Zip is its own inverse

This post is part of the programming series.

You can undo the zipping operation using zip itself. Let’s explore that in Python. By using the unpacking operator * you don’t have to manually specify the number of arguments (although I do assume the return of two components in the example below). In Python 3, the zip operator returns a generator instead of a list, so you need to explicitly cast to a list if you want one.

>>> a = [1, 3]
>>> b = [2, 4]
>>> c = list(zip(a,b))
>>> c
[(1, 2), (3, 4)]
>>> a, b = list(zip(*c))
>>> a
(1, 3)
>>> b
(2, 4)

The inverse operation will always return tuples in Python, so if your original input was a list, you also need to convert back the results to a list.

This operation is super handy. For example, I wrote a class to recommend relevant texts for a query document based on their distance in a vector space. The recommend() function of this class returns a list of recommended texts and a list of tuples containing relevant metadata about those texts. So the metadata will be a list of (distance, document_id, type) tuples. We may be interested in easily retrieving all distances, document_ids etc. as a list of their own. We can do that by using the unpack+zip trick:

distances, document_ids, types = zip(*meta)

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