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importostar

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User-defined Exception in Python

You are going to learn how to create a Python user defined exception.

There's all kinds of exceptions like ZeroDivisionError, IndexError, ImportError, TypeError and more.

Python is packed with several built-in exceptions like

  • ZeroDivisionError
  • ImportError
  • TypeError
  • ModuleNotFoundError

Exceptions

Python will show those exceptions when something goes wrong like:

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

or loading a non existing module

>>> import life
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'life'

Custom Exceptions

However you can Create your own personalized exceptions Python allows us to create our exception from the exception base class.

Python usually throws exceptions in the try-except statement.

    try:
        #statement or line of code
    except customError:
        #statement or line of code

All kinds of exceptions can be raised. So how do you make your own exception type?

You create a class that inherits from the exception class

>>> class CustomException(Exception):
...     def __init__(self, statement, s):
...         self.statement = statement
...         self.s = s
...         print(self.statement)
... 

You can also add a pass statement instead of a constructor.

Then you can raise your own exceptions:

>>> raise CustomException("went wrong", 3)
went wrong
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.CustomException: ('went wrong', 3)
>>> 

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