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Python Dictionaries

Published on 8/11/2024

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A dictionary in Python is an unordered collection of items with each item consisting of a key and a value, typically referred to as key:value pairs. In a dictionary, a key is unique and immutable, and the value is any data type and doesn’t need to be unique.

Basic dictionary syntax:

python
dict_example = {key1: value1, key2: value2}
student = { "name": "Andrew", "age": 22, "major": "Computer Science"}

Creating Dictionaries

Dictionaries can be created in multiple ways in Python.

Using curly braces {} to create a dictionary:

python
student = { "name": "Andrew", "age": 22, "major": "Computer Science"}

Using the dict() constructor to create a dictionary:

python
student = dict(name="Andrew", age=22, major="Computer Science")

Creating an empty dictionary:

python
empty_dict = {}

Creating a dictionary from a list of key-value tuples using the dict() constructor:

python
pairs = [("name", "Andrew"), ("age", 22), ("major", "Computer Science")]
my_dict = dict(pairs)

Accessing Dictionary Elements

Dictionary elements can be accessed by their keys, or with the get() method to avoid raising a KeyError if the key doesn’t exist.

Accessing values of a dictionary by key:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}

print(student["name"])  # Output: Andrew
print(student["age"])   # Output: 22

Accessing values of a dictionary with the get() method:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
print(student.get("major")) # Output: Computer Science

Modifying Dictionaries

Similar to lists, dictionaries are mutable, meaning the elements can be added, updated, or removed after its been created.

Adding a key-value pair to a dictionary:

python
student = {"name": "Andrew", "age": 22}
student["major"] = "Computer Science"
print(student) # Output: student = {"name": "Andrew", "age": 22, "major": "Computer Science"}

Updating an existing key of a dictionary

python
student = {"name": "Andrew", "age": 22}
student["age"] = 30
print(student) # Output: student = {"name": "Andrew", "age": 30}

Remove and return the value of a specified key with the pop() method:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
age = student.pop("age")
print(age)     # Output: 22
print(student) # Output: {"name": "Andrew", "major": "Computer Science"}

Remove and return the last inserted key-value pair as a tuple with the popitem() method:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
last_item = student.popitem()
print(last_item) # Output: ('major', 'Computer Science')

Delete a key-value pair with a specified key using the del statement:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
del student["age"]
print(student) # Output: student = {"name": "Andrew", "major": "Computer Science"}

Remove all elements from a dictionary with the clear() method:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
student.clear()
print(student) # Output: {}

Dictionary Operations

In Python there are several operations that allow you to analyze and manipulate data within dictionaries.

Checking for the existence of a key in a dictionary using the in keyword:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
print("name" in student) # Output: True
print("gpa" in student)  # Output: False

Find the number of key-value pairs in a dictionary using the len() function:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
print(len(student)) # Output: 3

Return the keys, values, and items of a dictionary with the key() , values(), and items() methods:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
print(student.keys())   # Output: dict_keys(['name', 'age', 'major'])
print(student.values()) # Output: dict_values(['Andrew', 22, 'Computer Science'])
print(student.items())  # Output: dict_items([('name', 'Andrew'), ('age', 22), ('major', 'Computer Science')])

The items() method returns a view object containing the dictionaries key-value pairs as tuples.

Looping Through Dictionaries:

Loops can be used to loop through a dictionaries keys, values, or key-value pairs.

Looping through the keys of a dictionary:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
for key in student:
	print(key, student[key])
	
# Output:
# name Andrew
# age 22
# major Computer Science

Looping through the values of a dictionary:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
for value in student.values():
	print(value)

# Output: 
# Andrew
# 22
# Computer Science

Looping through the key-value pairs of a dictionary:

python
student = {"name": "Andrew", "age": 22, "major": "Computer Science"}
for key, value in student.items():
	print(key, value)

# Output: 
# name Andrew
# age 22
# major Computer Science

Nested Dictionaries

Dictionaries can contain other dictionaries, lists, or any other data type as values. This allows you to create complex, nested data structures.

python
students = {
	"Andrew": {"age": 22, "major": "Computer Science"},
	"Jackie": {"age": 21, "major": "Supply Chain"}
}
print(students["Jackie"]["major"] # Output: Supply Chain

Advanced Dictionary Techniques

Python has several techniques for working with dictionaries.

To merge two dictionaries, use the update() method:

python
dict1 = {"x": 1, "y": 2}
dict2 = {"a": 3, "b": 4 }
dict1.update(dict2)
print(dict1) # Output: {'x': 1, 'y': 2, 'a': 3, 'b': 4}

To sort a dictionary by its keys or values, use the sorted() method, which returns a sorted list of tuples and not a dictionary.

Sorting a dictionary by keys:

python
my_dict = {"b": 2, "a": 1, "c": 3}
sorted_by_keys = dict(sorted(my_dict.items()))
print(sorted_by_keys)  # Output: {'a': 1, 'b': 2, 'c': 3}

Sorting a dictionary by values:

python
my_dict = {"b": 2, "a": 1, "c": 3}
sorted_by_values = dict(sorted(my_dict.items(), key=lambda item: item[1]))
print(sorted_by_values)  # Output: {'a': 1, 'b': 2, 'c': 3}

When To Use Dictionaries Vs. Other Data Structures

It is important to understand when to use a dictionary instead of another data structure like lists and tuples. Here are the key differences between dictionaries, lists, and tuples:

Dictionaries: Use when you need to associate keys with values, and especially if you need fast lookups
Lists: Use when you need an ordered collection of items that can you modify and where the order of the elements is important
Tuples: Use when you need an unordered collection of items that should not change
Sets: Use when you need an unordered collection of unique items.

Use the following links to learn more about each data structure in detail: Lists, Tuples, Sets.

Conclusion

Dictionaries are an incredibly powerful and flexible data structure in Python, and are ideal for mapping keys to values and organizing data. Use your new knowledge of dictionaries in Python to further build on your skills as a programmer.