Iterating over an unknown number of nested loops in python

Question

I have a variable number of user-defined lists, each containing words. For example, there may be three lists like the following:

list1 = ["THE", "A"]
list2 = ["ELEPHANT", "APPLE", "CAR"]
list3 = ["WALKED", "DROVE", "SAT"]

What I want is to iterate over every combination in each list, checking each against a dictionary of known words, to see which word-groupings are most like the dictionary. That means the iterations would be like:

[
    "THE ELEPHANT WALKED",
    "THE APPLE WALKED",
    "THE CAR WALKED",
    "THE ELEPHANT DROVE",
    "THE APPLE DROVE",
    "THE CAR DROVE",
    # ...
    "A CAR SAT",
]

The problem is that there can be any number of lists, and each list can contain a variable amount of items. I know that recursion could be used for this, but I need a solution without recursion. The problem I keep having is the fact that there can be a variable amount of lists, otherwise I would just write:

for a in list1:
    for b in list2:
        for c in list3:
            ...

But I won't know where to stop...

Eric · Accepted Answer · 2021-05-11 17:17:07Z

30

itertools.product does exactly what you want:

from itertools import product

lists = [
    ['THE', 'A'],
    ['ELEPHANT', 'APPLE', 'CAR'],
    ['WALKED', 'DROVE', 'SAT']
]

for items in product(*lists):
    print(items)

edited May 11, 2021 at 17:17

answered Nov 18, 2012 at 18:12

Eric

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5 Comments

Gareth Latty Over a year ago

Given he doesn't know the number of lists, product(*lists) might be more apt.

Eric Over a year ago

@Lattyware: Was doing that while you commented

John Red Over a year ago

I am curious as to how product is implemented. The only way I could think of to solve the problem was to use recursion. Does product use recursion itself?

rdt0086 Over a year ago

This method only goes down one layer. Is there a way to go down an unknown level of embedded lists. So something like [[[[1,2,3],[4,5,6]],['Test','All']]]?

Eric Over a year ago

@rdt0086: Your comment isn't enough for me to understand your question - I recommend asking it as a new question rather than commenting on this answer, and showing precisely what behavior you want.

Shawn Chin · Accepted Answer · 2012-11-19 17:05:17Z

1

Using python 3.2

from itertools import product

[" ".join(i) for i in product(list1,list2,list3)]

edited Nov 19, 2012 at 17:05

Shawn Chin

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answered Nov 19, 2012 at 14:54

raton

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