Week 2 — Variables & Data Types

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

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.

💻 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.

📝 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.")