Generating a list of EVEN numbers in Python

Question

Basically I need help in generating even numbers from a list that I have created in Python:

[1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597         


        

Answer 1

Instead of generating all Fibonacci numbers then filtering for evens, why not generate just the even values?

def even_fibs():
    a,b = 1,2
    while True:
        yield b
        a,b = a+2*b, 2*a+3*b

generates [2, 8, 34, 144, 610, 2584, 10946 ...]

then your sum code becomes:

total = 0
for f in even_fibs():
    if f >= 4000000:
        break
    else:
        total += f

or

from itertools import takewhile
total = sum(takewhile(lambda n: n<4000000, even_fibs()))