python list comprehension doesn't work when converted to nested for loops

Question

I'm trying to understand this snippet from itertools library

def product(*args, **kwds):
    # 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 = map(tuple, args) * kwds.get('repeat', 1)
    result = [[]]
    for pool in pools:
        result = [x+[y] for x in result for y in pool]
    for prod in result:
        yield tuple(prod)

What this function does is that it takes two arguments: a set of characters and a length of the output, then generates all possible permutations using the given set of characters. for example:

product('ab', repeat=3)
#produces
['a','a','a'],
['a','a','b'],
['a','b','a'],
['a','b','b'],
['b','a','a'],
['b','a','b'],
['b','b','a'],
['b','b','b']

all make sense to me except for this line:

for pool in pools:
    result = [x+[y] for x in result for y in pool]

i tried to uncompact it to normal for loops:

for pool in pools:
    for x in result:
        for y in pool:
            result.append(x+[y])

But i ended up with infinite loop!

How does it work?

Answer 1

This part is a list comprehension . You got the translation right. But in

for pool in pools:
    result = [x+[y] for x in result for y in pool]

you create a list, using result and pool , and then assign it to result .

In

for pool in pools:
    for x in result:
        for y in pool:
            result.append(x+[y])

you iterate over result ( for x in result ), and add elements to it, so it keeps growing.

If you want to create for loops, you can use a temporary variable to save the previous result , and build the new one:

for pool in pools:
    result2 = result[:]
    result = []
    for x in result2:
        for y in pool:
            result.append(x+[y])

Edit: Thanks to @razzak and @GregHewgill for correcting my mistakes