---
title: Python Dictionaries and sets
description: In Python, a dictionary is an insertion-ordered (from Python > 3.7) collection of key, value pairs.
---

In Python, a dictionary is an _ordered_ (from Python > 3.7) collection of `key`: `value` pairs.

The main operations on a dictionary are storing a value with some key and extracting the value given the key. It is also possible to delete a key:value pair with <code>del</code>.

# Dictionaries

## Set key, value using subscript operator `[]`

>>> my_cat = {
...  'size': 'fat',
...  'color': 'gray',
...  'disposition': 'loud',
... }
>>> my_cat['age_years'] = 2
>>> print(my_cat)
...
# {'size': 'fat', 'color': 'gray', 'disposition': 'loud', 'age_years': 2}

## Get value using subscript operator `[]`

In case the key is not present in dictionary <a target="_blank" href="https://docs.python.org/3/library/exceptions.html#KeyError">`KeyError`</a> is raised.

>>> my_cat = {
...  'size': 'fat',
...  'color': 'gray',
...  'disposition': 'loud',
... }
>>> print(my_cat['size'])
...
# fat
>>> print(my_cat['eye_color'])
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# KeyError: 'eye_color'

## `values()`

The `values()` method gets the **values** of the dictionary:

>>> pet = {'color': 'red', 'age': 42}
>>> for value in pet.values():
...     print(value)
...
# red
# 42

## `keys()`

The `keys()` method gets the **keys** of the dictionary:

>>> pet = {'color': 'red', 'age': 42}
>>> for key in pet.keys():
...     print(key)
...
# color
# age

There is no need to use **.keys()** since by default you will loop through keys:

>>> pet = {'color': 'red', 'age': 42}
>>> for key in pet:
...     print(key)
...
# color
# age

## `items()`

The `items()` method gets the **items** of a dictionary and returns them as a <router-link to=/cheatsheet/lists-and-tuples#the-tuple-data-type>Tuple</router-link>:

>>> pet = {'color': 'red', 'age': 42}
>>> for item in pet.items():
...     print(item)
...
# ('color', 'red')
# ('age', 42)

Using the `keys()`, `values()`, and `items()` methods, a for loop can iterate over the keys, values, or key-value pairs in a dictionary, respectively.

>>> pet = {'color': 'red', 'age': 42}
>>> for key, value in pet.items():
...     print(f'Key: {key} Value: {value}')
...
# Key: color Value: red
# Key: age Value: 42

## `get()`

The `get()` method returns the value of an item with the given key. If the key doesn't exist, it returns `None`:

>>> wife = {'name': 'Rose', 'age': 33}

>>> f'My wife name is {wife.get("name")}'
# 'My wife name is Rose'

>>> f'She is {wife.get("age")} years old.'
# 'She is 33 years old.'

>>> f'She is deeply in love with {wife.get("husband")}'
# 'She is deeply in love with None'

You can also change the default `None` value to one of your choice:

>>> wife = {'name': 'Rose', 'age': 33}

>>> f'She is deeply in love with {wife.get("husband", "lover")}'
# 'She is deeply in love with lover'

## Adding items with `setdefault()`

It's possible to add an item to a dictionary in this way:

>>> wife = {'name': 'Rose', 'age': 33}
>>> if 'has_hair' not in wife:
...     wife['has_hair'] = True

Using the `setdefault` method, we can make the same code more short:

>>> wife = {'name': 'Rose', 'age': 33}
>>> wife.setdefault('has_hair', True)
>>> wife
# {'name': 'Rose', 'age': 33, 'has_hair': True}

## Removing Items

### `del`

The `del` method removes an item based on a given key.

>>> wife = {'name': 'Rose', 'age': 33, 'hair': 'brown'}
>>> del wife['age']
>>> wife
# {'name': 'Rose', 'hair': 'brown'}

## Checking keys in a Dictionary

>>> person = {'name': 'Rose', 'age': 33}

>>> 'name' in person.keys()
# True

>>> 'height' in person.keys()
# False

>>> 'skin' in person # You can omit keys()
# False

## Checking values in a Dictionary

>>>  person = {'name': 'Rose', 'age': 33}

>>> 'Rose' in person.values()
# True

>>> 33 in person.values()
# True

## Pretty Printing

>>> import pprint

>>> wife = {'name': 'Rose', 'age': 33, 'has_hair': True, 'hair_color': 'brown', 'height': 1.6, 'eye_color': 'brown'}
>>> pprint.pprint(wife)
# {'age': 33,
#  'eye_color': 'brown',
#  'hair_color': 'brown',
#  'has_hair': True,
#  'height': 1.6,
#  'name': 'Rose'}

## Dict comprehension

>>> c = {'name': 'Pooka', 'age': 5}
>>> {v: k for k, v in c.items()}
{'Pooka': 'name', 5: 'age'}

# Sets

A set is an unordered collection with no duplicate elements. Basic uses include membership testing and eliminating duplicate entries.

## Initializing a set

There are two ways to create sets: using curly braces `{}` and the built-in function `set()`

> When creating set, be sure to not use empty curly braces <code>{}</code> or you will get an empty dictionary instead.

>>> s = {1, 2, 3}
>>> s = set([1, 2, 3])

>>> s = {}  # this will create a dictionary instead of a set
>>> type(s)
# <class 'dict'>

## Unordered collections of unique elements

A set automatically remove all the duplicate values.

>>> s = {1, 2, 3, 2, 3, 4}
>>> s
# {1, 2, 3, 4}

And as an unordered data type, they can't be indexed.

>>> s = {1, 2, 3}
>>> s[0]
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# TypeError: 'set' object does not support indexing

## set `add()` and `update()`

Using the `add()` method we can add a single element to the set.

>>> s = {1, 2, 3}
>>> s.add(4)
>>> s
# {1, 2, 3, 4}

And with `update()`, multiple ones:

>>> s = {1, 2, 3}
>>> s.update([2, 3, 4, 5, 6])
>>> s
# {1, 2, 3, 4, 5, 6}

## set `remove()` and `discard()`

Both methods will remove an element from the set, but `remove()` will raise a `key error` if the value doesn't exist.

>>> s = {1, 2, 3}
>>> s.remove(3)
>>> s
# {1, 2}

>>> s.remove(3)
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# KeyError: 3

`discard()` won't raise any errors.

>>> s = {1, 2, 3}
>>> s.discard(3)
>>> s
# {1, 2}
>>> s.discard(3)

## set `union()`

`union()` or `|` will create a new set that with all the elements from the sets provided.

>>> s1 = {1, 2, 3}
>>> s2 = {3, 4, 5}
>>> s1.union(s2)  # or 's1 | s2'
# {1, 2, 3, 4, 5}

## set `intersection()`

`intersection` or `&` will return a set with only the elements that are common to all of them.

>>> s1 = {1, 2, 3}
>>> s2 = {2, 3, 4}
>>> s3 = {3, 4, 5}
>>> s1.intersection(s2, s3)  # or 's1 & s2 & s3'
# {3}

## set `difference()`

`difference` or `-` will return only the elements that are unique to the first set (invoked set).

>>> s1 = {1, 2, 3}
>>> s2 = {2, 3, 4}

>>> s1.difference(s2)  # or 's1 - s2'
# {1}

>>> s2.difference(s1) # or 's2 - s1'
# {4}

## set `symetric_difference()`

`symetric_difference` or `^` will return all the elements that are not common between them.

>>> s1 = {1, 2, 3}
>>> s2 = {2, 3, 4}
>>> s1.symmetric_difference(s2)  # or 's1 ^ s2'
# {1, 4}

## Set comprehension

>>> b = {"abc", "def"}
>>> {s.upper() for s in b}
{"ABC", "DEF"}