Wrapping my head around generators

I am going on with refactoring my code base while migrating from Python 2 to Python 3. I am using generators to make reusable components this time as I am more comfortable with them this time then last time. But there are times when I am stuck at their use. Like below example. These two are essentially same with one infinitely generating and other with a limit.

Is there a way to make these as one function? Normally an extra optional parameter as limit will do the trick but with generators I am not sure how to use them. Is there some way to do that?

def fibonacci_inf(a, b):
    """Lazily generates Fibonacci numbers Infinitely"""
    while True:
        yield b
        a, b = b, a + b

def fibonacci(a, b, num):
    """Lazily generates Fibonacci numbers"""
    while b < num:
        yield b
        a, b = b, a + b

You could just implement the infinite generator, and use itertools.takewhile to limit it:

>>> list(itertools.takewhile(lambda x : x < 8, fibonacci_inf(0,1)))
[1, 1, 2, 3, 5]

StackExchange Code Review Q#45752, answer score: 10