What is the inverse function of zip in python?

Question

Possible Duplicate:
A Transpose/Unzip Function in Python

I've used the zip() function from the numpy library to sort tuples and now I have a list containing all the tuples. I had since modified that list and now I would like to restore the tuples so I can use my data. How can I do this?

Solution

lst1, lst2 = zip(*zipped_list)

should give you the unzipped list.

*zipped_list unpacks the zipped_list object. it then passes all the tuples from the zipped_list object to zip, which just packs them back up as they were when you passed them in.

so if:

a = [1,2,3]
b = [4,5,6]

then zipped_list = zip(a,b) gives you:

[(1,4), (2,5), (3,6)]

and *zipped_list gives you back

(1,4), (2,5), (3,6)

zipping that with zip(*zipped_list) gives you back the two collections:

[(1, 2, 3), (4, 5, 6)]

Comments

[jwg]

In other words lambda x: zip(*x) is self-inverse.

[fosco]

This doesn't seem to be working: l1 = [1,2,3], l2 = [4,5,6]; if I call a,b = zip(*zip(l1,l2)), then a = (1,2,3) != l1 = [1,2,3] (because tuple != list)

[JLDiaz]

@FoscoLoregian a,b = map(list, zip(*zip(l1,l2)))

[Omer Tuchfeld]

See also stackoverflow.com/a/6473724/791430

[John Smith Optional]

Wouldn't it fail for Python version 3.6 and older as soon as your list of zipped tuples contains more than 255 items ? (because of the maximum numbers of arguments that can be passed to a function) See: stackoverflow.com/questions/714475/…

[Adam Selker]

No, because the limit of 255 doesn't apply when you expand args using *; see Chris Colbert's answer to stackoverflow.com/questions/714475/…

[SomethingSomething]

I might mention the fact that you can look at the zip function and notice that it is almost naturally the inverse of itself

[daruma]

having an unzip function would have been nice (even if just an alias of zip...)

[mueslo]

This answer is not quite complete, if you start with two empty lists you will not get them back.

[Fırat Kıyak]

I thought that this method would work slowly but it actually worked better than all the other ways I imagined. Why is this so fast?