Python Tutorial - Exception Handling

Jinku Hu Jan 30, 2023
  1. try...except
  2. raise Exception
  3. try...finally
  4. Python Built-In Exceptions
Python Tutorial - Exception Handling

You will learn how to handle Python exceptions in this section. Further, you will learn to raise and catch exceptions.

When an exception occurs in a program, the program execution is terminated. But most of the time, you don’t prefer the abnormal termination of the program, therefore you could use Python exceptions handling to avoid the crash of your program.


In the following example, the user is asked to enter 10 integers. If the user enters another type rather than integers, an exception will be thrown and statements in except block is executed:

>>> n = 10
>>> i = 1
>>> sum = 0
>>> while i <= n:
		num = int(input('Enter a number: '))
		sum = sum + num
		i = i+1
		print('Please enter an integer!!')

Enter a number: 23
Enter a number: 12
Enter a number: Asd
Please enter an integer!!
>>> print('Sum of numbers =', sum)
Sum of numbers = 35

You can also catch specified exceptions, for example, if division by zero happens, you can catch the corresponding exception as:

    # statements
except ZeroDivisionError:
    # exception handling
    # all other exceptions are handled here

raise Exception

You can force a specific exception to occur by using raise keyword:

>>> raise ZeroDivisionError
Traceback (most recent call last):
  File "<pyshell#17>", line 1, in <module>
    raise ZeroDivisionError

>>> raise ValueError
Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    raise ValueError

If you need to add some customized information to the self-raised exception, you could add it in the parenthesis after exception error type.

>>> raise NameError("This is a customized exception")
Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    raise NameError("This is a customized exception")
NameError: This is a customized exception


The finally block is optional in the try block. If there is a finally block, it will be executed in any case.

This is demonstrated in the example below:

>>> try:
		raise ValueError
		print('You are in finally block')

You are in finally block
Traceback (most recent call last):
  File "<pyshell#23>", line 2, in <module>
    raise ValueError

The try...finally block is used for the clean-up actions such as when working with a file, it is a good practice to close the file. So the file will be closed in the finally block.

Python Built-In Exceptions

In this section, you will learn about the standard exceptions in Python programming.

Exceptions are the runtime errors that are raised during interpretation. For example, the division of any number by zero will create a runtime error called ZeroDivisionError.

In Python, there are some built-in exceptions that are described below:

Exception When it is raised?
AssertionError when the assert statement is failed
AttributeError when the attribute reference is failed
EOFError when no data is read by the input() functions and the end of file is reached.
FloatingPointError when the floating point calculation is failed
GeneratorExit When a coroutine or a generator is closed, GeneratorExit exception occurs
ImportError when import cannot find the module
ModuleNotFoundError when the module is not found by import. This exception is a subclass of ImportError.
IndexError when the index of a sequence is not in range.
KeyError when a dictionary key is not found in the dictionary
KeyboardInterrupt when an interrupt key is pressed.
MemoryError when there is less amount of memory for a specified operation
NameError when a variable is not found
NotImplementedError when the implementation of the abstract method is needed but not provided.
OSError when a system error is encountered
OverFlowError when a value is too large to be represented.
RecursionError when the recursion limit exceeds the maximum number
IndentationError when indentation is properly specified for the definition of any function, class, etc.
SystemError when a system related error is found
SystemExit when sys.exit() is used to exit interpreter.
TypeError when an operation is invalid for the specified data type
ValueError when a built-in function has valid arguments but the values provided are not valid.
RunTimeError when the generated error falls in no other category
IOError when an I/O operation is failed.
UnBoundLocalError when the local variable is accessed but has no value
SyntaxError when there is a syntax error
TabError when there is unnecessary tab indentation
UnicodeError when a Unicode related error is encountered
UnicodeEncodeError when a Unicode related error is encountered because of encoding
UnicodeDecodeError when a Unicode related error is encountered because of decoding
UnicodeTranslateError when a Unicode related error is encountered because of translating
ZeroDivisionError when a number is divided by zero
Author: Jinku Hu
Jinku Hu avatar Jinku Hu avatar

Founder of Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.

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