context-managers.md

Python by Example: Context Managers

The with statement manages resources that need setup and cleanup—files, network connections, locks. It calls __enter__ when entering the block and __exit__ when leaving, even if an exception occurs. Use contextlib.contextmanager to build your own with a generator function.

What you'll learn:

• Using with for automatic resource cleanup
• Building a context manager with @contextmanager
• Combining multiple managers in one with

from contextlib import contextmanager


# with guarantees cleanup even if an error occurs
with open("data.txt", "w") as f:
    f.write("hello")

with open("data.txt") as f:
    print(f.read())


@contextmanager
def managed(name):
    print(f"Opening {name}")
    try:
        yield name
    finally:
        print(f"Closing {name}")


with managed("connection") as conn:
    print(f"Using {conn}")


# Multiple managers in one with statement
with managed("db") as db, managed("cache") as cache:
    print(f"Using {db} and {cache}")

The code before yield is setup (__enter__); the finally block is cleanup (__exit__). Multiple managers on one line are cleaned up in reverse order.

To run this program:

$ python source/context-managers.py
hello
Opening connection
Using connection
Closing connection
Opening db
Opening cache
Using db and cache
Closing cache
Closing db

Tip: Any object with __enter__ and __exit__ works as a context manager—not just files. Examples: threading locks, database transactions, test fixtures.

Try it: Write a context manager that temporarily changes the current working directory and restores it on exit.

Source: context-managers.py

Next: Modules