python - Re assign a list efficiently -

this mwe of re-arrainging need do:

a = [[1,2,3], [4,5,6], [7,8,9], [10,11,12]] b = [[], [], []]  item in a:     b[0].append(item[0])         b[1].append(item[1])     b[2].append(item[2]) 

which makes b lool this:

b = [[1, 4, 7, 10], [2, 5, 8, 11], [3, 6, 9, 12]] 

i.e., every first item in every list inside a stored in first list in b , same lists 2 , 3 in b.

i need apply big a list, there more efficient way this?

there much better way transpose rows , columns:

b = zip(*a) 

demo:

>>> = [[1,2,3], [4,5,6], [7,8,9], [10,11,12]] >>> zip(*a) [(1, 4, 7, 10), (2, 5, 8, 11), (3, 6, 9, 12)] 

zip() takes multiple sequences arguments , pairs elements each form new lists. passing in a * splat argument, ask python expand a separate arguments zip().

note output gives list of tuples; map elements lists needed:

b = map(list, zip(*a))