contextlib (contextmanager, ExitStack)

contextlib is Reliability Tooling

contextlib helps you build predictable cleanup paths. This is critical for files, locks, and resource stacks.

@contextmanager

from contextlib import contextmanager

@contextmanager
def managed_resource(open_fn, close_fn):
    r = open_fn()
    try:
        yield r
    finally:
        close_fn(r)

ExitStack for Many Resources

from contextlib import ExitStack

with ExitStack() as stack:
    f1 = stack.enter_context(open("a.txt", "r", encoding="utf-8"))
    f2 = stack.enter_context(open("b.txt", "r", encoding="utf-8"))
    data = f1.read() + f2.read()

Operational Checklist

• Use ExitStack when number of resources is dynamic.
• Cleanup always happens in finally paths.
• Keep context managers small and focused.

Failure Modes

• Leaked handles: missing cleanup under exceptions.
• Complex nesting: deeply nested with blocks reduce readability.