Python Dictionary : How to store Ordered collection of Items

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Python dictionary is an ordered collection (starting from Python 3.7) of items. It stores elements in key/value pairs. Here, keys are unique identifiers that are associated with each value.

Let’s see an example,

If we want to store information about countries and their capitals, we can create a dictionary with country names as keys and capitals as values.

Keys	Values
Nepal	Kathmandu
Italy	Rome
England	London

Create a dictionary in Python

Here’s how we can create a dictionary in Swift.

capital_city = {"Nepal": "Kathmandu", "Italy": "Rome", "England": "London"}
print(capital_city)

Output

{'Nepal': 'Kathmandu', 'Italy': 'Rome', 'England': 'London'}

In the above example, we have created a dictionary named capital_city. Here,

1. Keys are "Nepal", "Italy", "England"
2. Values are "Kathmandu", "Rome", "London"

Note: Here, keys and values both are of string type. We can also have keys and values of different data types.

Example 1: Python Dictionary

# dictionary with keys and values of different data types
numbers = {1: "One", 2: "Two", 3: "Three"}
print(numbers)

Output

[3: "Three", 1: "One", 2: "Two"]

In the above example, we have created a dictionary named numbers. Here, keys are of integer type and values are of string type.

Add Elements to a Python Dictionary

We can add elements to a dictionary using the name of the dictionary with []. For example,

capital_city = {"Nepal": "Kathmandu", "England": "London"}
print("Initial Dictionary: ",capital_city)

capital_city["Japan"] = "Tokyo"

print("Updated Dictionary: ",capital_city)

Output

Initial Dictionary:  {'Nepal': 'Kathmandu', 'England': 'London'}
Updated Dictionary:  {'Nepal': 'Kathmandu', 'England': 'London', 'Japan': 'Tokyo'}

In the above example, we have created a dictionary named capital_city. Notice the line,

capital_city["Japan"] = "Tokyo"

Here, we have added a new element to capital_city with key: Japan and value: Tokyo.

Change Value of Dictionary

We can also use [] to change the value associated with a particular key. For example,

student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}
print("Initial Dictionary: ", student_id)

student_id[112] = "Stan"

print("Updated Dictionary: ", student_id)

Output

Initial Dictionary:  {111: 'Eric', 112: 'Kyle', 113: 'Butters'}
Updated Dictionary:  {111: 'Eric', 112: 'Stan', 113: 'Butters'}

In the above example, we have created a dictionary named student_id. Initially, the value associated with the key 112 is "Kyle". Now, notice the line,

student_id[112] = "Stan"

Here, we have changed the value associated with the key 112 to "Stan".

Accessing Elements from Dictionary

In Python, we use the keys to access their corresponding values. For example,

student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}

print(student_id[111])  # prints Eric
print(student_id[113])  # prints Butters

Here, we have used the keys to access their corresponding values.

If we try to access the value of a key that doesn’t exist, we’ll get an error. For example,

student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}
print(student_id[211])  

# Output: KeyError: 211

Removing elements from Dictionary

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

student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}

print("Initial Dictionary: ", student_id)

del student_id[111]

print("Updated Dictionary ", student_id)

Output

Initial Dictionary:  {111: 'Eric', 112: 'Kyle', 113: 'Butters'}
Updated Dictionary  {112: 'Kyle', 113: 'Butters'}

Here, we have created a dictionary named student_id. Notice the code,

del student_id[111]

The del statement removes the element associated with the key 111.

We can also delete the whole dictionary using the del statement,

student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}

# delete student_id dictionary
del student_id

print(student_id)

# Output: NameError: name 'student_id' is not defined

We are getting an error message because we have deleted the student_id dictionary and student_id doesn’t exist anymore.

Python Dictionary Methods

Methods that are available with a dictionary are tabulated below. Some of them have already been used in the above examples.

Function	Description
all()	Return True if all keys of the dictionary are True (or if the dictionary is empty).
any()	Return True if any key of the dictionary is true. If the dictionary is empty, return False.
len()	Return the length (the number of items) in the dictionary.
sorted()	Return a new sorted list of keys in the dictionary.
clear()	Removes all items from the dictionary.
keys()	Returns a new object of the dictionary’s keys.
values()	Returns a new object of the dictionary’s values

Dictionary Membership Test

We can test if a key is in a dictionary or not using the keyword in. Notice that the membership test is only for the keys and not for the values.

# Membership Test for Dictionary Keys
squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}

# Output: True
print(1 in squares) # prints True

print(2 not in squares) # prints True

# membership tests for key only not value
print(49 in squares) # prints false

Output

True
True
False

Iterating Through a Dictionary

We can iterate through each key in a dictionary using a for loop.

# Iterating through a Dictionary
squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}
for i in squares:
    print(squares[i])

Output

1
9
25
49
81

Here, we have iterated through each key in the squares dictionary using the for loop.