Python - Dictionaries

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Dictionaries in Python

In Python, a dictionary is a built-in data type that stores data in key-value pairs. It is an insertion-ordered and mutable collection. Each key must be unique and hashable, and values can be accessed using their corresponding keys. Dictionaries are often used to store data that is related, such as information associated with a specific entity or object, where you can quickly retrieve a value based on its key.

Python's dictionary is an example of a mapping type. A mapping object 'maps' the value of one object to another. To establish mapping between a key and a value, the colon (:) symbol is put between the two.

Each key-value pair is separated by a comma and enclosed within curly braces {}. The key and value within each pair are separated by a colon (:), forming the structure key:value.

Given below are some examples of Python dictionary objects −

capitals = {"Maharashtra":"Mumbai", "Gujarat":"Gandhinagar", "Telangana":"Hyderabad", "Karnataka":"Bengaluru"}
numbers = {10:"Ten", 20:"Twenty", 30:"Thirty",40:"Forty"}
marks = {"Savita":67, "Imtiaz":88, "Laxman":91, "David":49}

Key Features of Dictionaries

Following are the key features of dictionaries −

• Insertion-ordered − Dictionaries maintain the order in which items are inserted. This is guaranteed as a language feature starting from Python 3.7; earlier versions did not preserve insertion order. Note that items cannot be accessed by their position, only by their keys.

• Mutable − You can change, add, or remove items after the dictionary has been created.

• Key-based access − Dictionaries do not have numeric indexes. Instead, keys are used to access the associated values.

• Unique Keys − Each key in a dictionary must be unique. If you try to assign a value to an existing key, the old value will be replaced by the new value.

• Heterogeneous − Values in a dictionary can be of any data type. Keys can also be of different types, as long as each key is hashable.

Example 1

Only hashable objects can be used as keys, such as a number, string or tuple (containing only hashable elements). You can use an object of any type as the value. Hence following definitions of dictionary are also valid −

d1 = {"Fruit":["Mango","Banana"], "Flower":["Rose", "Lotus"]}
d2 = {('India, USA'):'Countries', ('New Delhi', 'New York'):'Capitals'}
print (d1)
print (d2)

It will produce the following output −

{'Fruit': ['Mango', 'Banana'], 'Flower': ['Rose', 'Lotus']}
{'India, USA': 'Countries', ('New Delhi', 'New York'): 'Capitals'}

Example 2

Python doesn't accept unhashable objects such as list as key, and raises TypeError.

d1 = {["Mango","Banana"]:"Fruit", "Flower":["Rose", "Lotus"]}
print (d1)

It will raise a TypeError −

Traceback (most recent call last):
   File "C:\Users\Sairam\PycharmProjects\pythonProject\main.py", line 8, in <module>
d1 = {["Mango","Banana"]:"Fruit", "Flower":["Rose", "Lotus"]}
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: unhashable type: 'list'

Example 3

You can assign a value to more than one keys in a dictionary, but a key cannot appear more than once in a dictionary.

d1 = {"Banana":"Fruit", "Rose":"Flower", "Lotus":"Flower", "Mango":"Fruit"}
d2 = {"Fruit":"Banana","Flower":"Rose", "Fruit":"Mango", "Flower":"Lotus"}
print (d1)
print (d2)

It will produce the following output −

{'Banana': 'Fruit', 'Rose': 'Flower', 'Lotus': 'Flower', 'Mango': 'Fruit'}
{'Fruit': 'Mango', 'Flower': 'Lotus'}

Creating a Dictionary

You can create a dictionary in Python by placing a comma-separated sequence of key-value pairs within curly braces {}, with a colon : separating each key and its associated value. Alternatively, you can use the dict() function.

Example

The following example demonstrates how to create dictionaries using both curly braces and the dict() function −

# Creating a dictionary using curly braces
sports_player = {
   "Name": "Sachin Tendulkar",
   "Age": 48,
   "Sport": "Cricket"
}
print ("Dictionary using curly braces:", sports_player)
# Creating a dictionary using the dict() function
student_info = dict(name="Alice", age=21, major="Computer Science")
print("Dictionary using dict():",student_info)  

The result produced is as shown below −

Dictionary using curly braces: {'Name': 'Sachin Tendulkar', 'Age': 48, 'Sport': 'Cricket'}
Dictionary using dict(): {'name': 'Alice', 'age': 21, 'major': 'Computer Science'}

Accessing Dictionary Items

You can access the value associated with a specific key using square brackets [] or the get() method −

student_info = {
   "name": "Alice",
   "age": 21,
   "major": "Computer Science"
}
# Accessing values using square brackets
name = student_info["name"]
print("Name:",name)  

# Accessing values using the get() method
age = student_info.get("age")
print("Age:",age)  

The result obtained is as follows −

Name: Alice
Age: 21

Modifying Dictionary Items

You can modify the value associated with a specific key or add a new key-value pair −

student_info = {
   "name": "Alice",
   "age": 21,
   "major": "Computer Science"
}
# Modifying an existing key-value pair
student_info["age"] = 22

# Adding a new key-value pair
student_info["graduation_year"] = 2023
print("The modified dictionary is:",student_info)

Output of the above code is as follows −

The modified dictionary is: {'name': 'Alice', 'age': 22, 'major': 'Computer Science', 'graduation_year': 2023}

Removing Dictionary Items

You can remove items using the del statement, the pop() method, or the popitem() method −

student_info = {
   "name": "Alice",
   "age": 22,
   "major": "Computer Science",
   "graduation_year": 2023
}
# Removing an item using the del statement
del student_info["major"]

# Removing an item using the pop() method
graduation_year = student_info.pop("graduation_year")

print(student_info) 

Following is the output of the above code −

{'name': 'Alice', 'age': 22}

Iterating Through a Dictionary

You can iterate through the keys, values, or key-value pairs in a dictionary using loops −

student_info = {
   "name": "Alice",
   "age": 22,
   "major": "Computer Science",
   "graduation_year": 2023
}
# Iterating through keys
for key in student_info:
   print("Keys:",key, student_info[key])

# Iterating through values
for value in student_info.values():
   print("Values:",value)

# Iterating through key-value pairs
for key, value in student_info.items():
   print("Key:Value:",key, value) 

After executing the above code, we get the following output −

Keys: name Alice
Keys: age 22
Keys: major Computer Science
Keys: graduation_year 2023
Values: Alice
Values: 22
Values: Computer Science
Values: 2023
Key:Value: name Alice
Key:Value: age 22
Key:Value: major Computer Science
Key:Value: graduation_year 2023

Properties of Dictionary Keys

Dictionary values have no restrictions. They can be any arbitrary Python object, either standard objects or user-defined objects. However, same is not true for the keys.

There are two important points to remember about dictionary keys −

• More than one entry per key not allowed. Which means no duplicate key is allowed. When duplicate keys encountered during assignment, the last assignment wins. For example −

dict = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'}
print ("dict['Name']: ", dict['Name'])

When the above code is executed, it produces the following result −

dict['Name']:  Manni

Keys must be hashable. Which means you can use strings, numbers or tuples (containing only hashable elements) as dictionary keys but something like ['key'] is not allowed. Following is a simple example −

dict = {['Name']: 'Zara', 'Age': 7}
print ("dict['Name']: ", dict['Name'])

When the above code is executed, it produces the following result −

Traceback (most recent call last):
   File "test.py", line 3, in <module>
      dict = {['Name']: 'Zara', 'Age': 7};
TypeError: unhashable type: 'list'

Python Dictionary Operators

In Python, following operators are defined to be used with dictionary operands. In the example, the following dictionary objects are used.

d1 = {'a': 2, 'b': 4, 'c': 30}
d2 = {'a1': 20, 'b1': 40, 'c1': 60}

Operator	Description	Example
dict[key]	Extract/assign the value mapped with key	print (d1['b']) retrieves 4 d1['b'] = 'Z' assigns new value to key 'b'
dict1|dict2	Union of two dictionary objects, returning new object (Python 3.9+)	d3=d1|d2 ; print (d3) {'a': 2, 'b': 4, 'c': 30, 'a1': 20, 'b1': 40, 'c1': 60}
dict1|=dict2	Augmented dictionary union operator (Python 3.9+)	d1|=d2; print (d1) {'a': 2, 'b': 4, 'c': 30, 'a1': 20, 'b1': 40, 'c1': 60}

Python Dictionary Methods

Python includes following dictionary methods −

Built-in Functions with Dictionaries

Following are the built-in functions we can use with Dictionaries −