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pythonlist-comprehensioniterable-unpacking

Python Tuple Unpacking

If I have

 nums_and_words = [(1, 'one'), (2, 'two'), (3, 'three')]

and would like

nums = [1, 2, 3]
words= ['one', 'two', 'three']

How would I do that in a Pythonic way? It took me a minute to realize why the following doesn't work

nums, words = [(el[0], el[1]) for el in nums_and_words]

I'm curious if someone can provide a similar manner of achieving the result I'm looking for.

Solution

  • Use zip, then unpack:

    nums_and_words = [(1, 'one'), (2, 'two'), (3, 'three')]
nums, words = zip(*nums_and_words)

    Actually, this "unpacks" twice: First, when you pass the list of lists to zip with *, then when you distribute the result to the two variables.

    You can think of zip(*list_of_lists) as 'transposing' the argument:

       zip(*[(1, 'one'), (2, 'two'), (3, 'three')])
== zip(  (1, 'one'), (2, 'two'), (3, 'three') )
== [(1, 2, 3), ('one', 'two', 'three')]

    Note that this will give you tuples; if you really need lists, you'd have to map the result:

    nums, words = map(list, zip(*nums_and_words))