a = [1,2,3,4]
b = [x for x in a]

a = [[1,2],[3,4],[5,6]]
b = [x for xs in a for x in xs]

[leaf for branch in tree for leaf in branch]

for branch in tree:
    for leaf in branch:
        yield leaf

+----------------a------------------+
      | +--xs---+ , +--xs---+ , +--xs---+ | for xs in a
      | | x , x |   | x , x |   | x , x | | for x in xs
a  =  [ [ 1 , 2 ] , [ 3 , 4 ] , [ 5 , 6 ] ]
b  =  [ x for xs in a for x in xs ] == [1,2,3,4,5,6] #a list of just the "x"s

b = []
for xs in a:
   for x in xs:
       b.append(x)

...for xs in a...]

...for x in xs]

b = []
for xs in a:
    for x in xs:
        b.append(x)

result = []
for xs in a:
    for x in xs:
        result.append(x)

______
row 1 |1 | 2 |
       ______
row 2 |3 | 4 |
       ______
row 3 |5 | 6 |
       ______

b = [element for row in matrix for element in row]

b = []
for xs in a:
    for x in xs:
        b.append(x)

dice_rolls = []
for roll1 in range(1,7):
    for roll2 in range(1,7):
        for roll3 in range(1,7):
            dice_rolls.append((roll1, roll2, roll3))

# becomes

dice_rolls = [(roll1, roll2, roll3) for roll1 in range(1, 7) for roll2 in range(1, 7)
              for roll3 in range(1, 7)]