Question

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?

Answer 1

For Python 3.7 or higher, then you can use the nullcontext for stand-in purposes:

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)

---

For older Python versions, you can build one yourself on-the-fly with an optional None:

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 None. The with will either produce FILE as a file object, or None. But the None type will have a proper __exit__

---

Another solution would be to just write it like this:

if f is not None:
    with open(f) as FILE:
        do_something(FILE)
else:
    do_something(f)

(file is a builtin btw )