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.

Practice

1. Which of the following is the base class for all built-in exceptions in Python?

easy

A. Exception

B. BaseException

C. Error

D. RuntimeError

Solution

1. Step 1: Understand Python's exception hierarchy

   All exceptions in Python inherit from BaseException, which is the root of the hierarchy.

2. Step 2: Identify the base class

   Exception inherits from BaseException, but BaseException is the top-level base class.

3. Final Answer:

   BaseException -> Option B

4. Quick Check:

   BaseException is the root of all exceptions [OK]

Hint: Remember: BaseException is the root of all exceptions [OK]

Common Mistakes:

• Confusing Exception with BaseException
• Thinking Error is a built-in base class
• Choosing RuntimeError as base

2. Which of the following is the correct syntax to catch all exceptions except system-exiting ones?

easy

A. except SystemExit:

B. except BaseException:

C. except Exception:

D. except RuntimeError:

Solution

1. Step 1: Recall exception hierarchy for catching exceptions

   Catching Exception catches most errors but excludes system-exiting exceptions like SystemExit and KeyboardInterrupt.

2. Step 2: Identify correct syntax

   except Exception: is the standard way to catch all regular exceptions safely.

3. Final Answer:

   except Exception: -> Option C

4. Quick Check:

   Use Exception to catch all but system-exiting exceptions [OK]

Hint: Use except Exception to avoid catching system-exit errors [OK]

Common Mistakes:

• Using except BaseException catches system exit too
• Catching only RuntimeError misses many exceptions
• Using except SystemExit catches only exit exceptions

3. What will be the output of this code?

try:
    x = 1 / 0
except ArithmeticError:
    print('ArithmeticError caught')
except ZeroDivisionError:
    print('ZeroDivisionError caught')

medium

A. ArithmeticError caught

B. ZeroDivisionError caught

C. No output, program crashes

D. SyntaxError

Solution

1. Step 1: Understand exception hierarchy for ZeroDivisionError

   ZeroDivisionError is a subclass of ArithmeticError.

2. Step 2: Check which except block matches first

   Since ArithmeticError comes before ZeroDivisionError, the first except block catches the exception.

3. Final Answer:

   ArithmeticError caught -> Option A

4. Quick Check:

   Parent exception catches before child [OK]

Hint: Parent exceptions catch before child exceptions in order [OK]

Common Mistakes:

• Expecting ZeroDivisionError block to run first
• Thinking exception order does not matter
• Assuming program crashes without handling

4. Find the error in this code snippet:

try:
    open('file.txt')
except IOError:
    print('File error')
except FileNotFoundError:
    print('File not found')

medium

A. open() needs a mode argument

B. IOError should be replaced with Exception

C. No error, code is correct

D. FileNotFoundError should come before IOError

Solution

1. Step 1: Understand exception hierarchy between IOError and FileNotFoundError

   FileNotFoundError is a subclass of IOError.

2. Step 2: Check order of except blocks

   The more specific exception (FileNotFoundError) must come before the more general (IOError) to avoid unreachable code.

3. Final Answer:

   FileNotFoundError should come before IOError -> Option D

4. Quick Check:

   Specific exceptions must precede general ones [OK]

Hint: Place child exceptions before parent exceptions in except blocks [OK]

Common Mistakes:

• Putting general exceptions before specific ones
• Assuming IOError and FileNotFoundError are unrelated
• Thinking open() requires mode argument always

5. You want to catch all exceptions except KeyboardInterrupt and SystemExit. Which is the best way to write the except block?

hard

A. except Exception:

B. except BaseException:

C. except (KeyboardInterrupt, SystemExit):

D. except (Exception, KeyboardInterrupt, SystemExit):

Solution

1. Step 1: Recall exception hierarchy for KeyboardInterrupt and SystemExit

   Both KeyboardInterrupt and SystemExit inherit directly from BaseException, not Exception.

2. Step 2: Choose except block that excludes these exceptions

   except Exception: catches all exceptions except KeyboardInterrupt and SystemExit, which is the desired behavior.

3. Final Answer:

   except Exception: -> Option A

4. Quick Check:

   Exception excludes system-exiting exceptions [OK]

Hint: Use except Exception to exclude system-exiting exceptions [OK]

Common Mistakes:

• Using except BaseException catches everything including system exit
• Catching KeyboardInterrupt and SystemExit explicitly when not needed
• Combining Exception with KeyboardInterrupt in except tuple