creating multiple generators inside a list comprehension

Question

I am trying to group cards of the same suit (color) and rank inside generators and store those generators inside a list comprehension.

The solution I came up with does that except for the fact that all the generators contain exactly the same cards. Any idea why?

Here is the code

deck=range(52)

gens=[(i for i in deck if i%13==v) for v in range(13)]

Based on this I would expect for example:

gens[1].next()
1
gens[1].next()
14


gens[10].next()
10
gens[10].next()
23

But instead I get

gens[1].next()
12

gens[1].next()
25

gens[1].next()
38

And all the generators in the list return the same results..

Answer 1

The problem is that the name v in your generator expression refers to that variable v in the list comprehension. So, when the code your generator expression actually runs (when you call next), it looks at the variable v and sees the value 12, no matter what the value of v was when you created the generator.

One workaround:

deck = range(52)

def select_kth(v):
    return (i for i in deck if i % 13 == v)

gens = [select_kth(v) for v in range(13)]

Because we defined a function, the name v gets to live in its own naming environment and so stays around unmodified.

If you really wanted, you could do this in one line:

 gens = [(lambda v: (i for i in deck if i % 13 == v))(v) for v in range(13)]