(input('b = '))
c = float(input('c = '))
a = b / c
except (ZeroDivisionError, ValueError) as er:
print(er)
else:
print('a = ', a)

При этом переменной er присваивается объект того исключения, которое было вызвано. В результате на экран выводятся сведения о конкретной ошибке.

Raise — самостоятельный вызов исключений

Исключения можно генерировать самостоятельно — для этого нужно запустить оператор raise.

min = 100
if min > 10:
raise Exception('min must be less than 10')

> Traceback (most recent call last):
> File "test.py", line 3, in <module>
> raise Exception('min value must be less than 10')
> Exception: min must be less than 10

Перехватываются такие сообщения точно так же, как и остальные:

min = 100

> Моя ошибка

Кроме того, ошибку можно обработать в блоке except и пробросить дальше (вверх по стеку) с помощью raise:

min = 100

> Моя ошибка
> Traceback (most recent call last):
> File "test.py", line 5, in <module>
> raise Exception('min must be less than 10')
> Exception: min must be less than 10

Как пропустить ошибку

Иногда ошибку обрабатывать не нужно. В этом случае ее можно пропустить с помощью pass:

try:
a = 7 / 0
except ZeroDivisionError:
pass

Исключения в lambda функциях

Обрабатывать исключения внутри lambda функций нельзя (так как lambda записывается в виде одного выражения). В этом случае нужно использовать именованную функцию.

20 типов встроенных исключений в Python

Иерархия классов для встроенных исключений в Python выглядит так:

BaseException
SystemExit
KeyboardInterrupt
GeneratorExit
Exception
ArithmeticError
AssertionError
...
...
...
ValueError
Warning

Все исключения в Python наследуются от базового BaseException:

• SystemExit — системное исключение, вызываемое функцией sys.exit() во время выхода из приложения;
• KeyboardInterrupt — возникает при завершении программы пользователем (чаще всего при нажатии клавиш Ctrl+C);
• GeneratorExit — вызывается методом close объекта generator;
• Exception — исключения, которые можно и нужно обрабатывать (предыдущие были системными и их трогать не рекомендуется).

От Exception наследуются:

1 StopIteration — вызывается функцией next в том случае если в итераторе закончились элементы;

2 ArithmeticError — ошибки, возникающие при вычислении, бывают следующие типы:

• FloatingPointError — ошибки при выполнении вычислений с плавающей точкой (встречаются редко);
• OverflowError — результат вычислений большой для текущего представления (не появляется при операциях с целыми числами, но может появиться в некоторых других случаях);
• ZeroDivisionError — возникает при попытке деления на ноль.

3 AssertionError — выражение, используемое в функции assert неверно;

4 AttributeError — у объекта отсутствует нужный атрибут;

5 BufferError — операция, для выполнения которой требуется буфер, не выполнена;

6 EOFError — ошибка чтения из файла;

7 ImportError — ошибка импортирования модуля;

8 LookupError — неверный индекс, делится на два типа:

• IndexError — индекс выходит за пределы диапазона элементов;
• KeyError — индекс отсутствует (для словарей, множеств и подобных объектов);

9 MemoryError — память переполнена;

10 NameError — отсутствует переменная с данным именем;

11 OSError — исключения, генерируемые операционной системой:

• ChildProcessError — ошибки, связанные с выполнением дочернего процесса;
• ConnectionError — исключения связанные с подключениями (BrokenPipeError, ConnectionResetError, ConnectionRefusedError, ConnectionAbortedError);
• FileExistsError — возникает при попытке создания уже существующего файла или директории;
• FileNotFoundError — генерируется при попытке обращения к несуществующему файлу;
• InterruptedError — возникает в том случае если системный вызов был прерван внешним сигналом;
• IsADirectoryError — программа обращается к файлу, а это директория;
• NotADirectoryError — приложение обращается к директории, а это файл;
• PermissionError — прав доступа недостаточно для выполнения операции;
• ProcessLookupError — процесс, к которому обращается приложение не запущен или отсутствует;
• TimeoutError — время ожидания истекло;

12 ReferenceError — попытка доступа к объекту с помощью слабой ссылки, когда объект не существует;

13 RuntimeError — генерируется в случае, когда исключение не может быть классифицировано или не подпадает под любую другую категорию;

14 NotImplementedError — абстрактные методы класса нуждаются в переопределении;

15 SyntaxError — ошибка синтаксиса;

16 SystemError — сигнализирует о внутренне ошибке;

17 TypeError — операция не может быть выполнена с переменной этого типа;

18 ValueError — возникает когда в функцию передается объект правильного типа, но имеющий некорректное значение;

19 UnicodeError — исключение связанное с кодирование текста в unicode, бывает трех видов:

• UnicodeEncodeError — ошибка кодирования;
• UnicodeDecodeError — ошибка декодирования;
• UnicodeTranslateError — ошибка перевода unicode.

20 Warning — предупреждение, некритическая ошибка.

💭 Посмотреть всю цепочку наследования конкретного типа исключения можно с помощью модуля inspect:

import inspect

> (<class 'TimeoutError'>, <class 'OSError'>, <class 'Exception'>, <class 'BaseException'>, <class 'object'>)

📄 Подробное описание всех классов встроенных исключений в Python смотрите в официальной документации.

Как создать свой тип Exception

В Python можно создавать свои исключения. При этом есть одно обязательное условие: они должны быть потомками класса Exception:

class MyError(Exception):
def __init__(self, text):
self.txt = text

> Моя ошибка

С помощью try/except контролируются и обрабатываются ошибки в приложении. Это особенно актуально для критически важных частей программы, где любые «падения» недопустимы (или могут привести к негативным последствиям). Например, если программа работает как «демон», падение приведет к полной остановке её работы. Или, например, при временном сбое соединения с базой данных, программа также прервёт своё выполнение (хотя можно было отловить ошибку и попробовать соединиться в БД заново).

Вместе с try/except можно использовать дополнительные блоки. Если использовать все блоки описанные в статье, то код будет выглядеть так:

try:
# попробуем что-то сделать
except (ZeroDivisionError, ValueError) as e:
# обрабатываем исключения типа ZeroDivisionError или ValueError
except Exception as e:
# исключение не ZeroDivisionError и не ValueError
# поэтому обрабатываем исключение общего типа (унаследованное от Exception)
# сюда не сходят исключения типа GeneratorExit, KeyboardInterrupt, SystemExit
else:
# этот блок выполняется, если нет исключений
# если в этом блоке сделать return, он не будет вызван, пока не выполнился блок finally
finally:
# этот блок выполняется всегда, даже если нет исключений else будет проигнорирован
# если в этом блоке сделать return, то return в блоке

Подробнее о работе с исключениями в Python можно ознакомиться в официальной документации.

Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: Raising and Handling Python Exceptions

A Python program terminates as soon as it encounters an error. In Python, an error can be a syntax error or an exception. In this article, you will see what an exception is and how it differs from a syntax error. After that, you will learn about raising exceptions and making assertions. Then, you’ll finish with a demonstration of the try and except block.

>>> print( 0 / 0 ))
  File "<stdin>", line 1
    print( 0 / 0 ))
                  ^
SyntaxError: invalid syntax

>>> print( 0 / 0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

x = 10
if x > 5:
    raise Exception('x should not exceed 5. The value of x was: {}'.format(x))

Traceback (most recent call last):
  File "<input>", line 4, in <module>
Exception: x should not exceed 5. The value of x was: 10

import sys
assert ('linux' in sys.platform), "This code runs on Linux only."

Traceback (most recent call last):
  File "<input>", line 2, in <module>
AssertionError: This code runs on Linux only.

def linux_interaction():
    assert ('linux' in sys.platform), "Function can only run on Linux systems."
    print('Doing something.')

try:
    linux_interaction()
except:
    pass

try:
    linux_interaction()
except:
    print('Linux function was not executed')

Linux function was not executed

try:
    linux_interaction()
except AssertionError as error:
    print(error)
    print('The linux_interaction() function was not executed')

Function can only run on Linux systems.
The linux_interaction() function was not executed

try:
    with open('file.log') as file:
        read_data = file.read()
except:
    print('Could not open file.log')

try:
    with open('file.log') as file:
        read_data = file.read()
except FileNotFoundError as fnf_error:
    print(fnf_error)

[Errno 2] No such file or directory: 'file.log'

try:
    linux_interaction()
    with open('file.log') as file:
        read_data = file.read()
except FileNotFoundError as fnf_error:
    print(fnf_error)
except AssertionError as error:
    print(error)
    print('Linux linux_interaction() function was not executed')

Function can only run on Linux systems.
Linux linux_interaction() function was not executed

[Errno 2] No such file or directory: 'file.log'

try:
    linux_interaction()
except AssertionError as error:
    print(error)
else:
    print('Executing the else clause.')

Doing something.
Executing the else clause.

try:
    linux_interaction()
except AssertionError as error:
    print(error)
else:
    try:
        with open('file.log') as file:
            read_data = file.read()
    except FileNotFoundError as fnf_error:
        print(fnf_error)

Doing something.
[Errno 2] No such file or directory: 'file.log'

try:
    linux_interaction()
except AssertionError as error:
    print(error)
else:
    try:
        with open('file.log') as file:
            read_data = file.read()
    except FileNotFoundError as fnf_error:
        print(fnf_error)
finally:
    print('Cleaning up, irrespective of any exceptions.')

Function can only run on Linux systems.
Cleaning up, irrespective of any exceptions.

Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: Raising and Handling Python Exceptions

Python exception handling is the process of identifying and responding to errors in a program. In other words, it is a way to deal with errors that might occur in your program. In this article, you will learn how to handle errors in Python by using the Python try and except keywords. It will also teach you how to create custom exceptions, which can be used to define your own specific error messages.

What is an exception?

An exception is a condition that arises during the execution of a program. It is a signal that something unexpected happened. Python represents exceptions by an object of a certain type.

In Python, all built-in, non-system-exiting exceptions are derived from the Exception class. Exceptions have their own, descriptive names. For example, if you try to divide a number by zero, you will get a ZeroDivisionError exception, which is also a subclass of the Exception class.

For a complete hierarchy of all exceptions, you can view the Python manual if you’re interested. Here’s a small excerpt from this hierarchy, just to illustrate:

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- Exception
      +-- ArithmeticError
      |    +-- FloatingPointError
      |    +-- OverflowError
      |    +-- ZeroDivisionError
      .....

In case your knowledge about objects, classes, and inheritance is a bit rusty, you may want to read my article on objects and classes and my article on inheritance first.

When something unexpected occurs, we can raise an exception at the point of the error. When an exception is raised, Python stops the current flow of execution and starts looking for an exception handler that can handle it. So what is an exception handler? Here’s where the try and except statements come into play.

As demonstrated in the illustration, we can create a block of code by starting with a try statement. This basically means: try to run this code, but an exception might occur.

After our try block, one or more except blocks must follow. This is where the magic happens. These except blocks can catch an exception, as we usually call this. In fact, many other programming languages use a statement called catch instead of except. Each except block can handle a specific type of exception.

Remember: classes are hierarchical. For that reason, exceptions are too. Hence, except blocks must go from most specific, like a ZeroDivisionError, to less specific, like an ArithmeticError.

To demonstrate this, imagine what happens when we would start with an except block that catches Exception. This first block would catch basically everything, because most exceptions inherit from this one, rendering the other except blocks useless.

Now let’s first go back to raising an exception. When an exception is raised, the exception handler that’s able to handle the exception can be nearby, but it can also be in a different module. What’s important to realize, is that Python won’t just scan your code at random for an exception handler. Instead, the handler should be somewhere in the call stack.

Please forget about else and finally for now. I’ll explain them in detail further down in this article. We first need to discuss call stacks, to truly understand how an exception finds its way to an exception handler.

Call stack

A call stack is an ordered list of functions that are currently being executed. For example, you might call function A, which calls function B, which calls function C. We now have a call stack consisting of A, B, and C. When C raises an exception, Python will look for an exception handler in this call stack, going backward from end to start. It can be in function C (closest to the exception), in function B (somewhat farther), in function A, or even at the top level of the program where we called function A.

If Python finds a suitable except block, it executes the code in that block. If it doesn’t find one, Python handles the exception by itself. This means it will print the exception and exit the program since it has no clue what to do with it.

I hope you’re still with me! If not, no worries. The examples on this page will hopefully make all this more clear. You might want to revisit this section once you’ve finished the entire article.

Catching exceptions with try except

Let’s finally write some actual code! To handle an exception, we need to catch it. As we just learned, we can catch an exception by using the try and except keywords. When an exception occurs while we are inside the try block, the code in the except block is executed.

A simple example

Let’s try a simple example first. As you hopefully know, we can’t divide by the number zero. If we do so anyway, Python will throw and exception called ZeroDivisionError, which is a subclass of ArithmeticError:

try:
    print(2/0)
except ZeroDivisionError:
    print("You can't divide by zero!")

If you call a Python function inside the try block, and an exception occurs in that function, the flow of code execution stops at the point of the exception and the code in the except block is executed. Try doing this again, without try and except. You’ll see that Python prints the exception for us. You can do so in the following code crumb:

Also, note that Python prints the error to stderr if you don’t handle the exception by yourself. In the crumb above, this is visible because the output appears in an ‘Error’ tab instead of an ‘Output’ tab.

Catching IOError

Let’s try another, more common example. After all, who divides a number by zero, right?

Exceptions are likely to occur when interacting with the outside world, e.g. when working with files or networks. For example, if you try to open a file with Python, but that file doesn’t exist, you will get an IOError exception. If you don’t have access to a file due to permissions, you will again get an IOError exception. Let’s see how to handle these exceptions.

Assignment

Please do the following:

1. Run the code below, notice the file name (it doesn’t exist). See what happens.
2. Alter the file name to myfile.txt file and run the code again. What happens now?

Alternatively, here’s the code to copy/paste:

try:
    # Open file in read-only mode
    with open("not_here.txt", 'r') as f:
        f.write("Hello World!")
except IOError as e:
    print("An error occurred:", e)

Answers

The file is not found in the first case. You should get this output:

An error occurred: [Errno 2] No such file or directory: 'not_here.txt'

In the second case, you’ll still get an error after creating the file. This time because we’re trying to write to a file that is opened in read-only mode. For more information on these modes, read the article on opening, reading, and writing files with Python. The error should look like this:

An error occurred: not writable

Although this is an error, it’s not written to the stderr output of the operating system. That’s because we handled the exception ourselves. If you removed the try.. except from the code completely and then try to write to the file in read-only mode, Python will catch the error, force the program to terminate, and show this message:

Traceback (most recent call last):
  File "tryexcept.py", line 3, in <module>
    f.write("Hello World!")
io.UnsupportedOperation: not writable

The finally and else blocks

Remember the other two blocks that I asked you to forget for a while? Let’s look at those now, starting with the finally block.

The finally block in try-except

The finally block is executed regardless of whether an exception occurs or not. Finally blocks are useful, for example, when you want to close a file or a network connection regardless of what happens. After all, you want to clean up resources to prevent memory leaks.

Here’s an example of this at work, in which we open a file without using the with statement, forcing us to close it ourselves:

try:
    # Open file in write-mode
    f = open("myfile.txt", 'w')
    f.write("Hello World!")
except IOError as e:
    print("An error occurred:", e)
finally:
    print("Closing the file now")
    f.close()

You can also try this interactive example:

You should see the ‘Closing the file now’ message printed on your screen. Now change the writing mode to ‘r’ instead of ‘w’. You’ll get an error since the file does not exist. Despite this exception, we try to close the file anyway thanks to the finally block. This in turn goes wrong too: a NameError exception is thrown because the file was never opened, and hence f does not exist. You can fix this with a nested try.. except NameError. Try it for yourself.

The else block in try-except

In addition to the except and finally blocks, you can add an else block. The else block executes only when no exception occurs. So it differs from the finally block, since finally executes even if an exception occurs.

When should you use the else block? And why shouldn’t you just add extra code to the try block? Good questions!

According to the Python manual, the use of the else clause is better than adding additional code to the try clause. But why? The reasoning is that it avoids accidentally catching an exception that wasn’t raised by the code being protected by the try and except statements in the first place. I admit I don’t use else blocks very often myself. Also, I find them somewhat confusing, especially for people coming from other languages.

Common Python exceptions

Some exceptions are so common that you’ll inevitably encounter them. Here are a few of the most common ones:

If you like, you can try to evoke these exceptions intentionally. I’ll promise you, you will encounter these countless times in your Python programming career. Knowing and understanding what they mean and when they occur will greatly help you debug your code.

Exception best practices

Now that we know the mechanics of handling exceptions, there are a couple of best practices I’d like to share with you.

Don’t use blank except blocks

I’ve written about this before in the blog post ‘How not to handle exceptions in Python‘. Don’t use a blank block when you want to catch a broad range of exceptions. By this, I mean something like:

try:
    ...
except:
    print("An error occurred:")

You might encounter this in code samples on the web. If you do, make a habit of improving the exception handling. Why should you, and how can you improve code like the example above?

All exceptions, including system exceptions, inherit from a class called BaseException. If an except clause mentions a particular class, that clause also handles any exception classes derived from that class. An empty except is equivalent to except BaseException, hence it will catch all possible exceptions.

So although the syntax is allowed, I don’t recommend it. E.g., you’ll also catch KeyboardInterrupt and SystemExit exceptions, which prevent your program from exiting. Instead, use a try block with a list of explicit exceptions that you know you can handle. Or, if you really need to, catch the Exception base class to handle almost all the regular exceptions, but not the system ones.

If you’re feeling adventurous, you can try to catch all exceptions and see what happens:

from time import sleep

while True:
    try:
        print("Try and stop me")
        sleep(1)
    except:
        print("Don't stop me now, I'm having such a good time!")

You’ll probably need to close your terminal to stop this program. Now change the except block to catch Exception. You will still catch almost all exceptions, but the program will exit on system exceptions like KeyboardInterrupt and SystemExit:

from time import sleep

while True:
    try:
        print("Try and stop me")
        sleep(1)
    except Exception:
        print("Something went wrong")

It’s better to ask for forgiveness

In Python, you’ll often see a pattern where people simply try if something works, and if it doesn’t, catch the exception. In other words, it’s better to ask for forgiveness than permission. This is in contrast to other languages, where you preferably ask for permission. E.g., in Java, exceptions can slow down your program and you “ask for permission” by doing checks on an object instead of simply trying.

To make this more concrete: in Python, we often just try to access the key in a dictionary. If the key doesn’t exist, we’ll get an exception and handle it. Suppose we just converted some externally provided JSON to a dictionary, and now start to use it:

import json

user_json = '{"name": "John", "age": 39}'
user = json.loads(user_json)

try:
    print(user['name'])
    print(user['age'])
    print(user['address'])
    ...
except KeyError as e:
    print("There are missing fields in the user object: ", e)
    # Properly handle the error
    ...

This will print the error:

There are missing fields in the user object: 'address'

We could have added three checks (if 'name' in user, if 'age' in user, etc.) to make sure that all the fields are there. But this is not a good practice. It potentially introduces a lot of code just to check if keys exist. Instead, we ask for forgiveness in our except block once, which is much cleaner and more readable. And if you worry about performance: exceptions don’t take up that many CPU cycles in Python. Lots of comparisons are in fact slower than catching a single exception (if it occurs at all!).

Create custom exceptions

All built-in, non-system-exiting exceptions are derived from the Exception class as we learned before. All user-defined exceptions should also be derived from this class. So if we want to create our own exceptions, we need to create a subclass of the Exception class.

For example, if you want to create an exception that indicates that a user was not found, you can create a UserNotFoundError exception. This would, in its most basic form, look like this:

class UserNotFoundError(Exception):
    pass

This inherits all the properties and methods of Exception, but we give it a new name to distinguish it from the Exception class. This way, we’ll be able to specifically catch it with an except block.

The name of this exception clearly tells us the type of problem that was encountered, so as an added bonus, it functions as a form of code documentation as well. Just like well-named variables and functions, a well-named exception can be a big difference when reading back your code.

We’ll use this class in the example that follows.

Raising (or throwing) exceptions

We know some built-in exceptions and how to create custom exceptions. We also know how to catch exceptions with try and except. What’s left, is what’s called raising or throwing an exception. You can raise an exception yourself with the raise keyword.

In the example below, we use your previously defined UserNotFoundError. We call a function, fetch_user, that fetches some user data from an imaginary database. If the user is not found, this database returns None. We decided that we don’t want to return None, which would force the caller to check for None every time. Instead, we use our custom UserNotFoundError.

class UserNotFoundError(Exception):
    pass

def fetch_user(user_id):
    # Here you would fetch from some kind of db, e.g.:
    # user = db.get_user(user_id)

    # To make this example runnable, let's set it to None
    user = None

    if user == None:
        raise UserNotFoundError(f'User {user_id} not in database')
    else:
        return user

users = [123, 456, 789]
for user_id in users:
    try:
        fetch_user(user_id)
    except UserNotFoundError as e:
        print("There was an error: ", e)

Assignment

Here’s a little assignment. We could have used a regular Exception object instead. That way, we don’t need to define a custom one. Why is this a bad idea?

Answer

You can in fact raise a regular exception, e.g. with raise Exception('User not found'). But if you do, you need to catch all exceptions of type Exception. And as we know, there are a lot of those. Chances are you inadvertently catch some other exception that you’re not able to handle. For example, the database client might throw a DatabaseAuthenticationError which is also a subclass of Exception.

How to print a Python exception

You can print exceptions directly as long as you catch them properly. You may have seen examples of this above already. To be clear, here’s an example of how to catch and print an exception:

try:
    ...
except Exception as e:
    print("There was an error: ", e)

If you’d like to print the call stack, just like Python does when you don’t catch the exception yourself, you can import the traceback module:

import traceback

try:
    ...
except Exception:
    traceback.print_exc()

Keep learning

Here are some more resources to deepen your knowledge:

• My blog article ‘How not to handle exceptions in Python‘
• Introduction to Python functions
• Objects and classes, and Python inheritance
• The official documentation on exceptions.
• The official documentation on errors.

Error in Python can be of two types i.e. Syntax errors and Exceptions. Errors are the problems in a program due to which the program will stop the execution. On the other hand, exceptions are raised when some internal events occur which changes the normal flow of the program.
Note: For more information, refer to Errors and Exceptions in Python
Some of the common Exception Errors are : 
 

• IOError: if the file can’t be opened
• KeyboardInterrupt: when an unrequired key is pressed by the user
• ValueError: when built-in function receives a wrong argument
• EOFError: if End-Of-File is hit without reading any data
• ImportError: if it is unable to find the module

Try Except in Python

Try and Except statement is used to handle these errors within our code in Python. The try block is used to check some code for errors i.e the code inside the try block will execute when there is no error in the program. Whereas the code inside the except block will execute whenever the program encounters some error in the preceding try block.
 

Syntax: 

try:
    # Some Code
except:
    # Executed if error in the
    # try block

How try() works? 
 

• First, the try clause is executed i.e. the code between try and except clause.
• If there is no exception, then only the try clause will run, except the clause is finished.
• If any exception occurs, the try clause will be skipped and except clause will run.
• If any exception occurs, but the except clause within the code doesn’t handle it, it is passed on to the outer try statements. If the exception is left unhandled, then the execution stops.
• A try statement can have more than one except clause

Code 1: No exception, so the try clause will run. 
 

Auxiliary Space: O(1)

Output : 
 

('Yeah ! Your answer is :', 1)

Code 1: There is an exception so only except clause will run. 
 

Output : 
 

Sorry ! You are dividing by zero

Else Clause

In python, you can also use the else clause on the try-except block which must be present after all the except clauses. The code enters the else block only if the try clause does not raise an exception.
 

Syntax:

try:
    # Some Code
except:
    # Executed if error in the
    # try block
else:
    # execute if no exception

Code:

Output:

-5.0
a/b result in 0

Finally Keyword in Python

Python provides a keyword finally, which is always executed after the try and except blocks. The final block always executes after normal termination of try block or after try block terminates due to some exceptions.
 

Syntax:

try:
    # Some Code
except:
    # Executed if error in the
    # try block
else:
    # execute if no exception
finally:
    # Some code .....(always executed)

Code:

Output:

Can't divide by zero
This is always executed

Related Articles: 
 

• Output Questions
• Exception Handling in Python
• User-Defined Exceptions

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