Python Exception Handling Cheat Sheet

Python error handling covering try/except/else/finally, catching multiple exceptions, raising and chaining errors, and custom exceptions.

1 PageBeginnerApr 12, 2026

try / except / else / finally

The full structure of Python's exception handling block.

python

try:    result = 10 / 0except ZeroDivisionError as e:    print(f"Error: {e}")else:    print("No error occurred")   # runs only if try succeededfinally:    print("Always runs")          # runs no matter what

Catching Multiple Exceptions

Handling several exception types with one or more except clauses.

python

try:    value = int(input("Enter a number: "))except (ValueError, TypeError) as e:    print(f"Invalid input: {e}")except Exception as e:    print(f"Unexpected error: {e}")  # broad fallback, keep specific

Raising & Chaining Exceptions

Raising your own errors and preserving the original cause.

python

def validate_age(age):    if age < 0:        raise ValueError("Age cannot be negative")    return agetry:    validate_age(-5)except ValueError as e:    raise RuntimeError("Validation failed") from e  # preserves e as __cause__

Custom Exception Classes

Defining domain-specific exceptions by subclassing Exception.

python

class InsufficientFundsError(Exception):    def __init__(self, balance, amount):        self.balance = balance        self.amount = amount        super().__init__(f"Cannot withdraw {amount}, balance is {balance}")def withdraw(balance, amount):    if amount > balance:        raise InsufficientFundsError(balance, amount)    return balance - amount

Common Built-in Exceptions

Frequently encountered exception types in the standard library.

ValueError- correct type but inappropriate value, e.g. int('abc')
TypeError- operation applied to an object of an inappropriate type
KeyError- dictionary key not found
IndexError- sequence index out of range
AttributeError- attribute reference or assignment fails
FileNotFoundError- file or directory does not exist
ZeroDivisionError- division or modulo by zero
StopIteration- raised by next() when an iterator is exhausted

Pro Tip

Never write a bare `except:` clause — it catches everything including KeyboardInterrupt and SystemExit, making your program hard to stop and hiding real bugs. Catch specific exception types, or at most `except Exception:`.

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