Instead of this:
file = open(f)
do_something(file)
file.close()
it's better to use this:
with open(f) as file:
do_something(file)
What if I have something like this?
if f is not None:
file = open(f)
else:
file = None
do_something(file)
if file is not None:
file.close()
Where do_something also has an if file is None clause, and still does something useful in that case - I don't want to just skip do_something if file is None.
Is there a sensible way of converting this to with/as form? Or am I just trying to solve the optional file problem in a wrong way?
If you are using Python 3.7 or higher, then you can declare the null context manager for stand-in purposes in a much simpler way:
import contextlib
none_context = contextlib.nullcontext()
You can read more about it here.
if f is not None:
with open(f) as FILE:
do_something(FILE)
else:
do_something(f)
Alternatively, here is a way to do an on-the-fly context with an optional None that won't crash:
from contextlib import contextmanager
none_context = contextmanager(lambda: iter([None]))()
# <contextlib.GeneratorContextManager at 0x1021a0110>
with (open(f) if f is not None else none_context) as FILE:
do_something(FILE)
It creates a context that returns a None value. The with will either produce FILE as a file object, or a None type. But the None type will have a proper __exit__.