Generators

A function containing yield becomes a generator. Calling it returns an iterator instead of running the whole body.

Each time the loop asks for the next value, the function resumes after the previous yield. Values are produced lazily, so memory stays small.
A generator can be used once. Create a new one to start again.

def count_up(limit):
    n = 1
    while n <= limit:
        yield n
        n += 1

for value in count_up(3):
    print(value)