Erik Bernoth wrote:
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)
1
True
>>> test(1, True) and test(2, True)
1
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:
http://mail.python.org/pipermail/python-list/2006-November/585783.html
As MRAB pointed out, the issue is not with evens, it's with the list
comprehension. The list comprehension doesn't know when to stop, only
which numbers to include.
~Ethan~
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