Itertools.chain.from_iterable

Question

Can anyone explain to me, what exactly this code snippet is doing?

chained_country_list = set(itertools.chain.from_iterable(country_and_countrycodes)) & set(         


        

Answer 1

Let's break down each significant element of the code:

itertools.chain.from_iterable:

Basically, this is used to flatten a nested list, like this:

l = [[0], [1, 2], [2], [3, 6], [4], [5, 10]]
list(itertools.chain.from_iterable(l))    

Output:

[0, 1, 2, 2, 3, 6, 4, 5, 10]

& operator between two sets:

Consider the follow example of sets a and b.

a = {1, 2, 3}
b = {2, 3, 4}
a & b

Output:

{2, 3}

So basically it gets the common elements between two sets. Here they're 2 and 3.

The code as a whole:

Let's say:

country_and_countrycodes = [('United States', 'US'), ('China', 'CH')]
all_countries = ['United States', 'Mongolia', 'Togo']

Now, the first part is:

set(itertools.chain.from_iterable(country_and_countrycodes))

which gives us:

{'CH', 'China', 'US', 'United States'}

So, it just gets us a flat set from the tuples.

Then, the second part is:

set(itertools.chain.from_iterable(country_and_countrycodes)) & set(all_countries)

which gives us:

{'United States'}

Basically, what we did was:

{'CH', 'China', 'US', 'United States'} & {'United States', 'Mongolia', 'Togo'}

Since the only common element here is 'United States', that's what we got as the output.