🟢 Beginner · Lesson 14

Dictionaries in Python

Python Dictionaries

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.

Python – dict.py

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

Aman 95

Access, Add, Update

Python – modify.py

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

{'name': 'Aman', 'marks': 98, 'class': 12}

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

Dictionary Methods

Method	Does
.keys()	all keys
.values()	all values
.items()	all key-value pairs
.get(k)	value of k (no error if missing)
.pop(k)	remove key k

Program 1: Student Record

Python – record.py

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

Name : Riya Roll : 12 Total: 259

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

Program 2: Loop Through a Dictionary

Python – loopdict.py

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

pen costs 10 book costs 50 bag costs 400

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

Program 3: Count Word Frequency

Python – freq.py

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

{'apple': 3, 'banana': 2, 'cherry': 1}

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

Python – phonebook.py

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

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

Whose number? Riya Riya : 91234

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

Make a dictionary of 3 students and their marks; print the topper.
Count how many times each character appears in a word.
Build a small English-Hindi dictionary and look up a word.
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.

Dictionary क्या है?

Dictionary data को key : value pairs के रूप में curly braces { } में रखती है। आप key से value ढूंढते हैं — असली dictionary की तरह जहाँ शब्द key है और अर्थ value।

Python – dict.py

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

Aman 95

Access, Add, Update

Python – modify.py

student = {"name": "Aman", "marks": 95}
student["class"] = 12        # नई key जोड़ें
student["marks"] = 98        # मौजूदा update करें
print(student)

{'name': 'Aman', 'marks': 98, 'class': 12}

नई key को assign करने पर वह जुड़ जाती है।
मौजूदा key को assign करने पर वह update होती है।

Dictionary Methods

Method	क्या करता है
.keys()	सभी keys
.values()	सभी values
.items()	सभी key-value pairs
.get(k)	k की value (missing पर error नहीं)
.pop(k)	key k हटाएं

Program 1: Student Record

Python – record.py

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

Name : Riya Roll : 12 Total: 259

Value खुद एक list हो सकती है — यहाँ marks में तीन numbers हैं जिन्हें sum() से जोड़ते हैं।

Program 2: Dictionary पर Loop

Python – loopdict.py

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

pen costs 10 book costs 50 bag costs 400

.items() हर key और value एक साथ देता है, इसलिए loop दोनों एक बार में पढ़ता है।

Program 3: Word Frequency गिनें

Python – freq.py

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

{'apple': 3, 'banana': 2, 'cherry': 1}

freq.get(w, 0) मौजूदा count या नया word होने पर 0 लौटाता है।
हर बार word आने पर 1 जोड़ते हैं — classic counting pattern।

Program 4: छोटा Phone Book

Python – phonebook.py

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

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

Whose number? Riya Riya : 91234

सामान्य गलतियाँ

missing key को dict[key] से access करने पर KeyError — .get() use करें।
Keys unique होनी चाहिए; दोहराई key पुरानी value overwrite करती है।
Lists keys नहीं हो सकतीं (keys immutable होनी चाहिए)।

Practice Tasks

3 students और उनके marks की dictionary बनाकर topper print करें।
किसी word में हर character कितनी बार आता है, गिनें।
छोटी English-Hindi dictionary बनाकर एक word ढूंढें।
Product prices store करके कुल bill print करें।

सारांश

Dictionaries { } में key:value pairs रखती हैं।
Key से access; assign करने पर जुड़ता या update होता है।
Loop के लिए .items(), KeyError से बचने के लिए .get()।
Records, counting और lookups के लिए बढ़िया।

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