There isn't one because you can't do it in the general case - what if you have a lazy infinite generator? For example:
def fib():
a, b = 0, 1
while True:
a, b = b, a + b
yield a
This never terminates but will generate the Fibonacci numbers. You can get as many Fibonacci numbers as you want by calling next().
If you really need to know the number of items there are, then you can't iterate through them linearly one time anyway, so just use a different data structure such as a regular list.
