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

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 KeyError 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 (Optional)

>>> 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 {} 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()

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

Math operators (Optional)

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"}

Keywords

• key-value (鍵值對): A pair consisting of a key and its associated value, as used in dictionaries.
• key (鍵): An immutable identifier used to access a corresponding value in a dictionary.
• value (值): The data associated with a key in a dictionary.
• dictionary (字典): A collection of key-value pairs where each key is unique.
• set (集合): An unordered collection of unique elements.
• dictionary comprehension (字典生成式): A concise way to create dictionaries from iterables using a single expression.
• set comprehension (集合生成式): A concise way to create sets from iterables using a single expression.