Python language basics 68: iterating over a dictionary with dictionary comprehension in Python

Introduction

In the previous post we looked at a Python language construct called comprehension. We saw that comprehensions are a very concise way of iterating over a collection, applying a function to each element and adding the resulting element to a new collection. All that is declared in a single comprehension statement.

In this post we’ll look at dictionary comprehensions.

Dictionary comprehension

Consider the following dictionary:

sizes = {"XS": "Extra-Small", "S": "Small", "M": "Medium", "L": "Large", "XL": "Extra-Large"}

Here’s how each key and value can be converted into lower case characters:

lower_case_sizes = {size_key.lower():size_name.lower() for size_key, size_name in sizes.items()}
print(lower_case_sizes)

{‘l’: ‘large’, ‘xl’: ‘extra-large’, ‘m’: ‘medium’, ‘xs’: ‘extra-small’, ‘s’: ‘small’}

The syntax is somewhat odd but here are the elements:

• The function you’d like to apply on the dictionary key of the new dictionary
• A colon
• The function you’d like to apply on the dictionary value of the new dictionary
• The standard for-each loop for dictionaries

In the above example we use the items() function to get both the key and the value of each item in the iteration. Items() returns a tuple with 2 elements: key and value, and they are saved in the “size_key” and “size_value”, that is we unpack the tuple.

The function applied on the key and value can of course be independent. Here we convert to values into their reversed counterparts:

lower_case_sizes = {size_key.lower():''.join(reversed(size_name)) for size_key, size_name in sizes.items()}

{‘l’: ‘egraL’, ‘xs’: ‘llamS-artxE’, ‘s’: ‘llamS’, ‘xl’: ‘egraL-artxE’, ‘m’: ‘muideM’}

We can also easily build the opposite dictionary where the value becomes the key and vice versa:

lsizes_flipped = {size_name:size_key for size_key, size_name in sizes.items()}

{‘Extra-Large’: ‘XL’, ‘Large’: ‘L’, ‘Extra-Small’: ‘XS’, ‘Small’: ‘S’, ‘Medium’: ‘M’}

In the next post we’ll look at filtering in comprehensions.

Read all Python-related posts on this blog here.