context_sample.py

from contextlib import contextmanager

@contextmanager
def mark_enabling(value):
    global mark
    try:
        mark = True
        yield value
    finally:
        mark = False

def test():
    global mark
    print 'Yes' if mark else 'No'

test()
# No
with mark_enabling('Salsa') as v:
    test()
    print 'Value:', v
# Yes
# Value: Salsa
test()
# No