itertools product(infinite iterator) hangs

[ast]

Le 14/09/2019 à 04:26, Oscar Benjamin a écrit :
> I've been staring at this for a little while:
> 
> from itertools import product
> 
> class Naturals:
>      def __iter__(self):
>          i = 1
>          while True:
>              yield i
>              i += 1
> 
> N = Naturals()
> print(iter(N))
> print(product(N))  # <--- hangs
> 
> When I run the above the call to product hangs but I can't see why. I
> would expect that since I'm not iterating over the product it would
> just call iter(N) but clearly not since iter(N) returns a generator
> instantly where as product(N) hangs.
> 
> What am I missing?
> 
> Oscar
> 

here is a pseudo code for product:

def product(*args, repeat=1):
     # product('ABCD', 'xy') --> Ax Ay Bx By Cx Cy Dx Dy
     # product(range(2), repeat=3) --> 000 001 010 011 100 101 110 111
     pools = [tuple(pool) for pool in args] * repeat
     result = [[]]
     for pool in pools:
         result = [x+[y] for x in result for y in pool]
     for prod in result:
         yield tuple(prod)


clearly "tuple(pool)" hangs with an infinite iterable pool