Python Exceptions

Summary: in this tutorial, you’ll learn about the Python exceptions and how to handle them gracefully in programs.

Introduction to Python exceptions

In Python, exceptions are objects of the exception classes. All exception classes are the subclasses of the BaseException class.

However, almost all built-in exception classes inherit from the Exception class, which is the subclass of the BaseException class:

python exceptions

This page shows a complete class hierarchy for built-in exceptions in Python.

The following example defines a list of three elements and attempts to access the fourth one:

colors = ['red', 'green', 'blue'] print(colors[3])
Code language: Python (python)

The invalid index caused the IndexError exception as expected:

IndexError: list index out of range
Code language: Python (python)

When an exception occurs, Python stops the program unless you handle it. To handle an exception, you use the try...except statement. For example:

colors = ['red', 'green', 'blue'] try: print(colors[3]) except IndexError as e: print(e) print('Continue to run')
Code language: Python (python)

Output:

<class 'IndexError'> - list index out of range Continue to run
Code language: plaintext (plaintext)

In this example, we use the try...except statement to handle the IndexError exception. As you can see from the output, the program continues to run after the try...except statement.

The IndexError class inherits from the LookupError class which inherits from the Exception class:

And you can catch either LookupError or Exception class when an IndexError exception occurs. For example:

colors = ['red', 'green', 'blue'] try: print(colors[3]) except LookupError as e: print(e.__class__, '-', e) print('Continue to run')
Code language: Python (python)

Output:

<class 'IndexError'> - list index out of range Continue to run
Code language: plaintext (plaintext)

In this example, the exception is still IndexError even though we catch the LookupError exception. Therefore, when you handle an exception, the exception handler will catch the exception type you specify and any of its subclasses.

The program runs the same if you use the Exception class instead of the LookupError class:

colors = ['red', 'green', 'blue'] try: print(colors[3]) except Exception as e: print(e.__class__, '-', e) print('Continue to run')
Code language: Python (python)

Output:

<class 'IndexError'> - list index out of range Continue to run
Code language: plaintext (plaintext)

In practice, you should catch the exceptions as specific as possible so that you know how to deal with each exception in a specific way.

Python exception handling example

The following example defines a division function that returns the result of a is divided by b:

def division(a, b): return a / b c = division(10, 0)
Code language: Python (python)

Output:

ZeroDivisionError: division by zero
Code language: plaintext (plaintext)

In this example, if b is zero, the ZeroDivisionError exception will occur. To handle the ZeroDivisionError exception, you use the try...except statement as follows:

def division(a, b): try: return { 'success': True, 'message': 'OK', 'result': a / b } except ZeroDivisionError as e: return { 'success': False, 'message': 'b cannot be zero', 'result': None } result = division(10, 0) print(result)
Code language: Python (python)

In this example, the function returns a dictionary that has three elements:

  • success is a boolean value that indicates whethere the operation is successful or not.
  • message indicates the error or success message.
  • result stores the result of a / b or None if b is zero.

The following shows the output if the ZeroDivisionError occurs:

{'success': False, 'message': 'b cannot be zero', 'result': None}
Code language: plaintext (plaintext)

Now, if you don’t catch the ZeroDivisionError exception but the more general exception like Exception class:

def division(a, b): try: return { 'success': True, 'message': 'OK', 'result': a / b } except Exception as e: return { 'success': False, 'message': 'b cannot be zero', 'result': None } result = division(10, 0) print(result)
Code language: Python (python)

The program works as before because the try...except also catches the exception type that is the subclass of the Exception class.

However, if you pass two strings instead of two numbers to the division() function, you’ll get the same message as if the ZeroDivisionError exception occurred:

def division(a, b): try: return { 'success': True, 'message': 'OK', 'result': a / b } except Exception as e: return { 'success': False, 'message': 'b cannot be zero', 'result': None } result = division('10', '2') print(result)
Code language: Python (python)

Output:

{'success': False, 'message': 'b cannot be zero', 'result': None}
Code language: plaintext (plaintext)

In this example, the exception is not ZeroDivisionError but the TypeError. However, the code still handles it like the ZeroDivisionError exception.

Therefore, you should always handle the exceptions from the most specific to the least specific. For example:

def division(a, b): try: return { 'success': True, 'message': 'OK', 'result': a / b } except TypeError as e: return { 'success': False, 'message': 'Both a & b must be numbers', 'result': None } except ZeroDivisionError as e: return { 'success': False, 'message': 'b cannot be zero', 'result': None } except Exception as e: return { 'success': False, 'message': str(e), 'result': None } result = division('10', '2') print(result)
Code language: Python (python)

In this example, we catch the TypeError, ZeroDivisionError, and Exception in the order that they appear in the try...except statement.

If the code that handles different exceptions are the same, you can group all exceptions into one as follows:

def division(a, b): try: return { 'success': True, 'message': 'OK', 'result': a / b } except (TypeError, ZeroDivisionError, Exception) as e: return { 'success': False, 'message': str(e), 'result': None } result = division(10, 0) print(result)
Code language: Python (python)

Output:

{'success': False, 'message': 'division by zero', 'result': None}
Code language: plaintext (plaintext)

Summary

  • Python exceptions are objects of classes, which are the subclasses of the BaseException class.
  • Do handle the exception from the most specific to lest specific.
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