How does exception handling work in Python using try/except?

Python uses a try/except block to catch and handle exceptions rather than crashing the program. Code that might raise an error goes inside try; the handler goes inside except.

try:
    result = int(input('Enter a number: '))
    print(100 / result)
except ValueError:
    print('Not a valid integer.')
except ZeroDivisionError:
    print('Cannot divide by zero.')
except Exception as e:
    print(f'Unexpected error: {e}')
else:
    print('Calculation succeeded.')  # runs only if no exception
finally:
    print('Always runs — good for cleanup.')  # always runs

Key rules: always catch the most specific exception first, broad ones last. Catching bare Exception is acceptable as a last resort, but catching BaseException is usually wrong because it swallows KeyboardInterrupt and SystemExit.

The else clause runs only when no exception was raised — a clean place to put code that should only execute on success. The finally clause always runs regardless of whether an exception occurred, making it the right place for cleanup (closing a file, releasing a lock).

You can raise your own exceptions with raise ValueError('message') and create custom exception classes by subclassing Exception. Re-raise a caught exception inside an except block with bare raise to preserve the original traceback.

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