Day 7: Dictionaries in Python – The Ultimate Key-Value Powerhouse!

 Day 7: Dictionaries in Python – The Ultimate Key-Value Powerhouse!

---

Heyy, Day 7 is here! You’ve already crushed Hello World, Variables, Operators, Strings, Lists, Tuples, and Sets. Now, Day 7 is all about Dictionaries – one of the most powerful and most used data structures in Python!

Think of it like a real dictionary:

Word → Meaning Name → Phone Number Roll No → Marks

In Python, it’s Key → Value pairs. Super fast, super clean, and super useful!

---

What is a Dictionary? (10 mins)

python

student = {
    "name": "Middle class blog",
    "age": 22,
    "grade": "A+",
    "is_pass": True
}

• Uses curly braces {}
• Format: "key": value
• Keys must be unique (if duplicate, last one wins)
• Values can be anything: string, int, list, even another dict!

Accessing Values:

python

print(student["name"])     # Ravi
print(student.get("age"))  # 22 (safe – returns None if key missing)

---

Must-Know Dictionary Methods (15 mins)

Method	What it does	Example
.keys()	Returns all keys	student.keys() → dict_keys(['name', 'age', ...])
.values()	Returns all values	student.values()
.items()	Returns (key, value) pairs	student.items()
.update()	Merge another dict	student.update({"city": "Hyderabad"})
.pop("key")	Remove a key	student.pop("grade")

Try This:

python

car = {"brand": "Toyota", "model": "Camry", "year": 2023}
print("Keys:", list(car.keys()))
print("Values:", list(car.values()))

# Add new key-value
car["color"] = "Red"
car.update({"price": 30000})

print(car)

Output:

text

Keys: ['brand', 'model', 'year']
Values: ['Toyota', 'Camry', 2023]
{'brand': 'Toyota', 'model': 'Camry', 'year': 2023, 'color': 'Red', 'price': 30000}

---

Mini Project: Student Grade Book (30–40 mins)

Let’s build a Student Grade Book! It will:

• Take student name
• Take 3 subject marks
• Calculate average
• Assign grade
• Print a clean report

Code (day7_gradebook.py):

python

# Student Grade Book
print("=== Student Grade Book ===")
name = input("Student name: ")

# Input marks
sub1 = float(input("Maths: "))
sub2 = float(input("Science: "))
sub3 = float(input("English: "))

# Store in dictionary
student = {
    "name": name,
    "marks": [sub1, sub2, sub3],
    "total": sub1 + sub2 + sub3,
    "average": round((sub1 + sub2 + sub3) / 3, 2)
}

# Grade logic
avg = student["average"]
if avg >= 90:
    grade = "A+"
elif avg >= 80:
    grade = "A"
elif avg >= 70:
    grade = "B"
elif avg >= 60:
    grade = "C"
else:
    grade = "Fail"

student["grade"] = grade

# Print Report
print("\n--- Final Report ---")
for key, value in student.items():
    print(f"{key.capitalize()}: {value}")

Run & Test:

text

=== Student Grade Book ===
Student name: Priya
Maths: 95
Science: 88
English: 92

--- Final Report ---
Name: Priya
Marks: [95.0, 88.0, 92.0]
Total: 275.0
Average: 91.67
Grade: A+

---

Common Mistakes & Fixes

Mistake	Error	Fix
student["phone"] (key not found)	KeyError	Use .get("phone", "N/A")
student["marks"] = 90 (should be list)	Wrong data	Use student["marks"] = [90, 85]
Duplicate keys	Last one overwrites	Avoid same key names

---

Day 7 Wrap-Up

You learned:

• Create & access dictionaries (dict["key"])
• Use .keys(), .values(), .items(), .update()
• Built a Student Grade Book with grade logic
• Looped through .items() with for loop

Track Progress (in your notebook):

Day 7 Done: Dictionaries are OP! Built a grade book. Doubt: .get() vs []? Answer: .get() is safe – no error if key missing, can give default value.

Resources:

• RealPython: Dictionaries
• W3Schools: Dictionaries
• YouTube: “Python Dictionaries freeCodeCamp” (first 15 mins)