Single Use Context Manager
Recipes
Most context managers are written in a way that means
they can only be used effectively in a with statement once.
These single use context managers must be created afresh each time they’re used - attempting to
use them a second time will trigger an exception or otherwise not work correctly.
This common limitation means that it is generally advisable to create context managers directly
in the header of the with statement where they are used.
Context managers created using contextmanager() are also single use context managers,
and will complain about the underlying generator failing to yield
if an attempt is made to use them a second time:
from contextlib import contextmanager
@contextmanager
def singleuse():
print("Before")
yield
print("After")
>>> cm = singleuse()
# Run first time
>>> with cm:
... pass
...
Before
After
# Run second time (failed)
>>> with cm:
... pass
...
RuntimeError: generator didn't yield