Variables, Data Types & Expressions

Why read this? In the last topic, you printed values directly. But real programs need to remember data — a user's name, a score, a calculation result. Variables are your program's "sticky notes" — named boxes where you store information for later use.

🎯 Learning Objectives

1. Create variables and assign values to them
2. Follow Python's rules for naming variables
3. Understand dynamic typing — how a variable's type can change
4. Write arithmetic expressions using operators
5. Use assignment operators (+=, -=, etc.) as shortcuts
6. Convert between data types using int(), float(), str()

📋 Prerequisites

• Introduction to Programming & Python — You need to understand print(), data types, and the REPL.
• Basic arithmetic: addition, subtraction, multiplication, division.

📖 Core Content

2.1 What Problem Do Variables Solve?

python
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# Without variables — repetitive and hard to change
print(10 + 5 + 3)
print("Total is", 10 + 5 + 3)  # typed the numbers again!
# With variables — clean and modifiable
price1 = 10
price2 = 5
price3 = 3
total = price1 + price2 + price3
print("Total is", total)

2.2 Variables as Labels

• The variable name is the label on the box.
• The value is what's stored inside the box.
• The assignment statement (=) is the act of putting something in the box.

2.3 Variable Assignment

python
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# runnable
name = "Alice"     # string variable
age = 25           # integer variable
height = 5.6       # float variable
is_student = True  # boolean variable
print(name)
print(age)
print(height)
print(is_student)

pseudo
Alice
25
5.6
True

• name = "Alice" — Python creates a box labeled name and puts the string "Alice" in it.
• age = 25 — Creates a box labeled age with the integer 25.
• When we print(name), Python looks in the box labeled name, finds "Alice", and displays it.

2.4 Variable Naming Rules

sql
and, as, assert, break, class, continue, def, del, elif, else, except,
False, finally, for, from, global, if, import, in, is, lambda, None,
nonlocal, not, or, pass, raise, return, True, try, while, with, yield

2.5 Dynamic Typing — Python's Special Feature

python
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# runnable
thing = "Hello"       # thing is a string
print(type(thing))    # <class 'str'>
thing = 42            # Now thing is an integer
print(type(thing))    # <class 'int'>
thing = 3.14          # Now thing is a float
print(type(thing))    # <class 'float'>
thing = True          # Now thing is a boolean
print(type(thing))    # <class 'bool'>

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

⚠️ Warning: Just because you can change a variable's type doesn't mean you should. It's usually best to keep a variable's type consistent throughout your program. Changing types unexpectedly is confusing and leads to bugs.

2.6 Multiple Assignment

python
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# runnable
# Assign multiple variables at once
x, y, z = 10, 20, 30
print(x, y, z)
# Assign the same value to multiple variables
a = b = c = 0
print(a, b, c)
# Swap two variables (neat Python trick!)
m = 5
n = 10
m, n = n, m    # swap!
print("m =", m, "n =", n)

pseudo
10 20 30
0 0 0
m = 10 n = 5

2.7 Arithmetic Operators

2.8 Worked Example 1: Basic Arithmetic

python
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# runnable
a = 15
b = 4
print("a + b =", a + b)    # 19
print("a - b =", a - b)    # 11
print("a * b =", a * b)    # 60
print("a / b =", a / b)    # 3.75  (float division)
print("a // b =", a // b)  # 3     (floor division: 15/4 = 3.75, rounded DOWN to 3)
print("a % b =", a % b)    # 3     (remainder: 15 = 4*3 + 3)
print("a ** b =", a ** b)  # 50625 (15*15*15*15)

pseudo
a + b = 19
a - b = 11
a * b = 60
a / b = 3.75
a // b = 3
a % b = 3
a ** b = 50625

2.9 Operator Precedence (Order of Operations)

python
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# runnable
print(2 + 3 * 4)       # 14  (3*4=12, then 2+12=14)
print((2 + 3) * 4)     # 20  (2+3=5, then 5*4=20)
print(10 - 2 ** 3)     # 2   (2**3=8, then 10-8=2)
print(10 / 2 * 3)      # 15.0 (left to right: 10/2=5, then 5*3=15)
print(10 * 2 ** 3)     # 80  (2**3=8, then 10*8=80)

pseudo
14
20
2
15.0
80

Pro tip: When in doubt, use parentheses () to make your intention clear. It's better to write (2 + 3) * 4 than to rely on remembering precedence rules.

2.10 Assignment Operators (Shortcuts)

python
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# runnable
x = 10
x += 5    # same as: x = x + 5
print("After x += 5:", x)   # 15
x -= 3    # same as: x = x - 3
print("After x -= 3:", x)   # 12
x *= 2    # same as: x = x * 2
print("After x *= 2:", x)   # 24
x //= 5   # same as: x = x // 5
print("After x //= 5:", x)  # 4
x **= 3   # same as: x = x ** 3
print("After x **= 3:", x)  # 64

pseudo
After x += 5: 15
After x -= 3: 12
After x *= 2: 24
After x //= 5: 4
After x **= 3: 64

2.11 Type Conversion (Casting)

python
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# runnable
# String to integer
print(int("42"))        # 42
print(int("42") + 8)    # 50
# String to float
print(float("3.14"))    # 3.14
# Float to integer (truncates decimal! Not rounding)
print(int(3.99))        # 3 (truncated, not rounded)
print(int(-2.7))        # -2 (truncated toward zero)
# Number to string
print(str(42))          # "42"
print(str(3.14))        # "3.14"
# Converting one type to another
num_str = "100"
num_int = int(num_str)
num_float = float(num_str)
print(num_int + 50)     # 150
print(num_float + 50)   # 150.0

pseudo
42
50
3.14
3
-2
42
3.14
150
150.0

⚠️ Warning: int("3.14") produces ValueError: invalid literal for int() with base 10: '3.14' — you can't convert a string with a decimal point directly to int. First convert to float: int(float("3.14")).

2.12 Worked Example 2: Temperature Converter

python
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# runnable
celsius = 100
fahrenheit = celsius * 9 / 5 + 32
print(celsius, "°C =", fahrenheit, "°F")
celsius = 0
fahrenheit = celsius * 9 / 5 + 32
print(celsius, "°C =", fahrenheit, "°F")
celsius = 37
fahrenheit = celsius * 9 / 5 + 32
print(celsius, "°C =", fahrenheit, "°F")

pseudo
100 °C = 212.0 °F
0 °C = 32.0 °F
37 °C = 98.6 °F

2.13 Worked Example 3: Area and Circumference of a Circle

python
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# runnable
pi = 3.14159
radius = 5
area = pi * radius ** 2
circumference = 2 * pi * radius
print("Radius:", radius)
print("Area:", area)
print("Circumference:", circumference)

pseudo
Radius: 5
Area: 78.53975
Circumference: 31.4159

• radius ** 2 means "radius squared" (radius × radius).
• pi * radius ** 2 follows precedence: exponent happens before multiplication.
• 2 * pi * radius multiplies in order: 2 × pi, then × radius.

2.14 Worked Example 4: Simple Interest Calculator

python
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# runnable
principal = 10000    # P: amount borrowed
rate = 8.5           # R: annual interest rate in percent
time = 3             # T: time in years
simple_interest = (principal * rate * time) / 100
total_amount = principal + simple_interest
print("Principal: ₹", principal)
print("Rate:", rate, "% per year")
print("Time:", time, "years")
print("Simple Interest: ₹", simple_interest)
print("Total Amount: ₹", total_amount)

pseudo
Principal: ₹ 10000
Rate: 8.5 % per year
Time: 3 years
Simple Interest: ₹ 2550.0
Total Amount: ₹ 12550.0

2.15 Worked Example 5: Swapping Values (The Pythonic Way)

python
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# runnable
# Method 1: Using a temporary variable (works in any language)
a = 5
b = 10
temp = a
a = b
b = temp
print("a =", a, "b =", b)
# Method 2: Python's tuple unpacking (elegant!)
x = 5
y = 10
x, y = y, x
print("x =", x, "y =", y)

pseudo
a = 10 b = 5
x = 10 y = 5

📐 Key Concepts Reference

⚠️ Common Pitfalls

Pitfall 1: Variable Name Starts with a Number

Pitfall 2: Using = Instead of == in Conditions

Pitfall 3: Confusing int() with Rounding

Pitfall 4: Trying to Convert Invalid Strings

📝 Practice Questions

Q1: What will this code output?

python
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x = 5
y = x + 2
x = 10
print(y)

Answer:

pseudo
7

When y = x + 2 runs, x is 5, so y becomes 7. Changing x later doesn't affect y — y already has its own value 7. Q2: Evaluate the expression step by step:

python
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result = (5 + 3) * 2 ** 2 // 4 - 1
print(result)

Answer:

pseudo
7

Step-by-step:

1. (5 + 3) → 8
2. 2 ** 2 → 4
3. 8 * 4 → 32
4. 32 // 4 → 8
5. 8 - 1 → 7 Q3: What's the difference between print(10 / 3) and print(10 // 3)?

Answer:

• 10 / 3 → 3.3333333333333335 (true division, returns float)
• 10 // 3 → 3 (floor division, rounds DOWN to nearest integer)

Use / when you need the exact decimal value. Use // when you need a whole number (e.g., splitting items into groups). Q4: Why does this code produce an error? Fix it.

python
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value = "100"
result = value + 50
print(result)

Answer: Error: TypeError: can only concatenate str (not "int") to str Why: value is a string "100". You can't add a string and an integer. Fix: Convert to int: result = int(value) + 50 → outputs 150 Q5: Find the errors in this variable naming:

python
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my-var = 10
2nd_try = 20
class = 30

Answer:

• my-var — Invalid: hyphens are not allowed. Use my_var instead.
• 2nd_try — Invalid: starts with a digit. Use second_try instead.
• class — Invalid: class is a Python keyword. Use class_name instead. Q6: What does this code output and why?

python
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a, b = 10, 20
a, b = b, a + b
print(a, b)

Answer:

pseudo
20 30

• On the second line, Python evaluates the right side FIRST: b is 20, a + b is 10 + 20 = 30.
• Then it assigns: a = 20, b = 30. Q7: Write a program that converts a given number of seconds into hours, minutes, and seconds.

Answer:

python
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# runnable
total_seconds = 3665

hours = total_seconds // 3600        # 1 hour = 3600 seconds
minutes = (total_seconds % 3600) // 60
seconds = total_seconds % 60

print(total_seconds, "seconds =", hours, "hours,", minutes, "minutes,", seconds, "seconds")

Output:

pseudo
3665 seconds = 1 hours, 1 minutes, 5 seconds

Q8: Predict the output:

python
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x = 10
x += 5
x *= 2
x -= 7
x //= 3
print(x)

Answer:

pseudo
7

Step by step:

• Start: x = 10
• x += 5 → x = 15
• x *= 2 → x = 30
• x -= 7 → x = 23
• x //= 3 → x = 7 (23 ÷ 3 = 7.666..., floored to 7) Q9: What is the value of 7 % 2 and 7 % 5?

Answer:

• 7 % 2 → 1 (7 ÷ 2 = 3 remainder 1)
• 7 % 5 → 2 (7 ÷ 5 = 1 remainder 2)

The modulus operator gives the remainder after division. Think of it as "how many are left over after making as many equal groups as possible." Q10: Write code to check if a number is even or odd using the modulus operator.

Answer:

python
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# runnable
number = 17
remainder = number % 2
print("Remainder when divided by 2:", remainder)
print("Is it even?", remainder == 0)

Output:

pseudo
Remainder when divided by 2: 1
Is it even? False

If number % 2 == 0, the number is even. If number % 2 == 1, it's odd. This is one of the most common uses of the modulus operator.

🔗 Cross-References

• Next Topic: Input, Output & Formatted Strings — Getting user input and formatting output with f-strings.
• Previous Topic: Introduction to Programming & Python
• BSCS1001 Computational Thinking: Variables connect to the concept of "state" in computational problems.
• Reference: Python for Everybody, Chapter 2 — "Variables, expressions, and statements"
• Video: L5: Variables & literals in python, L6: Data types part 1 Join Discord Previous1. Intro to ProgrammingNext3. Input & Output