example1.py

class gen():
    def __enter__(self):
        print('enter')

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('exit')

def test():
    with gen():
        for i in range(10):
            yield i

t = test()
print(next(t))
print(next(t))
del t
print('after del')

example2.py

import contextlib

@contextlib.contextmanager
def gen():
    print('enter')
    yield
    print('exit')


def test():
    with gen():
        for i in range(10):
            yield i

t = test()
print(next(t))
print(next(t))
del t
print('after del')

Perfect question!