Problem

Given an upper bound n, list all even fibonacci numbers less than or equal to the nth fibonacci number and return the sum

i.e., all fibonacci numbers less than or equal to 10th fibonacci number is: [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

Listing all even elements, we get [2, 8, 34] with sum 44

Before we everything else, we need to create a fibonacci sequence generator:

from functools import lru_cache


@lru_cache(maxsize=None)
def fib(n: int):
    if n < 0:
        return 0
    if n < 2:
        return 1
    return fib(n - 1) + fib(n-2)

def fib_sequence(upper_bound: int):
    return [ fib(i) for i in range(upper_bound)]

Picking all even elements:

def sum_even_fib(n: int):
    return sum([f for f in fib_sequence(n) if f % 2 == 0])

Test:

assert sum_even_fib(10) == 44
assert sum_even_fib(20) == 3382
assert sum_even_fib(30) == 1089154