Dictionaries in Python

What is a Dictionary?

A dictionary stores data as key : value pairs inside curly braces { }. You look up a value using its key — like a real dictionary where the word is the key and the meaning is the value.

student = {"name": "Aman", "class": 12, "marks": 95}
print(student["name"])
print(student["marks"])

Access, Add, Update

student = {"name": "Aman", "marks": 95}
student["class"] = 12        # add a new key
student["marks"] = 98        # update existing
print(student)

• Assigning to a new key adds it.
• Assigning to an existing key updates it.

Dictionary Methods

Program 1: Student Record

student = {
    "name": "Riya",
    "roll": 12,
    "marks": [88, 92, 79]
}
print("Name :", student["name"])
print("Roll :", student["roll"])
print("Total:", sum(student["marks"]))

A value can itself be a list — here marks holds three numbers we add with sum().

Program 2: Loop Through a Dictionary

prices = {"pen": 10, "book": 50, "bag": 400}
for item, price in prices.items():
    print(item, "costs", price)

.items() gives each key and value together, so the loop reads both at once.

Program 3: Count Word Frequency

text = "apple banana apple cherry banana apple"
words = text.split()
freq = {}
for w in words:
    freq[w] = freq.get(w, 0) + 1
print(freq)

• freq.get(w, 0) returns the current count or 0 if the word is new.
• We add 1 each time the word appears — a classic counting pattern.

Program 4: Simple Phone Book

book = {}
book["Aman"] = "98765"
book["Riya"] = "91234"

name = input("Whose number? ")
if name in book:
    print(name, ":", book[name])
else:
    print("Not found")

Common Mistakes

• Accessing a missing key with dict[key] causes a KeyError — use .get().
• Keys must be unique; a repeated key overwrites the old value.
• Lists cannot be keys (keys must be immutable).

Practice Tasks

1. Make a dictionary of 3 students and their marks; print the topper.
2. Count how many times each character appears in a word.
3. Build a small English-Hindi dictionary and look up a word.
4. Store product prices and print the total bill.

Summary

• Dictionaries store key:value pairs in { }.
• Access by key; assigning adds or updates.
• Use .items() to loop, .get() to avoid KeyError.
• Great for records, counting and lookups.