Python · The language, not just syntax

What is a context manager and how do you write one?

Mediumcontext managerswithcleanupUpdated Oct 2026

Short answer

A context manager defines __enter__ and __exit__ so the with statement can acquire a resource, run a block, and guarantee cleanup even when an exception occurs. contextlib.contextmanager builds one from a generator.

The with statement calls __enter__, binds its return value if there is an as target, runs the block, then calls __exit__ with exception details. __exit__ runs on normal exit and on exceptions, which is why it is the right place for closing files, releasing locks and rolling back transactions.

python

from contextlib import contextmanager

@contextmanager
def transaction(connection):
    try:
        yield connection
    except Exception:
        connection.rollback()
        raise
    else:
        connection.commit()

with transaction(conn) as active:
    active.execute("insert into events (kind) values (?)", ("signup",))

The generator yields once. Code after yield is the cleanup path, and the except clause decides whether to roll back.

Rules that matter

__exit__ receives the exception type, value and traceback
Returning True from __exit__ suppresses the exception; do this only deliberately
Nested with statements and contextlib.ExitStack handle several resources cleanly
A context manager is not automatically thread-safe or reentrant

The generator-based decorator is concise but easy to get wrong around exceptions. If the generator does not use try/finally, cleanup after yield may be skipped when the block raises. Explicit classes are clearer when the manager holds complex state or needs to support reentry.

Do not use a context manager just to shorten code. Its value is deterministic cleanup and a clear resource lifetime, especially with locks, sockets, files and transactions.

Follow-ups you should expect

Interviewers use these to check whether the first answer was understanding or memorisation.

1What arguments does __exit__ receive?
2How do you suppress an exception in a context manager, and when should you?
3What does contextlib.contextmanager do under the hood?
4How would you manage two resources that must be closed in reverse order?

Say it in the interview

A context manager implements __enter__ and __exit__, and the with statement guarantees __exit__ runs even if the block raises. I use it for files, locks, sockets and database transactions because cleanup must be deterministic. For simple cases I write a generator with contextlib.contextmanager and wrap the yield in try/finally. If the manager holds state or must be reentrant, I write an explicit class.

Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.

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Rules that matter

Follow-ups you should expect

Related questions

Now say it out loud.