product function is one of several handy combinatoric iterators included in the
itertools module. Combinatoric iterators are related to an area of mathematics called enumerative combinatorics, which is concerned with the number of ways a given pattern can be formed.
product gives us a way to quickly calculate the cartesian product of two or more collections. An example of the cartesian product can be found below:
product also frequently serves as an alternative approach to having multiple
for clauses inside list comprehensions.
In a previous post, we provided code for a list comprehension that would calculate all the possible roll combinations for two six-sided dice.
roll_combinations = [(d1, d2) for d1 in range(1, 7) for d2 in range(1, 7)]
We can do very much the same thing using the
from itertools import product dice_combinations = product(range(1, 7), repeat=2)
So what's going on here?
product function accepts any number of iterables as positional arguments, and has an optional keyword only parameter called
When we provide two or more iterables as arguments, the
product function will find all the ways we can match an element from one of these iterables to an item in every other iterable. For example, we might have a pair of lists like so:
list_1 = ["a", "b", "c"] list_2 = [1, 2, 3]
When we pass these lists to the product function, we get the following:
cartesian_product = product(list_1, list_2) # ('a', 1) ('a', 2) ('a', 3) ('b', 1) ('b', 2) ('b', 3) ('c', 1) ('c', 2) ('c', 3)
If we were to add a third iterable, every one of these tuples would be matched up to an item in this third iterable. For example, if we had a third list containing
"z", we would get output like this:
# ('a', 1, 'x') ('a', 1, 'y') ('a', 1, 'z') ('a', 2, 'x') ... etc.
repeat parameter is most useful for when we want to use the same iterable multiple times. We can see an example of this in our code for finding roll combinations. We can easily add more and more dice by increasing the value of
If we set a
repeat value of
2 or more when we have multiple iterables,
product will duplicate all of the iterables for the purposes of finding the cartesian product. The following functions are identical in terms of functionality:
c_product_1 = product(["a", "b", "c"], [1, 2, 3], repeat=2) c_product_2 = product(["a", "b", "c"], [1, 2, 3], ["a", "b", "c"], [1, 2, 3])
That just about wraps up our introduction to the
product function. I hope you learnt something new, and I encourage you to play around with the things we've covered here to really understand how it all works.
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itertools functions, so make sure to check back next week! Alternatively, you can give our Complete Python Course a try, where we go into depth on a number of these topics.
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