Python Context Managers

The with Statement

You have likely already used a context manager without naming it: opening a file with the with statement. It guarantees that setup code runs before your block and cleanup code runs after, even if an exception occurs inside the block.

with open("data.txt") as f:
    content = f.read()
    print(content)
# file is automatically closed here

__enter__ and __exit__

A context manager is any object with two special methods: __enter__, which runs at the start of the with block and returns the value used after "as", and __exit__, which runs at the end, even after an exception, and can suppress it.

class Timer:
    def __enter__(self):
        import time
        self.start = time.time()
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        import time
        elapsed = time.time() - self.start
        print(f"Took {elapsed:.4f} seconds")

with Timer():
    total = sum(range(1_000_000))

contextlib.contextmanager

Writing a full class just for a context manager can feel heavy. The contextlib module gives you a decorator that turns a simple generator function into a context manager: code before yield is __enter__, code after yield is __exit__.

from contextlib import contextmanager

@contextmanager
def open_section(title):
    print(f"--- start {title} ---")
    yield
    print(f"--- end {title} ---")

with open_section("Report"):
    print("writing report contents")