Infinite yield problem

Question

Here is my simple code

class Fibonacci:
    @staticmethod
    def series():
        fprev = 1
        fnext = 1
        yield fnext
        while True:
            yield fnext
            fprev,fnext = fnext,fprev+fnext

under10 = (i for i in Fibonacci.series() if i<10)
for i in under10 :
    print i

It's absolutely obvious, but...WHY interpreter is executing block

while True:
                yield fnext
                fprev,fnext = fnext,fprev+fnext

Forever? I specified in generator,that I want only elements<10

under10 = (i for i in Fibonacci.series() if i<10)

IMHO, it's a little bit misunderstanding Any way to prevent infinite execution without re-writing "series"?

Answer 1

How should the interpreter know that all future numbers will be < 10? It would have to either know (somehow) that it’s churning out the Fibonacci series, or it would have to inspect the whole series.

It can’t do the first, so it does the second.

You can fix this by using itertools.takewhile:

import itertools

under10 = itertools.takewhile(lambda n: n < 10, Fibonacci.series())