Here is a clear, clear, and exam-ready explanation of Dictionary in Python, perfect for BCA/MCA/B.Tech students.

---

Dictionary in Python

A dictionary in Python is an unordered, mutable collection of key–value pairs.

It is one of the most important data types in Python and is used when data needs to be stored in a structured way, like a real dictionary (word → meaning).

---

1. Features of a Dictionary

✔ Stores data in key : value format
✔ Keys must be unique
✔ Keys must be immutable (str, int, tuple)
✔ Values can be any data type
✔ Dictionary is mutable (can be changed)
✔ Unordered (in Python 3.7+, preserves insertion order)

---

2. Creating a Dictionary

Basic Dictionary

student = {
    "name": "John",
    "age": 20,
    "course": "BCA"
}

Empty Dictionary

d = {}

Using dict()

d = dict(name="John", age=20)

---

3. Accessing Dictionary Values

Use the key inside square brackets:

print(student["name"])   # John

OR use .get() method (safer):

print(student.get("age"))    # 20

If key does not exist:

• student["x"] → Error (KeyError)
• student.get("x") → Returns None

---

4. Modifying Dictionary

Add or update values

student["age"] = 21
student["city"] = "Delhi"   # New key-value pair

---

5. Removing Elements

Method	Description
pop(key)	Removes item by key
popitem()	Removes last inserted item
del dict[key]	Deletes specific key
clear()	Removes all items

Example:

student.pop("age")
del student["course"]
student.clear()

---

6. Looping Through a Dictionary

Loop through keys

for k in student:
    print(k)

Loop through values

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

Loop through both keys and values

for k, v in student.items():
    print(k, v)

---

7. Dictionary Methods (Important for Exams)

Method	Description
keys()	Returns all keys
values()	Returns all values
items()	Returns key–value pairs
get(key)	Returns value
update(dict)	Adds/updates multiple items
pop(key)	Removes item
popitem()	Removes last item
clear()	Clears dictionary
fromkeys()	Creates dictionary from keys

---

8. Checking Membership

Check if a key exists:

"name" in student

Check if a value exists:

"John" in student.values()

---

9. Nested Dictionary

A dictionary inside another dictionary.

students = {
    "101": {"name": "John", "age": 20},
    "102": {"name": "Amit", "age": 22}
}
print(students["101"]["name"])   # John

---

10. Dictionary Comprehension

Just like list comprehension.

Example:

squares = {x: x*x for x in range(1, 6)}
print(squares)

Output:

{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

---

11. Example Programs

Example 1: Merge two dictionaries

a = {"x": 1, "y": 2}
b = {"y": 3, "z": 4}
a.update(b)
print(a)

Example 2: Count frequency of elements

items = ["a", "b", "a", "c", "b", "a"]
freq = {}

for item in items:
    freq[item] = freq.get(item, 0) + 1

print(freq)

Output:

{'a': 3, 'b': 2, 'c': 1}

---

12. Summary Table: Dictionary

Property	Description
Ordered	✔ Yes (Python 3.7+)
Mutable	✔ Yes
Duplicate keys	❌ No
Duplicate values	✔ Yes
Keys	Must be immutable
Values	Any data type
Syntax	{ key: value }

---