How do context manager magic methods enter and exit work, and what does exit's return value control?

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Answer

__enter__(self) runs at the start of a with block and its return value is bound to the as target. __exit__(self, exc_type, exc_val, exc_tb) runs on block exit, receiving exception info if one occurred; returning True suppresses the exception, while returning False (or None) lets it propagate normally.

class Transaction:
    def __enter__(self):
        print('BEGIN')
        return self
    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_type:
            print('ROLLBACK')
            return False
        print('COMMIT')

with Transaction():
    print('doing work')

Real-world example A database Transaction class uses __enter__/__exit__ to automatically COMMIT on success and ROLLBACK if any exception occurs inside the with block, without repeating try/finally everywhere.

Common follow-ups: How does contextlib.contextmanager simplify writing this?;What happens if __exit__ itself raises an exception?

File I/O & Context Managers;Exception Handling

All 15 Magic Methods & Operator Overloading questions

How do context manager magic methods __enter__ and __exit__ work, and what does __exit__'s return value control?

How do context manager magic methods enter and exit work, and what does exit's return value control?