By Konstantinos Patronas —

Python: Dictionary Comprehension

Dictionary comprehension is a way of transforming ones dictionary key and values to another! it can be a powerful tool which makes your…

Python: Dictionary Comprehension
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Dictionary comprehension is a way of transforming ones dictionary key and values to another! it can be a powerful tool which makes your coder shorter and more readable! lets see how it works!

Learning the values(), keys() and items() methods

Lets create a simple dictionary at first! and explore some methods which are crucial to handle comprehensions

my_dict = {} 
my_dict['a'] = 1 
my_dict['b'] = 2 
my_dict['c'] = 3 
my_dict['d'] = 4

Printing this will print both key and values

print(my_dict) 
{'a': 1, 'b': 2, 'c': 3, 'd': 4}

To print only values we can use the .values() method of the dictionary

print(my_dict.values()) 
dict_values([1, 2, 3, 4])

Keep in mind that .value() returns an iterable list! we will discuss about this later!

To print only keys we can use the .keys() method of the dictionary, which also returns an iterable list

print(my_dict.keys()) 
dict_keys(['a', 'b', 'c', 'd'])

We can get all key and values in a list of tuples using the .items() method

print(my_dict.items()) 
dict_items([('a', 1), ('b', 2), ('c', 3), ('d', 4)])

Creating dictionary comprehensions

The basic template you need to know to create dictionary comprehensions is this

new_dict = {k:v for (k,v) in dictionary.items()}
  • The for statement iterates the pairs of the dictionary items and saves key and value of each per to k and v respectively
  • k:v are are stored as key and values to new_dict dictionary

Might seem like just copying one dictionary to another and yets is that exactly! but the magic starts here!

new_dict = {k:v*v for (k,v) in my_dict.items()} 
print(new_dict) 
{'a': 1, 'b': 4, 'c': 9, 'd': 16}

This way we are able to multiply each value to itself and store the result to the new dictionary! isn't that awesome? if we want to do the same with simple python we would end to something like this which is bigger and less clear that dictionary comprehension

new_dict = {} 
for (k,v) in my_dict.items(): 
    new_dict[k] = v * v

The usability of dictionary comprehensions become clearer when we need to add if statements inside the loop, assume that we want to create dictionaries where values are even numbers, it would be something like this!

new_dict = {} 
for (k,v) in my_dict.items(): 
    if v%2 == 0: 
        new_dict[k] = v * v

We can shorten this to one line with this dictionary comprehension, you can plase if statements to the right of the expressions in order to filter keys, values or both!

new_dict = {k:v*v for (k,v) in my_dict.items() if v%2 == 0} 
print(new_dict) 
{'b': 4, 'd': 16}

Conclussion

Dictionary comprehension is a powerful technique which easy to learn and will make your code shorted and more readable! I hope you enjoyed this article as much I enjoyed writing the article!

In Plain English

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