5. Use nullcontext for optional context managers

Use nullcontext() when a value is sometimes managed by a context manager and sometimes already available.

This keeps the control flow uniform: the code can always use with even when one branch does not need resource management. That usually reads better than splitting the main logic across two branches just to handle setup differently.

Note

Python 3.7+

5.1. Don’t do this

if filename is None:
    data = stream.read()
else:
    with open(filename) as stream:
        data = stream.read()

5.2. Do this

from contextlib import nullcontext

cm = open(filename) if filename is not None else nullcontext(stream)
with cm as fh:
    data = fh.read()