 Python Ziiiiiiip! [A helpful guide]

Many coders struggle with understanding the zip function. Let’s fix this for you.

The zip function takes a number of iterables and aggregates them to a single one by combining the i-th values of each iterable into a tuple for every i.

(An iter-able is an object that contains multiple elements over which you can iterate. Examples are lists, sets, or tuples.)

How to zip two lists in Python?

Say, you have two lists:

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

Now you zip them together and get the new list:

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

How to unzip a zipped list in Python?

You can unzip them by using the following trick: If you remove the outer bracket of the result (e.g. via the asterisk operator), you get the following three tuples:

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

When you zip those together, you get the new list:

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

So you (almost) have your two original lists again!

This is the idea in the following code snippet:

lst_1 = [1, 2, 3]
lst_2 = [4, 5, 6]

# Zip two lists together
zipped = list(zip(lst_1, lst_2))
print(zipped)
# [(1, 4), (2, 5), (3, 6)]

# Unzip to lists again
lst_1_new, lst_2_new = zip(*zipped)
print(list(lst_1_new))
print(list(lst_2_new))

Puzzle: what will be the output of the last two print statements?

How to zip lists of different lengths?

It simply ignores the remaining elements of the larger list. Here is an example:

print(list(zip([1,2,3],[1,2]))) # [(1, 1), (2, 2)]

How to use zip with a single argument?

You can do that. But the more interesting question is: what will happen?

Python still creates a tuple of the i-th elements — only that there is only one of those. Thus, the tuple has only a single element. The default output of a tuple with one element x is (x,).

print(list(zip([1,2,3]))) # [(1,), (2,), (3,)]

This strange output formatting caused some confusion among the readers of my Python email course (join us, it’s free!). I hope that you now understand completely that this is not a bug in Python but only a tuple with a single element. (Don’t ask me why they didn’t use the format ‘(x)’ instead of ‘(x,)’.)

What is a zip object in Python?

You will quickly realize that the result of the zip function is neither a list nor a tuple:

x = [[1,2],[3,4]]
print(zip(*x))

You would expect [(1,3),(2,4)] but the result is “<zip object at 0x000002E9D87CFD08>”. That’s weird, isn’t it?

Well — actually not. The result of the zip() function is an iterator, not a list.

An iterator in Python is an object that contains a fixed number of elements, and allows you to access each element in an ordered fashion (the next(iterator) function for an iterator). This is more efficient and more general-purpose — compared to creating a list and returning the list as a result.

To fix this, you have to convert the iterator object in the iterable you want (e.g. set, list, tuple):

x = [[1,2],[3,4]]
print(list(zip(*x)))
# [(1, 3), (2, 4)]

Finally, let me clarify one last thing: the asterisk operator is placed just before the iterable to be unpacked (not after it or anywhere else). If you put the asterisk operator anywhere else, Python will think it’s multiplication and throw an error (best case):

x = [[1,2],[3,4]]
y = zip*(x)
# NO!

y = zip(x*)
# NO!

y = *zip(x)
# No! (It's an iterator not an iterable)

y = zip(*x)
# Yes!

Where to go from here?

This article is based on one of my free Python Email lectures. I call it “Coffee Break Python Email Course” because it will improve your Python skills in your coffee break. You just have to read my daily emails and participate in the fun coding challenges. My >5,000 readers love it.