Python Exceptions

Overview

Exceptions signal runtime errors without crashing the program when handled. Python uses a hierarchy of exception classes; catching specific types is clearer and safer than bare except: clauses.

Syntax / Usage

try:
    result = 10 / 0
except ZeroDivisionError as exc:
    print(f"Math error: {exc}")
except (ValueError, TypeError):
    print("Bad value or type")
else:
    print("No exception occurred")
finally:
    print("Always runs (cleanup)")

# Raise exceptions
raise ValueError("Age must be positive")

# Custom exception
class AppError(Exception):
    pass

Use raise ... from cause to chain exceptions and preserve context.

Examples

Safe integer conversion from user input:

def parse_int(value: str) -> int:
    try:
        return int(value)
    except ValueError as exc:
        raise ValueError(f"Not a valid integer: {value!r}") from exc

parse_int("42")   # 42
# parse_int("abc") raises ValueError

Read a file with guaranteed cleanup:

def read_config(path):
    handle = None
    try:
        handle = open(path)
        return handle.read()
    except FileNotFoundError:
        return None
    finally:
        if handle:
            handle.close()

Common Mistakes

• Catching bare Exception or BaseException and swallowing errors silently
• Using exceptions for normal control flow instead of conditionals
• Forgetting that finally runs even when return is in try or except
• Not re-raising or chaining when wrapping exceptions loses debugging context

See Also

python-file-io python-context-managers python-functions