Separate lists of format M=[(a,x), (b,y)] into M1=[a,b] and M2=[x,y]

Question

I have a list M below which came from an external file (This is a large list in reality, len(M)>10000 ).

M=[(1, 100), (2, 200), (3, 300), (4, 400)]

However, I want to separate it into M1=[1,2,3,4] and M2=[100,200,300,400] . Here is how I do it at the moment,

 M1 = [] M2 = [] for M1,M2 in M: M1.append(M1) M2.append(M2) 

I was wondering if there is a simpler and more efficient way to do it, a solution that uses built-in functions in Python.

Answer 1

Just do it:

M1, M2 = zip(*M)

M1 and M2 will be tuples. You can transform them into lists if you want:

M1, M2 = map(list, zip(*M))

or with a genexp:

M1, M2 = (list(i) for i in zip(*M))

Answer 2

One solution, more efficient than yours because it uses implicit loops is:

>>> [m1 for m1, _ in M]
>>> [1, 2, 3, 4]
>>> [m2 for _, m2 in M]
>>> [100, 200, 300, 400]

But better is using the * operator and zip :

>>> M1, M2 = zip(*M)
>>> M1
(1, 2, 3, 4)
>>> M2
(100, 200, 300, 400)

Answer 3

You could do

M1, M2 = zip(*M)

That works for tuples as well as lists.

If you really have a long external file though, your initial version might be faster, as this approach will send 10k arguments to zip, and hence keep all the content in memory.