Python Dictionary

A Python dictionary is a collection of items, similar to lists and tuples. However, unlike lists and tuples, each item in a dictionary is a key-value pair (consisting of a key and a value).

Create a Dictionary

We create a dictionary by placing key: value pairs inside curly brackets {}, separated by commas. For example,

# creating a dictionary
country_capitals = {
  "Germany": "Berlin", 
  "Canada": "Ottawa", 
  "England": "London"
}

# printing the dictionary
print(country_capitals)

Output

{'Germany': 'Berlin', 'Canada': 'Ottawa', 'England': 'London'}

The country_capitals dictionary has three elements (key-value pairs), where 'Germany' is the key and 'Berlin' is the value assigned to it and so on.

Key Value Pairs in a Dictionary — Python Dictionary

Notes:

Dictionary keys must be immutable, such as tuples, strings, integers, etc. We cannot use mutable (changeable) objects such as lists as keys.

We can also create a dictionary using a Python built-in function dict(). To learn more, visit Python dict().

Valid and Invalid Dictionaries

Keys of a dictionary must be immutable

Immutable objects can't be changed once created. Some immutable objects in Python are integer, tuple and string.

# valid dictionary
# integer as a key
my_dict = {1: "one", 2: "two", 3: "three"}

# valid dictionary
# tuple as a key
my_dict = {(1, 2): "one two", 3: "three"}

# invalid dictionary
# Error: using a list as a key is not allowed
my_dict = {1: "Hello", [1, 2]: "Hello Hi"}

# valid dictionary
# string as a key, list as a value
my_dict = {"USA": ["Chicago", "California", "New York"]}

In this example, we have used integers, tuples, and strings as keys for the dictionaries. When we used a list as a key, an error message occurred due to the list's mutable nature.

Note: Dictionary values can be of any data type, including mutable types like lists.

Keys of a dictionary must be unique

The keys of a dictionary must be unique. If there are duplicate keys, the later value of the key overwrites the previous value.

hogwarts_houses = {
    "Harry Potter": "Gryffindor",
    "Hermione Granger": "Gryffindor",
    "Ron Weasley": "Gryffindor",
    # duplicate key with a different house
    "Harry Potter": "Slytherin"
}

print(hogwarts_houses)

Output

{'Harry Potter': 'Slytherin', 'Hermione Granger': 'Gryffindor', 'Ron Weasley': 'Gryffindor'}

Here, the key Harry Potter is first assigned to Gryffindor. However, there is a second entry where Harry Potter is assigned to Slytherin.

As duplicate keys are not allowed in a dictionary, the last entry Slytherin overwrites the previous value Gryffindor.

Access Dictionary Items

We can access the value of a dictionary item by placing the key inside square brackets.

country_capitals = {
  "Germany": "Berlin", 
  "Canada": "Ottawa", 
  "England": "London"
}

# access the value of keys
print(country_capitals["Germany"])    # Output: Berlin
print(country_capitals["England"])    # Output: London

Note: We can also use the get() method to access dictionary items.

Add Items to a Dictionary

We can add an item to a dictionary by assigning a value to a new key. For example,

country_capitals = {
  "Germany": "Berlin", 
  "Canada": "Ottawa", 
}

# add an item with "Italy" as key and "Rome" as its value
country_capitals["Italy"] = "Rome"

print(country_capitals)

Output

{'Germany': 'Berlin', 'Canada': 'Ottawa', 'Italy': 'Rome'}

Remove Dictionary Items

We can use the del statement to remove an element from a dictionary. For example,

country_capitals = {
  "Germany": "Berlin", 
  "Canada": "Ottawa", 
}

# delete item having "Germany" key
del country_capitals["Germany"]

print(country_capitals)

Output

{'Canada': 'Ottawa'}

Note: We can also use the pop() method to remove an item from a dictionary.

If we need to remove all items from a dictionary at once, we can use the clear() method.

country_capitals = {
  "Germany": "Berlin", 
  "Canada": "Ottawa", 
}

# clear the dictionary
country_capitals.clear()

print(country_capitals)

Output

{}

Change Dictionary Items

Python dictionaries are mutable (changeable). We can change the value of a dictionary element by referring to its key. For example,

country_capitals = {
  "Germany": "Berlin", 
  "Italy": "Naples", 
  "England": "London"
}

# change the value of "Italy" key to "Rome"
country_capitals["Italy"] = "Rome"

print(country_capitals)

Output

{'Germany': 'Berlin', 'Italy': 'Rome', 'England': 'London'}

Note: We can also use the update() method to add or change dictionary items.

Iterate Through a Dictionary

A dictionary is an ordered collection of items (starting from Python 3.7), therefore it maintains the order of its items.

We can iterate through dictionary keys one by one using a for loop.

country_capitals = {
  "United States": "Washington D.C.", 
  "Italy": "Rome" 
}

# print dictionary keys one by one
for country in country_capitals:
    print(country)

print()

# print dictionary values one by one
for country in country_capitals:
    capital = country_capitals[country]
    print(capital)

Output

United States
Italy

Washington D.C.
Rome

Find Dictionary Length

We can find the length of a dictionary by using the len() function.

country_capitals = {"England": "London", "Italy": "Rome"}

# get dictionary's length
print(len(country_capitals))   # Output: 2

numbers = {10: "ten", 20: "twenty", 30: "thirty"}

# get dictionary's length
print(len(numbers))            # Output: 3

countries = {}

# get dictionary's length
print(len(countries))          # Output: 0