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

MRAB python at mrabarnett.plus.com
Thu Aug 13 13:23:50 CEST 2009


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.
> 
It's equivalent to:

     L = []
     for n in evens():
         if n < 15
             L.append(n)



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