Python Dictionaries

A dictionary in python is an associative array. This means you can use it to hold an array of things, and associate names, or keys with those things so they can be retrieved. Dictionaries use curly brackets in their declaration:

my_dict = {'pi': 3.14, 'e': 2.71, 'gravityAccel': 9.8}

to access a given element, you use square brackets on the key:


You can process a dictionary using a for loop and the “in” keyword:

for key in my_dict:
  print(key, "->", my_dict[key])

You can also loop through the keys and values together, using the items() method on your dictionary:

for key, value in my_dict.items():
  print(key, "->", value)

The elements of a dictionary can also be accessed by methods:

dictionary_values = my_dict.values()
dictionary_keys = my_dict.keys()

Quickly examine just the first 5 items in your dictionary, take a slice of a list:


Comprehensions are often used for processing dictionaries. For instance, reversing the keys and values:

my_reverse_dict = {v: k for k, v in my_dict.items()}

That’s a lot – creating a new dictionary, looping through the old dictionary, and reversing the keys and values all in one line.

And this can be combined with conditional statements such as “if”:

my_filtered_reverse_dict = {v: k for k, v in my_dict.items() if v > 3}

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