Explanation of how nested list comprehension works?
b = [x for xs in a for x in xs]
is similar to following nested loop.
b = []
for xs in a:
for x in xs:
b.append(x)
if a = [[1,2],[3,4],[5,6]]
, then if we unroll that list comp, we get:
+----------------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
Ah, the incomprehensible "nested" comprehensions. Loops unroll in the same order as in the comprehension.
[leaf for branch in tree for leaf in branch]
It helps to think of it like this.
for branch in tree:
for leaf in branch:
yield leaf
The PEP202 asserts this syntax with "the last index varying fastest" is "the Right One", notably without an explanation of why.