Exception hierarchy in Python

Introduction

Exceptions help your program handle errors without crashing. The exception hierarchy shows how different errors are related, making it easier to catch and fix them.

When you want to catch all errors of a certain type or group.

When you create your own error types to organize problems clearly.

When you want to handle specific errors differently but still catch general errors.

When debugging to understand what kind of error happened.

When writing code that needs to be safe and not stop unexpectedly.

Syntax

Python

BaseException
 ├── SystemExit
 ├── KeyboardInterrupt
 ├── Exception
      ├── ArithmeticError
      ├── LookupError
      ├── ... (other built-in exceptions)

The top of the hierarchy is BaseException, which is the parent of all errors.

Most user errors come from Exception, which is a child of BaseException.

Examples

This catches a specific error: dividing by zero.

Python

try:
    x = 1 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")

This catches an error when converting a string to a number fails.

Python

try:
    x = int('abc')
except ValueError:
    print("Invalid number!")

This catches any error that is a child of Exception, including ZeroDivisionError.

Python

try:
    x = 1 / 0
except Exception:
    print("Some error happened")

Sample Program

This program shows how different exceptions from the hierarchy are caught separately or generally.

Python

def test_errors(value):
    try:
        if value == 'zero':
            result = 1 / 0
        elif value == 'key':
            raise KeyError('missing key')
        else:
            result = int(value)
        print(f'Result is {result}')
    except ZeroDivisionError:
        print('Caught a ZeroDivisionError')
    except KeyError:
        print('Caught a KeyError')
    except Exception:
        print('Caught a general exception')

print('Test with zero:')
test_errors('zero')
print('Test with key:')
test_errors('key')
print('Test with invalid int:')
test_errors('abc')
print('Test with valid int:')
test_errors('10')

Important Notes

Always catch specific exceptions before general ones to avoid hiding errors.

Use Exception to catch most errors, but avoid catching BaseException unless you really want to catch system-exiting events.

You can create your own exceptions by inheriting from Exception or its children.

Summary

Exceptions are organized in a tree called the exception hierarchy.

Catching exceptions from higher in the hierarchy catches more errors.

Use specific exceptions to handle errors clearly and safely.