what is it, that I don't understand about python and lazy evaluation?

Erik Bernoth erik.bernoth at googlemail.com
Thu Aug 13 11:32:53 CEST 2009

Hi List,

look at the following code:

def evens():
    # iterator returning even numbers
    i = 0
    while True:
        yield i
        i += 2

# now get all the even numbers up to 15
L = [n for n in evens() if n < 15]

Isn't it strange, that this code runs (in a lazy language) for eternity? I
would expect python to to spit out (in no time):
>> L
[0, 2, 4, 6, 8, 10, 12, 14]

after 14 it is not nessesary to evaluate evens() any further.

I really started to ask myself if python really is lazy, but everything else
I wrote in lazy style still worked. Example:
>> def test(txt, retval):
..    print(txt)
..    return retval
>>> test(1, True) or test(2, True)
>>> test(1, True) and test(2, True)

Can anybody explain what happens with evens()?

best regards
Erik Bernoth

PS: The code comes from a list post from 2006. You find it here:
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