Python Dictionaries

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

From the Python 3 documentation 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 del.

Example Dictionary:

my_cat = {
    'size': 'fat',
    'color': 'gray',
    'disposition': 'loud'
}

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 Tuple:

>>> 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

pop()

The pop() method removes and returns an item based on a given key.

>>> wife = {'name': 'Rose', 'age': 33, 'hair': 'brown'}
>>> wife.pop('age')
# 33
>>> wife
# {'name': 'Rose', 'hair': 'brown'}

popitem()

The popitem() method removes the last item in a dictionary and returns it.

>>> wife = {'name': 'Rose', 'age': 33, 'hair': 'brown'}
>>> wife.popitem()
# ('hair', 'brown')
>>> wife
# {'name': 'Rose', 'age': 33}

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'}

clear()

Theclear() method removes all the items in a dictionary.

>>> wife = {'name': 'Rose', 'age': 33, 'hair': 'brown'}
>>> wife.clear()
>>> wife
# {}

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'}

Merge two dictionaries

For Python 3.5+:

>>> dict_a = {'a': 1, 'b': 2}
>>> dict_b = {'b': 3, 'c': 4}
>>> dict_c = {**dict_a, **dict_b}
>>> dict_c
# {'a': 1, 'b': 3, 'c': 4}