[FIXED] How can I get multiple lists as separate results from a list comprehension?

Issue

Suppose I have this code:

def f(x):
    return 2*x,x*x

x = range(3)
xlist, ylist = [f(value) for value in x]

How can I neatly get a result like this?

xlist = [0, 2, 4]
ylist = [0, 1, 4]

Solution

Note that return 2*x,x is short for return (2*x,x), i.e. a tuple. Your list comprehension thus generates a list of tuples, not a tuple of lists. The nice thing of zip however is that you can easily use it in reverse with the asterisk:

xlist,ylist = zip(*[f(value) for value in x])
#                 ^ with asterisk

Note that xlist and ylist will be tuples (since zip will be unpacked). If you want them to be lists, you can for instance use:

xlist,ylist = map(list, zip(*[f(value) for value in x]))

which results in:

>>> xlist
[0, 2, 4]
>>> ylist
[0, 1, 4]

Another way to do this is with separate list comprehensions:

xlist = [f(value)[0] for value in x]
ylist = [f(value)[1] for value in x]

Of course, this repeats the work of f, which is inelegant and can be inefficient.

Answered By - Willem Van Onsem