How to Use the zip Function in Python

A commonly used function in Python is the zip function. You can use it to turn two lists of equal length into a dictionary. Naturally, it only makes sense if the elements of one list can be turned to keys and the elements of the other list to values of a dictionary.

So, what we want to achieve is turn the following two lists:

countries = ["Germany", "Japan", "Australia"]
capitals = ["Berlin", "Tokyo", "Canberra"]

into the following dictionary:

{'Germany': 'Berlin', 'Japan': 'Tokyo', 'Australia': 'Canberra'}

So,  you have two lists: one with countries and the other with their capitals. They’re good candidates to be turned into a dictionary.

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for Loop

Actually, there are several ways you can do it. You can achieve that without the zip function, for example by using the for loop like so:

countries = ["Germany", "Japan", "Australia"]
capitals = ["Berlin", "Tokyo", "Canberra"]

a = {}

for n in range(len(countries)):
    a[countries[n]] = capitals[n]

print(a)

The output is:

{'Germany': 'Berlin', 'Japan': 'Tokyo', 'Australia': 'Canberra'}

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zip Function with Iterator

Another approach is to use the zip function and an iterator: Here is the function with two arguments, which are the two lists:

it = zip(countries, capitals) # it - an iterator

The function returns a list iterator. Then we have to use the list function to convert the list iterator into a list:

pairs = list(it)

If we now print pairs in interactive mode, we’ll get a list of 2-tuples:

>>> pairs
[('Germany', 'Berlin'), ('Japan', 'Tokyo'), ('Australia', 'Canberra')]

We can convert this list into a dictionary using the dict(list) function:

print(country_capital_dict)

Here’s the output:

{'Germany': 'Berlin', 'Japan': 'Tokyo', 'Australia': 'Canberra'}

Here is the full code:

countries = ["Germany", "Japan", "Australia"]
capitals = ["Berlin", "Tokyo", "Canberra"]

it = zip(countries, capitals) # it - an iterator
pairs = list(it)  # list of 2-tuples
country_capital_dict = dict(pairs)  # dictionary

print(country_capital_dict)

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zip Function Without Iterator

We can achieve the same in a simpler way. We can pass zip as the argument of the dict function:

countries = ["Germany", "Japan", "Australia"]
capitals = ["Berlin", "Tokyo", "Canberra"]

country_capital_dict = dict(zip(countries, capitals))  # dictionary

print(country_capital_dict)

There is one thing we have to know about the zip function. It returns an iterator, not a list. In iterators the elements get used up, so that they can’t be used again. Here we have two lists again:

countries = ["Germany", "Japan", "Australia"]
capitals = ["Berlin", "Tokyo", "Canberra"] 

Let’s zip them:

>>> it = zip(countries, capitals)

Now we can use the iterator to make a list:

>>> list1 = list(it)

And now let’s make another list using the same iterator:

>>> list2 = list(it)

The iterator gets used up when it is used in the first list, so the elements go to the first list:

>>> list1
[('Germany', 'Berlin'), ('Japan', 'Tokyo'), ('Australia', 'Canberra')]

and there is nothing left for the second list. The second list is empty:

>>> list2
[]

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