Python Error Handling

Overview

Python uses exceptions to signal errors and exceptional conditions. Proper handling of these exceptions lets your programs recover gracefully, log useful information, and maintain a clean control flow.

Exceptions

All built‑in exceptions inherit from BaseException. Most user‑visible errors derive from Exception.

Copyclass MyError(Exception):
    """Custom error for demonstration"""
    pass

Try‑Except

The try block encloses code that may raise an exception. except clauses catch specific exception types.

Copytry:
    result = 10 / divisor
except ZeroDivisionError as e:
    print('Cannot divide by zero:', e)
except Exception as e:
    print('Unexpected error:', e)

Finally

The finally block runs no matter what—useful for cleanup.

Copyfile = open('data.txt')
try:
    data = file.read()
finally:
    file.close()

Else

The else clause runs only if no exception was raised.

Copytry:
    value = int(user_input)
except ValueError:
    print('Invalid number')
else:
    print('You entered', value)

Custom Exceptions

Define your own exception types to convey domain‑specific errors.

Copyclass ValidationError(Exception):
    def __init__(self, field, msg):
        super().__init__(f'{field}: {msg}')
        self.field = field
        self.msg = msg

Common Patterns

• Validate inputs early and raise ValueError or custom exceptions.
• Wrap external resources (files, network) in with statements or use try/finally for cleanup.
• Log exceptions using the logging module before re‑raising if needed.