Python Notes

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Python Variables

Python variables ko samjhein — declaration, assignment, naming rules, scope, multiple assignment, aur memory model ke saath complete guide.

Hindi: Variable matlab ek naam-wala container — jaise ek dabbe par label lagana. Jaise aap keh sakte hain "is dabbe mein atta hai" — Python mein flour = 5 matlab "flour naam ke container mein 5 rakhna."

2. Creating Variables

In Python, you don't need to declare a variable before using it. Just assign a value!

Hindi: Python mein variable banane ke liye sirf naam = value likhna hota hai. Koi int, string ya var type declare nahi karna padta — Python khud samajh jaata hai!

python example

# Creating variables with assignment operator (=)
name = "Rahul Kumar"       # string
age = 25                   # integer
height = 5.9               # float
is_student = True          # boolean

# Print all
print(name)
print(age)
print(height)
print(is_student)

Output

Rahul Kumar
25
5.9
True

3. Python is Dynamically Typed

Python figures out the type automatically based on the value you assign.

Hindi: Python "dynamically typed" hai — matlab aapko type batane ki zaroorat nahi. x = 10 likhne par Python samjh jaata hai ki yeh integer hai. Ek hi variable ko alag types dene bhi kar sakte hain!

Example

x = 10 print(type(x)) # <class 'int'> x = "Hello" print(type(x)) # <class 'str'> x = 3.14 print(type(x)) # <class 'float'> x = True print(type(x)) # <class 'bool'> x = [1, 2, 3] print(type(x)) # <class 'list'>

Output

<class 'int'>
<class 'str'>
<class 'float'>
<class 'bool'>
<class 'list'>

Dynamic vs Static Typing Comparison

Feature	Python (Dynamic)	Java/C++ (Static)
Type declaration	Not needed	Required
Example	`x = 10`	`int x = 10;`
Change type	Allowed	Not allowed
Speed	Slower	Faster
Flexibility	High	Low
Type checking	Runtime	Compile-time

4. Variable Assignment

Simple Assignment

python example

# Single assignment
city = "Mumbai"
population = 20_667_656    # underscore for readability (1000s separator)
area = 603.4

print(f"{city}: Population = {population:,}, Area = {area} km²")

Output

Mumbai: Population = 20,667,656, Area = 603.4 km²

Multiple Assignment (Same Value)

python example

# Assign same value to multiple variables
a = b = c = 0
print(a, b, c)    # 0 0 0

# Useful for initialization
x = y = z = None
print(x, y, z)    # None None None

Output

0 0 0
None None None

Multiple Assignment (Different Values)

Example

# Assign different values in one line (tuple unpacking) name, age, city = "Priya", 22, "Delhi" print(name, age, city) # Unpacking from a list coordinates = [28.6139, 77.2090] latitude, longitude = coordinates print(f"Delhi: {latitude}°N, {longitude}°E")

Output

Priya 22 Delhi
Delhi: 28.6139°N, 77.209°E

Swap Variables

python example

# Traditional swap (other languages use temp variable)
# temp = a; a = b; b = temp

# Python's elegant swap — NO temp variable needed!
a = 10
b = 20
print(f"Before: a={a}, b={b}")

a, b = b, a    # ← Python magic!

print(f"After:  a={a}, b={b}")

Output

Before: a=10, b=20
After:  a=20, b=10

Hindi: Python mein do variables swap karna bahut aasaan hai — a, b = b, a bas itna! Doosri languages mein 3 lines lagti hain.

5. Augmented Assignment Operators

python example

score = 100

score += 50    # score = score + 50
print(f"After += 50: {score}")

score -= 30    # score = score - 30
print(f"After -= 30: {score}")

score *= 2     # score = score * 2
print(f"After *= 2: {score}")

score //= 3    # score = score // 3 (integer division)
print(f"After //= 3: {score}")

score **= 2    # score = score ** 2 (power)
print(f"After **= 2: {score}")

score %= 1000  # score = score % 1000 (remainder)
print(f"After %= 1000: {score}")

Output

After += 50: 150
After -= 30: 120
After *= 2: 240
After //= 3: 80
After **= 2: 6400
After %= 1000: 400

6. Variable Naming Rules

Rules (Mandatory)

Example

✅ MUST follow these rules: 1. Start with letter (a-z, A-Z) or underscore (_) 2. Can contain letters, digits, underscores 3. Cannot be a Python keyword 4. No spaces, hyphens, or special characters VALID: INVALID: ────────── ───────────── name 1name (starts with digit) _age my-name (hyphen) count1 my name (space) myVar class (keyword) TOTAL @value (special char) __init__ 2fast (starts with digit)

python example

# Valid examples
student_name = "Amit"
_private = "secret"
__very_private = "more secret"
count99 = 100
MAX_SIZE = 1000
myVariable = "camelCase also valid"
print(student_name, count99, MAX_SIZE)

Output

Amit 100 1000

Naming Conventions (PEP 8 Best Practices)

python example

# Variables and functions — snake_case
student_name = "Rahul"
total_marks = 450
is_passed = True

def calculate_percentage(marks, total):
    return (marks / total) * 100

# Constants — UPPER_SNAKE_CASE
PI = 3.14159
GRAVITY = 9.81
MAX_STUDENTS = 60
DATABASE_URL = "localhost:5432"

# Classes — PascalCase (CamelCase)
class StudentRecord:
    pass

class BankAccount:
    pass

# Private — single underscore prefix
_internal_counter = 0

# "Very private" — double underscore prefix
__secret_key = "abc123"

result = calculate_percentage(total_marks, 500)
print(f"{student_name}: {result}%")

Output

Rahul: 90.0%

7. Python Memory Model

Hindi: Python mein variables sirf "labels" hain — woh directly data store nahi karte, balki memory mein rakhe object ki taraf point karte hain. Yeh samajhna bahut important hai!

Example

# Variables are references to objects a = [1, 2, 3] b = a # b points to SAME object as a print(f"a = {a}") print(f"b = {b}") print(f"id(a) = {id(a)}") print(f"id(b) = {id(b)}") print(f"a is b: {a is b}") # True — same object! b.append(4) # modifying via b also affects a! print(f"\nAfter b.append(4):") print(f"a = {a}") # [1, 2, 3, 4] print(f"b = {b}") # [1, 2, 3, 4]

Output

a = [1, 2, 3]
b = [1, 2, 3]
id(a) = 140234567890  (some memory address)
id(b) = 140234567890  (SAME address)
a is b: True

After b.append(4):
a = [1, 2, 3, 4]
b = [1, 2, 3, 4]

ASCII Diagram — Variable References

Example

x = 10 y = x x ──────────────────► [ 10 ] ▲ y ──────────────────────── ┘ Both x and y point to the SAME object! id(x) == id(y) → True

python example

# For integers (immutable), this behavior is safe:
x = 10
y = x
y = 20      # y now points to new object 20, x unchanged

print(f"x = {x}")    # 10 — unchanged
print(f"y = {y}")    # 20
print(f"id(x) = {id(x)}")
print(f"id(y) = {id(y)}")   # different!

Output

x = 10
y = 20
id(x) = 9788896  (some address)
id(y) = 9789216  (different address)

8. Variable Scope

python example

# Global variable — accessible everywhere
global_var = "I am global"

def demo_scope():
    # Local variable — only inside function
    local_var = "I am local"
    print(global_var)   # can access global
    print(local_var)

demo_scope()
print(global_var)       # accessible outside
# print(local_var)      # ❌ NameError — not accessible!

Output

I am global
I am local
I am global

The `global` Keyword

python example

counter = 0    # global variable

def increment():
    global counter    # tell Python: use the global counter
    counter += 1
    print(f"Inside function: counter = {counter}")

increment()
increment()
increment()
print(f"Outside function: counter = {counter}")

Output

Inside function: counter = 1
Inside function: counter = 2
Inside function: counter = 3
Outside function: counter = 3

LEGB Rule — Variable Lookup Order

Python searches variables in this order: ┌─────────────────────────────────────────┐ │ L — Local scope (inside function) │ │ E — Enclosing scope (nested functions) │ │ G — Global scope (module level) │ │ B — Built-in scope (Python built-ins) │ └─────────────────────────────────────────┘

python example

x = "global"       # G — Global

def outer():
    x = "enclosing"    # E — Enclosing

    def inner():
        x = "local"    # L — Local
        print(x)       # prints "local"

    inner()
    print(x)           # prints "enclosing"

outer()
print(x)               # prints "global"

Output

local
enclosing
global

9. Constants in Python

Python doesn't have true constants, but by convention we use UPPER_CASE:

python example

# Constants — uppercase with underscores
PI = 3.14159265358979
SPEED_OF_LIGHT = 299_792_458   # m/s
GRAVITY = 9.81                  # m/s²
MAX_RETRIES = 3
DEFAULT_TIMEOUT = 30            # seconds
BASE_URL = "https://api.wohotech.com"

def calculate_circle_area(radius):
    return PI * radius ** 2

def calculate_circle_circumference(radius):
    return 2 * PI * radius

r = 7
print(f"Area: {calculate_circle_area(r):.4f}")
print(f"Circumference: {calculate_circle_circumference(r):.4f}")

Output

Area: 153.9380
Circumference: 43.9823

10. Type Annotations (Python 3.5+)

python example

# Optional type hints — doesn't enforce types but improves readability
name: str = "Rahul"
age: int = 25
gpa: float = 3.8
is_enrolled: bool = True

# Function with type annotations
def calculate_bmi(weight: float, height: float) -> float:
    """Calculate BMI given weight (kg) and height (m)."""
    return round(weight / (height ** 2), 2)

bmi = calculate_bmi(70.5, 1.75)
print(f"Name: {name}, Age: {age}")
print(f"BMI: {bmi}")

Output

Name: Rahul, Age: 25
BMI: 23.02

11. The Walrus Operator `:=` (Python 3.8+)

Example

# Walrus operator — assign AND use in one expression import random # Traditional way numbers = [random.randint(1, 100) for _ in range(10)] result = max(numbers) if result > 80: print(f"High score found: {result}") # Walrus operator way numbers = [random.randint(1, 100) for _ in range(10)] if (result := max(numbers)) > 80: print(f"High score found: {result}") else: print(f"Max was only: {result}")

Output

High score found: 95   (random, may vary)

12. Deleting Variables

python example

x = 100
y = 200
z = 300

print(x, y, z)

del x       # delete single variable
print(y, z)

del y, z    # delete multiple variables
# print(x)  # ❌ NameError: name 'x' is not defined
print("Variables deleted successfully")

Output

100 200 300
200 300
Variables deleted successfully

Real-World Example: Student Grade System

python example

# Student information variables
student_name = "Anjali Verma"
roll_number = 2301
subject_1_marks = 87
subject_2_marks = 92
subject_3_marks = 78
subject_4_marks = 95
subject_5_marks = 83

# Constants
TOTAL_SUBJECTS = 5
MAX_MARKS_PER_SUBJECT = 100
PASSING_PERCENTAGE = 33

# Calculations
total_marks = (subject_1_marks + subject_2_marks + subject_3_marks +
               subject_4_marks + subject_5_marks)
max_possible = MAX_MARKS_PER_SUBJECT * TOTAL_SUBJECTS
percentage = (total_marks / max_possible) * 100
is_passed = percentage >= PASSING_PERCENTAGE

# Grade calculation
if percentage >= 90:
    grade = "A+"
elif percentage >= 80:
    grade = "A"
elif percentage >= 70:
    grade = "B"
elif percentage >= 60:
    grade = "C"
elif percentage >= 33:
    grade = "D"
else:
    grade = "F"

# Output
print("=" * 40)
print(f"STUDENT REPORT CARD")
print("=" * 40)
print(f"Name:       {student_name}")
print(f"Roll No:    {roll_number}")
print(f"Total:      {total_marks}/{max_possible}")
print(f"Percentage: {percentage:.1f}%")
print(f"Grade:      {grade}")
print(f"Result:     {'PASS ✅' if is_passed else 'FAIL ❌'}")
print("=" * 40)

Output

========================================
STUDENT REPORT CARD
========================================
Name:       Anjali Verma
Roll No:    2301
Total:      435/500
Percentage: 87.0%
Grade:      A
Result:     PASS ✅
========================================

Practice Exercises

Exercise 1 — Variable Basics

Create variables for:

Your name, age, city, height (float), is_employed (bool)
Print them all in a formatted sentence

Exercise 2 — Swap Without Temp

Given x = "Hello" and y = "World", swap their values and print both.

Exercise 3 — Multiple Assignment

Unpack this list into variables: data = ["Rahul", 22, "Delhi", 9.2] (name, age, city, gpa)

Exercise 4 — Augmented Assignment

Start with balance = 10000. Apply these operations:

Add 5000 (salary)
Subtract 1200 (rent)
Subtract 800 (groceries)
Multiply remaining by 1.04 (4% interest)
Print final balance

Exercise 5 — Scope

Write a function that modifies a global user_count variable using the global keyword.

Interview Questions

Q1. Is Python statically typed or dynamically typed? What's the difference? > Python is dynamically typed — variable types are determined at runtime based on assigned values. You don't declare types. In contrast, statically typed languages (Java, C++) require explicit type declarations and check types at compile time.

Q2. What is the difference between =, ==, and is in Python? > = is assignment (stores value), == checks value equality, is checks identity (same memory address/object). Example: a = [1,2]; b = [1,2]; a == b is True (same values), but a is b is False (different objects).

Q3. Explain variable scope and the LEGB rule. > Scope determines where a variable is accessible. Python uses LEGB: Local (inside function) → Enclosing (outer function) → Global (module level) → Built-in (Python's built-ins). Python searches in this order.

Q4. What happens when you do b = a with a list? > Both a and b reference the same list object. Modifying through b affects a too. To create an independent copy, use b = a.copy() or b = a[:].

Q5. How do Python constants differ from true constants? > Python has no built-in constant mechanism. By convention, variables in UPPER_CASE are treated as constants, but they can technically be changed. For true immutability, you can use typing.Final (Python 3.8+) or put constants in a frozen dataclass.

⚠️ Common Mistakes

1. Variable naam digit se shuru karna ❌

python example

# Wrong ❌
1st_name = "Rahul"    # SyntaxError!

# Correct ✅
first_name = "Rahul"

Hindi: Variable ka naam kabhi bhi number se shuru nahi ho sakta — hamesha letter ya underscore _ se shuru karein.

2. Assignment `=` aur Comparison `==` confuse karna ❌

python example

# Wrong ❌ — yeh assign karega, compare nahi
if x = 10:     # SyntaxError!
    print("yes")

# Correct ✅
if x == 10:
    print("yes")

Hindi: = matlab value assign karna, == matlab compare karna. Bahut common mistake hai beginners mein!

3. Undefined variable use karna ❌

python example

# Wrong ❌
print(salary)   # NameError: name 'salary' is not defined

# Correct ✅
salary = 50000
print(salary)

Hindi: Variable ko use karne se pehle usse value assign karna zaroori hai. Python mein sirf declare karna possible nahi — assign karna padta hai.

4. Mutable objects ko copy samajhna ❌

Example

# Wrong ❌ — dono same list ko point karte hain a = [1, 2, 3] b = a b.append(4) print(a) # [1, 2, 3, 4] — a bhi change ho gaya! 😱 # Correct ✅ — independent copy banao a = [1, 2, 3] b = a.copy() b.append(4) print(a) # [1, 2, 3] — a safe hai ✅

Hindi: Lists, dictionaries jaise mutable objects mein b = a likhne se dono SAME object ko point karte hain. Independent copy chahiye toh .copy() ya slicing [:] use karein.

5. `global` keyword bhoolna ❌

python example

count = 0

# Wrong ❌
def increment():
    count += 1   # UnboundLocalError!

# Correct ✅
def increment():
    global count
    count += 1

Hindi: Function ke andar global variable ko modify karne ke liye global keyword zaroori hai, warna Python use local maanegi aur error degi.

6. Case sensitivity ignore karna ❌

python example

# Wrong ❌ — yeh ALAG variables hain!
Name = "Rahul"
name = "Priya"
NAME = "Amit"

print(Name, name, NAME)   # Rahul Priya Amit — teeno different!

Hindi: Python case-sensitive hai — Name, name, aur NAME teeno alag variables hain. Consistent naming follow karein (prefer snake_case).

7. Python keywords ko variable naam rakhna ❌

python example

# Wrong ❌
class = "10th"       # SyntaxError!
for = "loop"         # SyntaxError!
import = "module"    # SyntaxError!

# Correct ✅
class_name = "10th"
loop_type = "for"
module_name = "os"

Hindi: Python ke reserved words (if, else, class, for, while, import, etc.) ko variable naam ke roop mein use nahi kar sakte.

✅ Key Takeaways

📦 Variable ek label hai — yeh memory mein kisi object ko point karta hai, directly data store nahi karta.
🔄 Python dynamically typed hai — aapko type declare nahi karna padta, Python khud value dekh kar type decide karta hai.
📝 Assignment operator = se variable banate hain — naam = value bas itna hi kaafi hai.
🐍 Naming rules yaad rakhein — letter/underscore se shuru, digits allowed baad mein, no spaces/special chars, no keywords.
🎯 PEP 8 conventions follow karein — variables ke liye snake_case, constants ke liye UPPER_CASE, classes ke liye PascalCase.
🔗 b = a (mutable objects) se dono same object point karte hain — independent copy ke liye .copy() use karein.
🌍 Scope matters — LEGB rule (Local → Enclosing → Global → Built-in) samjhein. Function mein global modify karne ke liye global keyword use karein.
🔀 Swap karna easy hai — a, b = b, a Python ka elegant approach hai, temp variable ki zaroorat nahi.
🏷️ Type annotations optional hain — code readability badhate hain lekin Python enforce nahi karta.
🗑️ del statement se variables delete kar sakte hain — memory management mein helpful hai.

❓ FAQ

Q1. Kya Python mein variable declare karna zaroori hai? > Nahi! Python mein alag se declaration ki zaroorat nahi hoti. Aap directly x = 10 likh sakte hain — Python automatically type infer kar leta hai. Yeh Java/C++ se alag hai jahan int x = 10; likhna padta hai.

Q2. Ek variable ki type runtime pe change kar sakte hain kya? > Haan! Python dynamically typed hai, toh x = 10 ke baad x = "Hello" likh sakte hain — koi error nahi aayega. Lekin yeh best practice nahi hai kyunki code confusing ho sakta hai.

Q3. is aur == mein kya difference hai? > == value compare karta hai (dono ki value same hai ya nahi), jabki is identity check karta hai (dono same memory object hain ya nahi). Example: [1,2] == [1,2] → True, lekin [1,2] is [1,2] → False (different objects).

Q4. Global variable ko function ke andar kaise modify karein? > Function ke andar global variable_name likhein, phir modify karein. Bina global keyword ke, Python function ke andar ek naya local variable bana dega ya UnboundLocalError dega.

Q5. Python mein constants kaise banate hain? > Python mein true constants nahi hote. Convention hai ki UPPER_CASE mein likhe variables ko constant maana jaata hai (e.g., MAX_SIZE = 100). Python 3.8+ mein typing.Final use kar sakte hain type checker ke liye, lekin runtime pe phir bhi change ho sakta hai.

Q6. Variable naming mein _ (single underscore) aur __ (double underscore) ka kya matlab hai? > Single underscore _var ka matlab hai "private/internal use" (convention). Double underscore __var Python mein name mangling trigger karta hai — class ke andar variable ka naam change ho jaata hai taaki subclass accidentally override na kare.

Q7. Kya ek line mein multiple variables assign kar sakte hain? > Haan! Do tarike hain: (1) Same value — a = b = c = 0, (2) Different values — x, y, z = 1, 2, 3. Yeh tuple unpacking ke concept par based hai.

Q8. del statement kab use karna chahiye? > del tab use karein jab aapko explicitly kisi variable ko memory se hatana ho — jaise large data structures jinka kaam ho gaya. Normally Python ka garbage collector khud handle karta hai, lekin memory-intensive programs mein del helpful hai.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Python Variables.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Python Master Course topic.

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Python Notes

Python Variables

Python variables ko samjhein — declaration, assignment, naming rules, scope, multiple assignment, aur memory model ke saath complete guide.

Hindi: Variable matlab ek naam-wala container — jaise ek dabbe par label lagana. Jaise aap keh sakte hain "is dabbe mein atta hai" — Python mein flour = 5 matlab "flour naam ke container mein 5 rakhna."

2. Creating Variables

In Python, you don't need to declare a variable before using it. Just assign a value!

Hindi: Python mein variable banane ke liye sirf naam = value likhna hota hai. Koi int, string ya var type declare nahi karna padta — Python khud samajh jaata hai!

python example

# Creating variables with assignment operator (=)
name = "Rahul Kumar"       # string
age = 25                   # integer
height = 5.9               # float
is_student = True          # boolean

# Print all
print(name)
print(age)
print(height)
print(is_student)

Output

Rahul Kumar
25
5.9
True

3. Python is Dynamically Typed

Python figures out the type automatically based on the value you assign.

Hindi: Python "dynamically typed" hai — matlab aapko type batane ki zaroorat nahi. x = 10 likhne par Python samjh jaata hai ki yeh integer hai. Ek hi variable ko alag types dene bhi kar sakte hain!

Example

Output

<class 'int'>
<class 'str'>
<class 'float'>
<class 'bool'>
<class 'list'>

Dynamic vs Static Typing Comparison

Feature	Python (Dynamic)	Java/C++ (Static)
Type declaration	Not needed	Required
Example	`x = 10`	`int x = 10;`
Change type	Allowed	Not allowed
Speed	Slower	Faster
Flexibility	High	Low
Type checking	Runtime	Compile-time

4. Variable Assignment

Simple Assignment

python example

# Single assignment
city = "Mumbai"
population = 20_667_656    # underscore for readability (1000s separator)
area = 603.4

print(f"{city}: Population = {population:,}, Area = {area} km²")

Output

Mumbai: Population = 20,667,656, Area = 603.4 km²

Multiple Assignment (Same Value)

python example

# Assign same value to multiple variables
a = b = c = 0
print(a, b, c)    # 0 0 0

# Useful for initialization
x = y = z = None
print(x, y, z)    # None None None

Output

0 0 0
None None None

Multiple Assignment (Different Values)

Example

Output

Priya 22 Delhi
Delhi: 28.6139°N, 77.209°E

Swap Variables

python example

# Traditional swap (other languages use temp variable)
# temp = a; a = b; b = temp

# Python's elegant swap — NO temp variable needed!
a = 10
b = 20
print(f"Before: a={a}, b={b}")

a, b = b, a    # ← Python magic!

print(f"After:  a={a}, b={b}")

Output

Before: a=10, b=20
After:  a=20, b=10

Hindi: Python mein do variables swap karna bahut aasaan hai — a, b = b, a bas itna! Doosri languages mein 3 lines lagti hain.

5. Augmented Assignment Operators

python example

score = 100

score += 50    # score = score + 50
print(f"After += 50: {score}")

score -= 30    # score = score - 30
print(f"After -= 30: {score}")

score *= 2     # score = score * 2
print(f"After *= 2: {score}")

score //= 3    # score = score // 3 (integer division)
print(f"After //= 3: {score}")

score **= 2    # score = score ** 2 (power)
print(f"After **= 2: {score}")

score %= 1000  # score = score % 1000 (remainder)
print(f"After %= 1000: {score}")

Output

After += 50: 150
After -= 30: 120
After *= 2: 240
After //= 3: 80
After **= 2: 6400
After %= 1000: 400

6. Variable Naming Rules

Rules (Mandatory)

Example

python example

# Valid examples
student_name = "Amit"
_private = "secret"
__very_private = "more secret"
count99 = 100
MAX_SIZE = 1000
myVariable = "camelCase also valid"
print(student_name, count99, MAX_SIZE)

Output

Amit 100 1000

Naming Conventions (PEP 8 Best Practices)

python example

# Variables and functions — snake_case
student_name = "Rahul"
total_marks = 450
is_passed = True

def calculate_percentage(marks, total):
    return (marks / total) * 100

# Constants — UPPER_SNAKE_CASE
PI = 3.14159
GRAVITY = 9.81
MAX_STUDENTS = 60
DATABASE_URL = "localhost:5432"

# Classes — PascalCase (CamelCase)
class StudentRecord:
    pass

class BankAccount:
    pass

# Private — single underscore prefix
_internal_counter = 0

# "Very private" — double underscore prefix
__secret_key = "abc123"

result = calculate_percentage(total_marks, 500)
print(f"{student_name}: {result}%")

Output

Rahul: 90.0%

7. Python Memory Model

Hindi: Python mein variables sirf "labels" hain — woh directly data store nahi karte, balki memory mein rakhe object ki taraf point karte hain. Yeh samajhna bahut important hai!

Example

Output

a = [1, 2, 3]
b = [1, 2, 3]
id(a) = 140234567890  (some memory address)
id(b) = 140234567890  (SAME address)
a is b: True

After b.append(4):
a = [1, 2, 3, 4]
b = [1, 2, 3, 4]

ASCII Diagram — Variable References

Example

python example

# For integers (immutable), this behavior is safe:
x = 10
y = x
y = 20      # y now points to new object 20, x unchanged

print(f"x = {x}")    # 10 — unchanged
print(f"y = {y}")    # 20
print(f"id(x) = {id(x)}")
print(f"id(y) = {id(y)}")   # different!

Output

x = 10
y = 20
id(x) = 9788896  (some address)
id(y) = 9789216  (different address)

8. Variable Scope

python example

# Global variable — accessible everywhere
global_var = "I am global"

def demo_scope():
    # Local variable — only inside function
    local_var = "I am local"
    print(global_var)   # can access global
    print(local_var)

demo_scope()
print(global_var)       # accessible outside
# print(local_var)      # ❌ NameError — not accessible!

Output

I am global
I am local
I am global

The `global` Keyword

python example

counter = 0    # global variable

def increment():
    global counter    # tell Python: use the global counter
    counter += 1
    print(f"Inside function: counter = {counter}")

increment()
increment()
increment()
print(f"Outside function: counter = {counter}")

Output

Inside function: counter = 1
Inside function: counter = 2
Inside function: counter = 3
Outside function: counter = 3

LEGB Rule — Variable Lookup Order

python example

x = "global"       # G — Global

def outer():
    x = "enclosing"    # E — Enclosing

    def inner():
        x = "local"    # L — Local
        print(x)       # prints "local"

    inner()
    print(x)           # prints "enclosing"

outer()
print(x)               # prints "global"

Output

local
enclosing
global

9. Constants in Python

Python doesn't have true constants, but by convention we use UPPER_CASE:

python example

# Constants — uppercase with underscores
PI = 3.14159265358979
SPEED_OF_LIGHT = 299_792_458   # m/s
GRAVITY = 9.81                  # m/s²
MAX_RETRIES = 3
DEFAULT_TIMEOUT = 30            # seconds
BASE_URL = "https://api.wohotech.com"

def calculate_circle_area(radius):
    return PI * radius ** 2

def calculate_circle_circumference(radius):
    return 2 * PI * radius

r = 7
print(f"Area: {calculate_circle_area(r):.4f}")
print(f"Circumference: {calculate_circle_circumference(r):.4f}")

Output

Area: 153.9380
Circumference: 43.9823

10. Type Annotations (Python 3.5+)

python example

# Optional type hints — doesn't enforce types but improves readability
name: str = "Rahul"
age: int = 25
gpa: float = 3.8
is_enrolled: bool = True

# Function with type annotations
def calculate_bmi(weight: float, height: float) -> float:
    """Calculate BMI given weight (kg) and height (m)."""
    return round(weight / (height ** 2), 2)

bmi = calculate_bmi(70.5, 1.75)
print(f"Name: {name}, Age: {age}")
print(f"BMI: {bmi}")

Output

Name: Rahul, Age: 25
BMI: 23.02

11. The Walrus Operator `:=` (Python 3.8+)

Example

Output

High score found: 95   (random, may vary)

12. Deleting Variables

python example

x = 100
y = 200
z = 300

print(x, y, z)

del x       # delete single variable
print(y, z)

del y, z    # delete multiple variables
# print(x)  # ❌ NameError: name 'x' is not defined
print("Variables deleted successfully")

Output

100 200 300
200 300
Variables deleted successfully

Real-World Example: Student Grade System

python example

# Student information variables
student_name = "Anjali Verma"
roll_number = 2301
subject_1_marks = 87
subject_2_marks = 92
subject_3_marks = 78
subject_4_marks = 95
subject_5_marks = 83

# Constants
TOTAL_SUBJECTS = 5
MAX_MARKS_PER_SUBJECT = 100
PASSING_PERCENTAGE = 33

# Calculations
total_marks = (subject_1_marks + subject_2_marks + subject_3_marks +
               subject_4_marks + subject_5_marks)
max_possible = MAX_MARKS_PER_SUBJECT * TOTAL_SUBJECTS
percentage = (total_marks / max_possible) * 100
is_passed = percentage >= PASSING_PERCENTAGE

# Grade calculation
if percentage >= 90:
    grade = "A+"
elif percentage >= 80:
    grade = "A"
elif percentage >= 70:
    grade = "B"
elif percentage >= 60:
    grade = "C"
elif percentage >= 33:
    grade = "D"
else:
    grade = "F"

# Output
print("=" * 40)
print(f"STUDENT REPORT CARD")
print("=" * 40)
print(f"Name:       {student_name}")
print(f"Roll No:    {roll_number}")
print(f"Total:      {total_marks}/{max_possible}")
print(f"Percentage: {percentage:.1f}%")
print(f"Grade:      {grade}")
print(f"Result:     {'PASS ✅' if is_passed else 'FAIL ❌'}")
print("=" * 40)

Output

========================================
STUDENT REPORT CARD
========================================
Name:       Anjali Verma
Roll No:    2301
Total:      435/500
Percentage: 87.0%
Grade:      A
Result:     PASS ✅
========================================

Practice Exercises

Exercise 1 — Variable Basics

Create variables for:

Your name, age, city, height (float), is_employed (bool)
Print them all in a formatted sentence

Exercise 2 — Swap Without Temp

Given x = "Hello" and y = "World", swap their values and print both.

Exercise 3 — Multiple Assignment

Unpack this list into variables: data = ["Rahul", 22, "Delhi", 9.2] (name, age, city, gpa)

Exercise 4 — Augmented Assignment

Start with balance = 10000. Apply these operations:

Add 5000 (salary)
Subtract 1200 (rent)
Subtract 800 (groceries)
Multiply remaining by 1.04 (4% interest)
Print final balance

Exercise 5 — Scope

Write a function that modifies a global user_count variable using the global keyword.

Interview Questions

⚠️ Common Mistakes

1. Variable naam digit se shuru karna ❌

python example

# Wrong ❌
1st_name = "Rahul"    # SyntaxError!

# Correct ✅
first_name = "Rahul"

Hindi: Variable ka naam kabhi bhi number se shuru nahi ho sakta — hamesha letter ya underscore _ se shuru karein.

2. Assignment `=` aur Comparison `==` confuse karna ❌

python example

# Wrong ❌ — yeh assign karega, compare nahi
if x = 10:     # SyntaxError!
    print("yes")

# Correct ✅
if x == 10:
    print("yes")

Hindi: = matlab value assign karna, == matlab compare karna. Bahut common mistake hai beginners mein!

3. Undefined variable use karna ❌

python example

# Wrong ❌
print(salary)   # NameError: name 'salary' is not defined

# Correct ✅
salary = 50000
print(salary)

Hindi: Variable ko use karne se pehle usse value assign karna zaroori hai. Python mein sirf declare karna possible nahi — assign karna padta hai.

4. Mutable objects ko copy samajhna ❌

Example

Hindi: Lists, dictionaries jaise mutable objects mein b = a likhne se dono SAME object ko point karte hain. Independent copy chahiye toh .copy() ya slicing [:] use karein.

5. `global` keyword bhoolna ❌

python example

count = 0

# Wrong ❌
def increment():
    count += 1   # UnboundLocalError!

# Correct ✅
def increment():
    global count
    count += 1

Hindi: Function ke andar global variable ko modify karne ke liye global keyword zaroori hai, warna Python use local maanegi aur error degi.

6. Case sensitivity ignore karna ❌

python example

# Wrong ❌ — yeh ALAG variables hain!
Name = "Rahul"
name = "Priya"
NAME = "Amit"

print(Name, name, NAME)   # Rahul Priya Amit — teeno different!

Hindi: Python case-sensitive hai — Name, name, aur NAME teeno alag variables hain. Consistent naming follow karein (prefer snake_case).

7. Python keywords ko variable naam rakhna ❌

python example

# Wrong ❌
class = "10th"       # SyntaxError!
for = "loop"         # SyntaxError!
import = "module"    # SyntaxError!

# Correct ✅
class_name = "10th"
loop_type = "for"
module_name = "os"

Hindi: Python ke reserved words (if, else, class, for, while, import, etc.) ko variable naam ke roop mein use nahi kar sakte.

✅ Key Takeaways

📦 Variable ek label hai — yeh memory mein kisi object ko point karta hai, directly data store nahi karta.
🔄 Python dynamically typed hai — aapko type declare nahi karna padta, Python khud value dekh kar type decide karta hai.
📝 Assignment operator = se variable banate hain — naam = value bas itna hi kaafi hai.
🐍 Naming rules yaad rakhein — letter/underscore se shuru, digits allowed baad mein, no spaces/special chars, no keywords.
🎯 PEP 8 conventions follow karein — variables ke liye snake_case, constants ke liye UPPER_CASE, classes ke liye PascalCase.
🔗 b = a (mutable objects) se dono same object point karte hain — independent copy ke liye .copy() use karein.
🌍 Scope matters — LEGB rule (Local → Enclosing → Global → Built-in) samjhein. Function mein global modify karne ke liye global keyword use karein.
🔀 Swap karna easy hai — a, b = b, a Python ka elegant approach hai, temp variable ki zaroorat nahi.
🏷️ Type annotations optional hain — code readability badhate hain lekin Python enforce nahi karta.
🗑️ del statement se variables delete kar sakte hain — memory management mein helpful hai.

❓ FAQ

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Python Variables.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Python Master Course topic.

Search Terms

python-master-course, python master course, python, master, course, basics, variables, python variables

Continue learning this concept

PreviousPython Type Casting NextBreak, Continue and Pass in Python

2. Creating Variables

3. Python is Dynamically Typed

Dynamic vs Static Typing Comparison

4. Variable Assignment

Simple Assignment

Multiple Assignment (Same Value)

Multiple Assignment (Different Values)

Swap Variables

5. Augmented Assignment Operators

6. Variable Naming Rules

Rules (Mandatory)

Naming Conventions (PEP 8 Best Practices)

7. Python Memory Model

ASCII Diagram — Variable References

8. Variable Scope

The global Keyword

LEGB Rule — Variable Lookup Order

9. Constants in Python

10. Type Annotations (Python 3.5+)

11. The Walrus Operator := (Python 3.8+)

12. Deleting Variables

Real-World Example: Student Grade System

Practice Exercises

Exercise 1 — Variable Basics

Exercise 2 — Swap Without Temp

Exercise 3 — Multiple Assignment

Exercise 4 — Augmented Assignment

Exercise 5 — Scope

Interview Questions

⚠️ Common Mistakes

1. Variable naam digit se shuru karna ❌

2. Assignment = aur Comparison == confuse karna ❌

3. Undefined variable use karna ❌

4. Mutable objects ko copy samajhna ❌

5. global keyword bhoolna ❌

6. Case sensitivity ignore karna ❌

7. Python keywords ko variable naam rakhna ❌

✅ Key Takeaways

❓ FAQ

Continue learning this concept

2. Creating Variables

3. Python is Dynamically Typed

Dynamic vs Static Typing Comparison

4. Variable Assignment

Simple Assignment

Multiple Assignment (Same Value)

Multiple Assignment (Different Values)

Swap Variables

5. Augmented Assignment Operators

6. Variable Naming Rules

Rules (Mandatory)

Naming Conventions (PEP 8 Best Practices)

7. Python Memory Model

ASCII Diagram — Variable References

8. Variable Scope

The global Keyword

LEGB Rule — Variable Lookup Order

9. Constants in Python

10. Type Annotations (Python 3.5+)

11. The Walrus Operator := (Python 3.8+)

12. Deleting Variables

Real-World Example: Student Grade System

Practice Exercises

Exercise 1 — Variable Basics

Exercise 2 — Swap Without Temp

Exercise 3 — Multiple Assignment

Exercise 4 — Augmented Assignment

Exercise 5 — Scope

Interview Questions

⚠️ Common Mistakes

1. Variable naam digit se shuru karna ❌

2. Assignment = aur Comparison == confuse karna ❌

3. Undefined variable use karna ❌

4. Mutable objects ko copy samajhna ❌

5. global keyword bhoolna ❌

6. Case sensitivity ignore karna ❌

7. Python keywords ko variable naam rakhna ❌

✅ Key Takeaways

❓ FAQ

Continue learning this concept

The `global` Keyword

11. The Walrus Operator `:=` (Python 3.8+)

2. Assignment `=` aur Comparison `==` confuse karna ❌

5. `global` keyword bhoolna ❌

The `global` Keyword

11. The Walrus Operator `:=` (Python 3.8+)

2. Assignment `=` aur Comparison `==` confuse karna ❌

5. `global` keyword bhoolna ❌