List to Dictionary Conversions in Python

I recently published a couple articles on data type conversions. If you would like to read them, here they are:

– String to Number Conversions

– Converting String Representations of Binary, Octal and Hexadecimal Integers

– Number to String Conversions

– String to List Conversions

– Sequence Conversions

– Dictionary Conversions

Today we’ll learn how to convert lists to dictionaries. We’ll learn how to use the functions zip and dict.

Here’s the video version of this article:

Just as we can convert a dictionary to a list, we can convert a list to a dictionary as well. Suppose we have two lists: one with English words and the other with their German counterparts. We can make a dictionary out of them in a couple of steps:

>>> english_words = ["car", "hat", "house"]
>>> german_words = ["Auto", "Hut", "Haus"] 

First we need the method zip(list1, list2):

>>> english_german_iterator = zip(english_words, german_words)

The method returns a list iterator:

>>> english_german_iterator
<zip object at 0x0000023A5504D448> 

If you want to read more about iterators, feel free to read this article.

Then we have to use the list method to convert the list iterator into a list:

>>> english_german_translations = list(english_german_iterator)
>>> english_german_translations
[('car', 'Auto'), ('hat', 'Hut'), ('house', 'Haus')] 

What we get is a list of 2-tuples, where the first element in each tuple corresponds to a key in the dictionary we’re gonna convert the lists into and the second element will be the value. We can convert this list into a dictionary using the method dict(list):

>>> translations_dictionary = dict(english_german_translations)
>>> translations_dictionary
{'car': 'Auto', 'hat': 'Hut', 'house': 'Haus'} 

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

>>> dict(zip(english_words, german_words))
{'car': 'Auto', 'hat': 'Hut', 'house': 'Haus'} 

Lists it Doesn’t Work For

It doesn’t work for all lists. The keys in a dictionary must be an immutable data type. So, if we have a mutable data type (like lists) in the first list, they can’t be converted to keys in the dictionary. We get an error:

>>> odd_triplets = [[1,3,5], [7, 9, 11]]
>>> middles = [3, 9]
>>> dict(zip(odd_triplets, middles))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list' 

Lists with Different Numbers of Elements

And what happens if the numbers of elements in the two lists are different? The superfluous elements will be lost:

>>> positions = [1, 2, 3, 4, 5]
>>> rewards = ["gold medal", "silver medal", "bronze medal"] 

The two superfluous positions 4 and 5 will be lost:

>>> dict(zip(positions, rewards))
{1: 'gold medal', 2: 'silver medal', 3: 'bronze medal'} 

Iterators Get Used Up

There is one thing we have to know about the method zip. 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:

>>> seas = ["Baltic Sea", "Sea of Japan", "Arabian Sea"]
>>> oceans = ["Atlantic Ocean", "Pacific Ocean", "Indian Ocean"] 

Let’s zip them:

>>> seas_iterator = zip(seas, oceans)

Let’s use the iterator to make a list:

>>> sea_list1 = list(seas_iterator)

Let’s make another list using the same iterator:

>>> sea_list2 = list(seas_iterator)

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

>>> sea_list1
[('Baltic Sea', 'Atlantic Ocean'), ('Sea of Japan', 'Pacific Ocean'), ('Arabian Sea', 'Indian Ocean')] 

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

>>> sea_list2
[]