Why is the context manager closed after it is yielded?

Question

I am trying to yield a context manager that is created in a with statement. However, something that I don't understand happens: the context manager is closed before it is yielded, even though the execution in the generator did not exit the scope of the with. For example:

class CM:
  def __enter__(self):
    print('enter cm')
    return self
  def __exit__(self, exc_type, exc_val, exc_tb):
    print('exit cm')


def make_foo():
  with CM() as cm:
    print('before yield')
    yield cm
  print('after yield')


print('before make_foo')
foo = next(make_foo())
print('after make_foo')

outputs

before make_foo
enter cm
before yield
exit cm
after make_foo

I saw this thread on a related topic, and the answer is about time when the object is garbage collected -- however why would cm be garbage collected before it is returned to be used by the caller here?

EDIT

When writing instead

foo_maker = make_foo()
foo = next(foo_maker)

then the CM is not closed -- so it seems that the CM is indeed GC because the generator is GC. But shouldn't it left alone since it is returned and potentially used after?

the contxt manager isn't garbage collected, the generator object is, which `.close`es it, which causes the `with` statement to execute `__exit__` — juanpa.arrivillaga, Oct 14 '20 at 12:16
You only called `next()` once, so `make_foo()` never resumed after the `yield`. The `with` block therefore never got a chance to exit. — jasonharper, Oct 14 '20 at 12:18
This is an interesting question. I'll add that there isn't any garbage collection in cpython, there is ref counting. Their diferent. — Pedro Rodrigues, Oct 14 '20 at 12:47

score 1 · Accepted Answer · answered Oct 14 '20 at 12:51

I think this is because of your method definition and how you call it.

if you instead of foo = next(make_foo()) use this :

for i in make_foo():
  print('thanks I saw cm')

the result would be this :

enter cm
before yield
thanks I saw cm
exit cm
after yield
after make_foo

Why is the context manager closed after it is yielded?

1 Answers1