Error Types

Complete reference for Python exception types and error handling. Understanding different error types helps you write more robust code and debug issues effectively.

Built-in Exception Hierarchy

BaseException
 ├── SystemExit
 ├── KeyboardInterrupt
 ├── GeneratorExit
 └── Exception
     ├── StopIteration
     ├── StopAsyncIteration
     ├── ArithmeticError
     │   ├── FloatingPointError
     │   ├── OverflowError
     │   └── ZeroDivisionError
     ├── AssertionError
     ├── AttributeError
     ├── BufferError
     ├── EOFError
     ├── ImportError
     │   └── ModuleNotFoundError
     ├── LookupError
     │   ├── IndexError
     │   └── KeyError
     ├── MemoryError
     ├── NameError
     │   └── UnboundLocalError
     ├── OSError
     │   ├── BlockingIOError
     │   ├── ChildProcessError
     │   ├── ConnectionError
     │   │   ├── BrokenPipeError
     │   │   ├── ConnectionAbortedError
     │   │   ├── ConnectionRefusedError
     │   │   └── ConnectionResetError
     │   ├── FileExistsError
     │   ├── FileNotFoundError
     │   ├── InterruptedError
     │   ├── IsADirectoryError
     │   ├── NotADirectoryError
     │   ├── PermissionError
     │   ├── ProcessLookupError
     │   └── TimeoutError
     ├── ReferenceError
     ├── RuntimeError
     │   ├── NotImplementedError
     │   └── RecursionError
     ├── SyntaxError
     │   └── IndentationError
     │       └── TabError
     ├── SystemError
     ├── TypeError
     ├── ValueError
     │   └── UnicodeError
     │       ├── UnicodeDecodeError
     │       ├── UnicodeEncodeError
     │       └── UnicodeTranslateError
     └── Warning
         ├── DeprecationWarning
         ├── PendingDeprecationWarning
         ├── RuntimeWarning
         ├── SyntaxWarning
         ├── UserWarning
         └── FutureWarning

Common Exception Types

Arithmetic Errors

Exception	Cause	Example	Solution
ZeroDivisionError	Division by zero	`5 / 0`	Check divisor before operation
OverflowError	Result too large	Large exponential calculations	Use appropriate data types
FloatingPointError	Floating point operation failed	Platform-specific	Handle with try-except

Type Errors

Exception	Cause	Example	Solution
TypeError	Wrong type for operation	`"hello" + 5`	Convert types or check types
AttributeError	Object has no attribute	`5.append(1)`	Check object type and available methods

Value Errors

Exception	Cause	Example	Solution
ValueError	Correct type, wrong value	`int("hello")`	Validate input before conversion
IndexError	Index out of range	`[1, 2, 3][5]`	Check list length before accessing
KeyError	Key not found in dictionary	`{'a': 1}['b']`	Use `get()` method or check key existence

Name and Import Errors

Exception	Cause	Example	Solution
NameError	Variable not defined	Using `undefined_var`	Define variable before use
UnboundLocalError	Local variable referenced before assignment	Local var accessed before assignment	Initialize variable properly
ImportError	Module import failed	`import nonexistent_module`	Check module name and installation
ModuleNotFoundError	Module not found	`import missing_module`	Install module or check name

File and OS Errors

Exception	Cause	Example	Solution
FileNotFoundError	File doesn't exist	Opening non-existent file	Check file existence first
PermissionError	Access denied	Writing to read-only file	Check file permissions
IsADirectoryError	Expected file, got directory	Opening directory as file	Verify path is a file
FileExistsError	File already exists	Creating existing file with 'x' mode	Check existence or use different mode

Best Practices Summary

Be specific: Catch specific exceptions rather than using broad except clauses
Provide context: Include meaningful error messages and logging
Don't ignore errors: Avoid empty except blocks that hide problems
Use finally for cleanup: Ensure resources are properly released
Log appropriately: Use proper logging levels and include stack traces when needed
Fail fast: Validate inputs early and raise exceptions for invalid states
Document exceptions: Clearly document what exceptions your functions might raise
Test error paths: Write tests to verify exception handling works correctly
Use context managers: Prefer with statements for resource management
Chain exceptions: Use raise ... from ... to preserve error context

Online Python

Error Types

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