Weeks 2–3 — Variables & Data Types · Operators & I/O

What you'll learn

• 1Declare and reassign variables
• 2Distinguish int, float, str, and bool
• 3Use type() to inspect a value's type
• 4Convert between types with int(), float(), str()
• 5Understand dynamic typing in Python
• 6Use all arithmetic operators including // and %
• 7Compare values with ==, !=, <, >, <=, >=
• 8Combine conditions with and, or, not
• 9Format output with f-strings
• 10Chain augmented assignment operators (+=, -=, *=)

Week 2 — Variables & Data Types

Explore Python's four core data types, learn how to assign variables, use type() to inspect values, and perform basic type conversions.

1. Variables

A variable is a named container for a value. Python uses dynamic typing — you don't declare the type, Python infers it.

x = 10
name = "Alice"
is_student = True
print(type(x))     # <class 'int'>

2. Core Data Types

age = 25            # int
price = 9.99        # float
greeting = "Hi!"    # str
active = True       # bool
print(type(age))    # <class 'int'>

3. Type Conversion

Convert between types using built-in functions: int(), float(), str(), bool().

num_str = "42"
num = int(num_str)       # "42" → 42
pi = float("3.14")       # "3.14" → 3.14
text = str(100)          # 100 → "100"
print(bool(0), bool(1))  # False True

4. User Input

input() always returns a string. Remember to convert when you need a number.

name = input("Enter your name: ")
age  = int(input("Enter your age: "))
print(f"Hello, {name}! You are {age} years old.")

5. Common Mistakes

1. Adding a number to a string without conversion — TypeError: can only concatenate str to str. Use str(num) + "text".
2. int("3.7") → ValueError. Use int(float("3.7")) instead.
3. Forgetting that input() returns str — age > 18 fails if age is a string.

Week 3 — Operators & I/O

Master Python's arithmetic, comparison, and logical operators, format output with f-strings, and build interactive programs with input().

1. Arithmetic Operators

2. Comparison & Logical Operators

print(5 > 3)        # True
print(5 == 5)       # True
print(5 != 3)       # True
print(True and False)  # False
print(True or False)   # True
print(not True)        # False

3. f-strings

f-strings (f"...") are the modern way to embed expressions inside string literals.

name = "Alice"
age = 20
pi = 3.14159
print(f"Hello, {name}!")
print(f"Age: {age}")
print(f"Pi ≈ {pi:.2f}")   # 2 decimal places

4. Augmented Assignment

x = 10
x += 5   # x = x + 5 = 15
x -= 3   # x = 12
x *= 2   # x = 24
print(x) # 24

5. Common Mistakes

1. = vs ==: = assigns, == compares. if x = 5 is a SyntaxError.
2. 7 / 2 → 3.5 (float), not 3. Use 7 // 2 for integer division.
3. "5" + 5 → TypeError. Convert first: int("5") + 5.

💻 Examples

Run these examples and check the output yourself.

x = 42
pi = 3.14
name = "Alice"
flag = True
print(x, type(x))
print(pi, type(pi))
print(name, type(name))
print(flag, type(flag))

42 <class 'int'>
3.14 <class 'float'>
Alice <class 'str'>
True <class 'bool'>

print(int("100"))      # 100
print(float("3.14"))   # 3.14
print(str(42))         # "42"
print(bool(0))         # False
print(bool("hi"))      # True

100
3.14
42
False
True

name = input("Name: ")
year = int(input("Birth year: "))
age = 2024 - year
print(f"{name} is about {age} years old.")

Name: Alice
Birth year: 2000
Alice is about 24 years old.

a, b = 17, 5
print(f"{a} + {b} = {a+b}")
print(f"{a} - {b} = {a-b}")
print(f"{a} * {b} = {a*b}")
print(f"{a} / {b} = {a/b:.2f}")
print(f"{a} // {b} = {a//b}")
print(f"{a} % {b} = {a%b}")
print(f"{a} ** {b} = {a**b}")

17 + 5 = 22
17 - 5 = 12
17 * 5 = 85
17 / 5 = 3.40
17 // 5 = 3
17 % 5 = 2
17 ** 5 = 1419857

name = "Alice"
score = 92.567
print(f"Student: {name}")
print(f"Score:   {score:.1f}")
print(f"Passed:  {score >= 60}")

Student: Alice
Score:   92.6
Passed:  True

a = float(input("First number: "))
b = float(input("Second number: "))
print(f"{a} + {b} = {a+b}")
print(f"{a} - {b} = {a-b}")
print(f"{a} * {b} = {a*b}")
if b != 0:
    print(f"{a} / {b} = {a/b:.4f}")
else:
    print("Division by zero!")

📝 Exercises

Try them yourself first, then open the solution to compare.

a = 10
b = 3.14
c = "hello"
d = True
e = None
for v in [a, b, c, d, e]:
    print(type(v).__name__, "→", v)

int → 10
float → 3.14
str → hello
bool → True
NoneType → None

Name: Alice
Birth year: 1998
Hello, Alice! You are 26 years old.

name = input("Name: ")
year = int(input("Birth year: "))
age = 2024 - year
print(f"Hello, {name}! You are {age} years old.")

Enter a number: 7
7 is odd

n = int(input("Enter a number: "))
if n % 2 == 0:
    print(f"{n} is even")
else:
    print(f"{n} is odd")

Weight (kg): 65
Height (m): 1.70
BMI: 22.49

w = float(input("Weight (kg): "))
h = float(input("Height (m): "))
bmi = w / h**2
print(f"BMI: {bmi:.2f}")