Python Dictionaries

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

Example Dictionary:

# Dictionary: collection of key-value pairs
my_cat = {
    'size': 'fat',          # key: 'size', value: 'fat'
    'color': 'gray',         # key: 'color', value: 'gray'
    'disposition': 'loud'    # key: 'disposition', value: 'loud'
}

Set key, value using subscript operator []

# Add or update dictionary entry using subscript operator
my_cat = {
 'size': 'fat',
 'color': 'gray',
 'disposition': 'loud',
}
my_cat['age_years'] = 2  # Add new key-value pair
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:

# Iterate over dictionary values using .values() method
pet = {'color': 'red', 'age': 42}
for value in pet.values():  # Loop through all values
    print(value)

red
42

keys()

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

# Iterate over dictionary keys using .keys() method
pet = {'color': 'red', 'age': 42}
for key in pet.keys():  # Loop through all keys
    print(key)

color
age

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

# Iterating over dictionary directly loops through keys (default behavior)
pet = {'color': 'red', 'age': 42}
for key in pet:  # Equivalent to for key in pet.keys()
    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.

# Iterate over key-value pairs using .items() method
pet = {'color': 'red', 'age': 42}
for key, value in pet.items():  # Unpack tuple into key and value
    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:

# .get() method: safely retrieve value, returns None if key doesn't exist
wife = {'name': 'Rose', 'age': 33}

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

'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_b, **dict_a}
dict_c

{'a': 1, 'b': 3, 'c': 4}

Relevant links

• Python Data Types Blog Post
• Python \*args and \*\*kwargs Made Easy
• Comprehensions
• Args and Kwargs
• Python Comprehensions Step-by-Step
• dict()
• len()
• iter()
• zip()