How do you handle exceptions in Python?

Use try/except to catch specific exceptions, else for code that runs only on success, and finally for cleanup.

Python

Overview

Python uses `try/except` blocks for error handling. Unlike JavaScript, Python makes a strong distinction between the `except` clause (error path), `else` (success path), and `finally` (always runs). Catching the right exception type is key.

Key Points

•`except ExceptionType` catches that specific exception always prefer specific types over bare `except`.
•`except (TypeError, ValueError)` catches multiple exception types.
•`else` block runs only if no exception was raised in `try` keeps success code separate.
•`finally` always runs use it for cleanup (closing files, releasing locks).
•Use `raise` to re-raise the current exception, `raise ExceptionType("msg")` to throw a new one.

Examples

Basic try/except

python

try:
    value = int(input("Enter a number: "))
except ValueError:
    print("That's not a valid number.")

Full try/except/else/finally

python

try:
    file = open("data.json")
    data = json.load(file)
except FileNotFoundError:
    print("File not found")
except json.JSONDecodeError as e:
    print(f"Invalid JSON: {e}")
else:
    process(data)  # only runs if no exception
finally:
    file.close()   # always runs (or use 'with' to avoid this)

Context manager (better than manual finally)

python

# Preferred: 'with' handles closing automatically
with open("data.json") as file:
    data = json.load(file)  # file is closed even if an exception occurs

Custom exception class

python

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

try:
    raise InsufficientFundsError(100, 50)
except InsufficientFundsError as e:
    print(e)

Pro Tip

Never use bare `except:` it catches `SystemExit`, `KeyboardInterrupt`, and `GeneratorExit`, which you almost never want to catch. At minimum use `except Exception:`, but specific exception types are best.