Python Exception Handling

Python has many built-in exceptions that are raised when a program encounters an error, and most external libraries, like the popular Requests, include his own custom exceptions that we will need to deal to.

Basic exception handling

You can’t divide by zero, that is a mathematical true, and if you try to do it in Python, the interpreter will raise the built-in exception ZeroDivisionError:

>>> def divide(dividend , divisor):
...     print(dividend / divisor)
...
>>> divide(dividend=10, divisor=5)
# 5

>>> divide(dividend=10, divisor=0)
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# ZeroDivisionError: division by zero

Let’s say we don’t want our program to stop its execution or show the user an output he will not understand. Say we want to print a useful and clear message, then we need to handle the exception with the try and except keywords:

>>> def divide(dividend , divisor):
...     try:
...         print(dividend / divisor)
...     except ZeroDivisionError:
...         print('You can not divide by 0')
...
>>> divide(dividend=10, divisor=5)
# 5

>>> divide(dividend=10, divisor=0)
# You can not divide by 0

Handling Multiple exceptions using one exception block

You can also handle multiple exceptions in one line like the following without the need to create multiple exception blocks.

>>> def divide(dividend , divisor):
...     try:
...         var = 'str' + 1
...         print(dividend / divisor)
...     except (ZeroDivisionError, TypeError) as error:
...         print(error)
...
>>> divide(dividend=10, divisor=5)
# 5

>>> divide(dividend=10, divisor=0)
# `division by zero` Error message
# `can only concatenate str (not "int") to str` Error message

Finally code in exception handling

The code inside the finally section is always executed, no matter if an exception has been raised or not:

>>> def divide(dividend , divisor):
...     try:
...         print(dividend / divisor)
...     except ZeroDivisionError:
...         print('You can not divide by 0')
...     finally:
...         print('Execution finished')
...
>>> divide(dividend=10, divisor=5)
# 5
# Execution finished

>>> divide(dividend=10, divisor=0)
# You can not divide by 0
# Execution finished

Custom Exceptions

Custom exceptions initialize by creating a class that inherits from the base Exception class of Python, and are raised using the raise keyword:

>>> class MyCustomException(Exception):
...     pass
...
>>> raise MyCustomException
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# __main__.MyCustomException

To declare a custom exception message, you can pass it as a parameter:

>>> class MyCustomException(Exception):
...     pass
...
>>> raise MyCustomException('A custom message for my custom exception')
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# __main__.MyCustomException: A custom message for my custom exception

Handling a custom exception is the same as any other:

>>> try:
...     raise MyCustomException('A custom message for my custom exception')
>>> except MyCustomException:
...     print('My custom exception was raised')
...
# My custom exception was raised