Lesson 32 of 70 – Python Dictionaries

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Python Dictionaries

A dictionary is a collection in Python that stores data in key-value pairs. Dictionaries are useful when you want to store and retrieve values using meaningful keys.

Note: Dictionaries are written using curly braces { }. Each item contains a key and its corresponding value separated by a colon :.

1. What is a Dictionary?

A dictionary stores information in the form of key-value pairs.

student = {
    "name": "Amit",
    "age": 20,
    "course": "Python"
}

print(student)

Output:
{'name': 'Amit', 'age': 20, 'course': 'Python'}

2. Key-Value Pair

Each dictionary item has two parts:

Key – identifies the value.
Value – stores the actual data.

student = {
    "name": "Rahul",
    "age": 21
}

Here, name and age are keys, while Rahul and 21 are their values.

3. Creating a Dictionary

person = {
    "name": "Priya",
    "city": "Patna",
    "age": 22
}

print(person)

Output:
{'name': 'Priya', 'city': 'Patna', 'age': 22}

4. Empty Dictionary

An empty dictionary can be created using curly braces.

student = {}

print(student)

Output:
{}

The type() function confirms that it is a dictionary.

student = {}

print(type(student))

Output:
<class 'dict'>

5. Accessing Dictionary Values

You can access a value by using its key inside square brackets.

student = {
    "name": "Amit",
    "age": 20
}

print(student["name"])
print(student["age"])

Output:
Amit
20

6. Using get() to Access Values

The get() method can also be used to retrieve a value.

student = {
    "name": "Rahul",
    "age": 21
}

print(student.get("name"))

Output:
Rahul

7. Accessing a Missing Key

Using square brackets with a key that does not exist raises a KeyError.

student = {
    "name": "Amit"
}

print(student["age"])

Result:
KeyError: 'age'

Using get() avoids this error and returns None by default.

print(student.get("age"))

Output:
None

8. Adding a New Item

You can add a new key-value pair by assigning a value to a new key.

student = {
    "name": "Amit",
    "age": 20
}

student["course"] = "Python"

print(student)

Output:
{'name': 'Amit', 'age': 20, 'course': 'Python'}

9. Updating a Dictionary Value

If a key already exists, assigning a new value updates that value.

student = {
    "name": "Amit",
    "age": 20
}

student["age"] = 21

print(student)

Output:
{'name': 'Amit', 'age': 21}

10. Dictionary Keys Must Be Unique

Dictionary keys must be unique. If the same key is written more than once, the later value replaces the earlier value.

student = {
    "name": "Amit",
    "name": "Rahul"
}

print(student)

Output:
{'name': 'Rahul'}

11. Dictionary Values Can Be Duplicated

Values in a dictionary do not have to be unique.

students = {
    "student1": "Amit",
    "student2": "Amit",
    "student3": "Rahul"
}

print(students)

Output:
{'student1': 'Amit', 'student2': 'Amit', 'student3': 'Rahul'}

12. Dictionary Length

Use the len() function to find the number of key-value pairs.

student = {
    "name": "Amit",
    "age": 20,
    "course": "Python"
}

print(len(student))

Output:
3

13. Checking Whether a Key Exists

Use the in operator to check whether a key exists.

student = {
    "name": "Amit",
    "age": 20
}

print("name" in student)
print("course" in student)

Output:
True
False

14. Checking a Key with not in

student = {
    "name": "Amit",
    "age": 20
}

print("course" not in student)

Output:
True

15. Dictionary Can Store Different Data Types

Dictionary values can contain different data types.

student = {
    "name": "Amit",
    "age": 20,
    "marks": 85.5,
    "passed": True
}

print(student)

Output:
{'name': 'Amit', 'age': 20, 'marks': 85.5, 'passed': True}

16. Dictionary with a List

A dictionary value can also be a list.

student = {
    "name": "Amit",
    "subjects": ["Python", "SQL", "HTML"]
}

print(student["subjects"])

Output:
['Python', 'SQL', 'HTML']

17. Dictionary with a Nested Dictionary

A dictionary can contain another dictionary as a value. This is called a nested dictionary.

student = {
    "name": "Amit",
    "details": {
        "age": 20,
        "city": "Patna"
    }
}

print(student["details"]["city"])

Output:
Patna

18. Looping Through a Dictionary

A for loop can be used to iterate through the keys of a dictionary.

student = {
    "name": "Amit",
    "age": 20,
    "course": "Python"
}

for key in student:
    print(key)

Output:
name
age
course

19. Looping Through Dictionary Values

Use the values() method to iterate through dictionary values.

student = {
    "name": "Amit",
    "age": 20,
    "course": "Python"
}

for value in student.values():
    print(value)

Output:
Amit
20
Python

20. Looping Through Keys and Values

Use the items() method to get both keys and values.

student = {
    "name": "Amit",
    "age": 20
}

for key, value in student.items():
    print(key, ":", value)

Output:
name : Amit
age : 20

21. Dictionary Keys

The keys() method returns a view containing the dictionary's keys.

student = {
    "name": "Amit",
    "age": 20,
    "course": "Python"
}

print(student.keys())

Output:
dict_keys(['name', 'age', 'course'])

22. Dictionary Values

The values() method returns a view containing the dictionary's values.

student = {
    "name": "Amit",
    "age": 20,
    "course": "Python"
}

print(student.values())

Output:
dict_values(['Amit', 20, 'Python'])

23. Dictionary Items

The items() method returns a view containing key-value pairs.

student = {
    "name": "Amit",
    "age": 20
}

print(student.items())

Output:
dict_items([('name', 'Amit'), ('age', 20)])

24. Dictionary del Keyword

The del keyword can be used to remove a specific key-value pair.

student = {
    "name": "Amit",
    "age": 20,
    "course": "Python"
}

del student["age"]

print(student)

Output:
{'name': 'Amit', 'course': 'Python'}

25. Key Points

A dictionary stores data in key-value pairs.
Dictionaries are written using { }.
Keys must be unique within a dictionary.
Dictionary values can be duplicated.
Values can be of different data types.
Use square brackets or get() to access values.
Use in to check whether a key exists.
Use keys() to access keys.
Use values() to access values.
Use items() to access keys and values together.
Dictionaries can contain lists, tuples, sets, or other dictionaries as values.
Dictionaries are mutable, so their contents can be changed.

🧠 Quick Quiz

Question: Which syntax is used to access a dictionary value using its key?

student["name"]
student("name")
student.name
student<name>

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