examples/euler2.py

# https://projecteuler.net/problem=2
# Sum the even Fibonacci terms not exceeding 4,000,000.
# Recursive function calling, generators and Python idioms
def fib():
    a, b = 1, 2
    while True:
        yield a
        a, b = b, a + b

def evens(it):
    for x in it:
        if x % 2 == 0:
            yield x

def upto(limit, it):
    for x in it:
        if x > limit:
            return
        yield x

def euler2(limit=4000000):
    return sum(evens(upto(limit, fib())))

print(euler2())