Python Built-in Exceptions Reference

Python raises specific built-in exceptions for different problems, organised in a class hierarchy under BaseException. Recognising them quickly — and knowing which one to catch or raise — makes debugging faster and error handling more precise.

This reference lists the exceptions you will meet most often, explains what causes each one, and shows code that triggers and handles them.

The Hierarchy

BaseException is the root. Directly under it are SystemExit, KeyboardInterrupt and GeneratorExit, which normal code should not catch. Everything else derives from Exception, grouped into families such as ArithmeticError (ZeroDivisionError, OverflowError), LookupError (IndexError, KeyError) and OSError (FileNotFoundError, PermissionError, TimeoutError, ConnectionError...). Catching a family catches all its members.

Most Common Exceptions

The ones you will see daily:

• SyntaxError / IndentationError — code cannot be parsed.
• NameError — a name is not defined (typo or missing import).
• TypeError — operation on the wrong type, or wrong number of arguments.
• ValueError — right type, invalid value (int("abc")).
• AttributeError — object has no such attribute or method.
• IndexError — sequence index out of range; KeyError — missing dict key.
• ZeroDivisionError, OverflowError — arithmetic problems.
• FileNotFoundError, PermissionError, IsADirectoryError — file system problems.
• ImportError / ModuleNotFoundError — import failures.
• StopIteration — iterator exhausted; RecursionError — too deep recursion.
• AssertionError, NotImplementedError, RuntimeError, MemoryError, UnicodeDecodeError.

Reading Tracebacks

Read a traceback from the bottom up: the last line shows the exception type and message; the lines above show the chain of calls, with the most recent call last. Modern Python versions underline the exact failing expression and suggest fixes such as "Did you mean: 'append'?".

Examples

import math

tests = [
    lambda: undefined_variable,
    lambda: "5" + 5,
    lambda: int("abc"),
    lambda: [1, 2, 3][10],
    lambda: {"a": 1}["b"],
    lambda: 10 / 0,
    lambda: "text".push("x"),
    lambda: open("missing_file.txt"),
    lambda: __import__("not_a_real_module"),
    lambda: next(iter([])),
    lambda: math.exp(1000),
    lambda: b"\xff".decode("utf-8"),
]
for test in tests:
    try:
        test()
    except Exception as e:
        # OSError numbers differ between platforms, so show only the message
        message = f"{e.strerror}: {e.filename!r}" if isinstance(e, OSError) else e
        print(f"{type(e).__name__:<20} {message}")

NameError            name 'undefined_variable' is not defined
TypeError            can only concatenate str (not "int") to str
ValueError           invalid literal for int() with base 10: 'abc'
IndexError           list index out of range
KeyError             'b'
ZeroDivisionError    division by zero
AttributeError       'str' object has no attribute 'push'
FileNotFoundError    No such file or directory: 'missing_file.txt'
ModuleNotFoundError  No module named 'not_a_real_module'
StopIteration
OverflowError        math range error
UnicodeDecodeError   'utf-8' codec can't decode byte 0xff in position 0: invalid start byte

def safe_get(container, key):
    try:
        return container[key]
    except LookupError as e:              # covers IndexError and KeyError
        return f"missing ({type(e).__name__})"

print(safe_get([1, 2], 5), safe_get({"a": 1}, "z"), safe_get("abc", 1))

for exc in (IndexError, KeyError, FileNotFoundError, ZeroDivisionError, KeyboardInterrupt):
    print(exc.__name__, "->", " > ".join(c.__name__ for c in exc.__mro__[1:-1]))

missing (IndexError) missing (KeyError) b
IndexError -> LookupError > Exception > BaseException
KeyError -> LookupError > Exception > BaseException
FileNotFoundError -> OSError > Exception > BaseException
ZeroDivisionError -> ArithmeticError > Exception > BaseException
KeyboardInterrupt -> BaseException

import traceback

def level_two():
    return {"user": None}["user"]["name"]

def level_one():
    return level_two()

try:
    level_one()
except TypeError:
    lines = traceback.format_exc().strip().splitlines()
    print(lines[0])
    print(lines[-1])

Traceback (most recent call last):
TypeError: 'NoneType' object is not subscriptable