Python has manybuilt-in exceptions that are raised when a program encounters an error, and most external libraries, like the popularRequests, include his owncustom 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 exceptionZeroDivisionError:

>>>defdivide(dividend, divisor):...print(dividend/ divisor)...>>> divide(dividend=10, divisor=5)# 2>>> 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 tohandle the exception with thetry andexcept keywords:

>>>defdivide(dividend, divisor):...try:...print(dividend/ divisor)...except ZeroDivisionError:...print('You can not divide by 0')...>>> divide(dividend=10, divisor=5)# 2>>> 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.

>>>defdivide(dividend, divisor):...try:...if(dividend==10):...           var='str'+1...else:...print(dividend/ divisor)...except(ZeroDivisionError, TypeError)as error:...print(error)...>>> divide(dividend=20, divisor=5)# 4>>> divide(dividend=10, divisor=5)# `can only concatenate str (not "int") to str` Error message>>> divide(dividend=10, divisor=0)# `division by zero` Error message

Finally code in exception handling

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

>>>defdivide(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 aclass that inherits from the baseException class of Python, and are raised using theraise keyword:

>>>classMyCustomException(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:

>>>classMyCustomException(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

Relevant links

• Control Flow
• breakpoint()
• isinstance()
• issubclass()

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