So what's said here is :

from contextlib import contextmanager

@contextmanager
def custom_open(filename):
    f = open(filename)
    yield f
    f.close()

with custom_open('file') as f:
    contents = f.read()

But as far as I undestand, we shoud wrap the yield into a try: block with the close in the finally:, otherwise a exception might allow leaving the with block without calling close. I might do a PR for this, but I wanted to check with someone if there was a reason the code was shown this way and/or if I was mistaken altogether...