I find that I frequently need the "null" (no-op) context manager.  For example, in code such as:

with transaction or contextlib.null():
    ...

Since there is no easy expression to create a null context manager, we must resort to workarounds, such as:

if transaction:
    with transaction:
        ... code ...
else:
    ... duplicated code ...

(with the usual options of moving the duplicated code to a function—but still.)

Or by creating ad-hoc null context managers with the help of contextlib.contextmanager:

if transaction is None:
    transaction = contextlib.contextmanager(lambda: iter([None])()
with transaction:
    ...

Adding a "null" context manager would be both practical and elegant.  I have attached a patch for contextlib.py