python double for in list comprehension

Question

I have checked the other questions but here I found something very strange.

if I make the two for loop in a list comprehension it works well.

pools = [(0, 1, 2), (0, 1, 2)]
result = [[]]
for pool in pools:
    result = [x + [y] for x in result for y in pool]

# print(result)
# result = [[0, 0], [0, 1], [0, 2], [1, 0], [1, 1], [1, 2], [2, 0], [2, 1], [2, 2]]

But if I break it down into normal nested for loop it will be an endless code.

pools = [(0, 1, 2), (0, 1, 2)]
result = [[]]
for pool in pools:
    for x in result:
        for y in pool:
            result.append(x + [y])

# This will be an endless looping

I guess it may because there are some hidden code in the python list comprehension? Really appreciate for your kindness help.

I have modified the nested for loop but still seems not work.

pools = [(0, 1, 2), (0, 1, 2)]
result = [[]]
temp = [[]]
for pool in pools:
    for x in result:
        for y in pool:
            temp.append(x + [y])
result = temp

# result = [[], [0], [1], [2], [0], [1], [2]]

Answer 1

In the first code sample the list comprehension is executed first and the finally created list is assigned to result .

In the second example the list referenced by result is modified while the second for-loop iterates over it. Always appending new items to result in the loop ensures that the for-loop won't ever exhaust the list.

The list comprehension can be rewritten with conventional for-loops as:

pools = [(0, 1, 2), (0, 1, 2)]
result = [[]]
for pool in pools:
    temp = []     # Begin of rewritten list comprehension
    for x in result:
        for y in pool:
            temp.append(x + [y])

    result = temp # Final assignment of constructed list