once.py

import inspect

DONE_ONCE = set()


def only_once():
    """Use as a generator to run a block only once."""
    frame = inspect.stack()[1]
    # frame is (frame object, path, line, ...)
    key = (frame[1], frame[2])
    if key not in DONE_ONCE:
        DONE_ONCE.add(key)
        yield


for i in range(2):
    for _ in only_once():
        print "iteration", i

# prints:
# iteration 0