Variables, Names, and Casting

What You'll Learn

How to create variables, choose good names, and convert values between different types (casting).

What Is a Variable?

A variable is a named container for a value. Think of it like a labelled box.

score = 100       # the box is named "score" and holds 100
name = "Alice"    # the box is named "name" and holds "Alice"

You can change what's in the box at any time:

score = 100
print(score)   # 100

score = 200
print(score)   # 200

Creating Variables

Python creates variables automatically when you assign a value. No var, let, or int keyword needed:

age = 25
height = 1.75
username = "alice"
is_active = True

Multiple Assignment

Assign the same value to multiple variables:

x = y = z = 0
print(x, y, z)   # 0 0 0

Assign different values in one line:

first, second, third = 10, 20, 30
print(first, second, third)   # 10 20 30

Naming Rules

Python variable names must follow these rules:

Rule	Example
Letters, numbers, underscores only	user_name, score1
Cannot start with a number	❌ 1score → ✅ score1
Case-sensitive	Name and name are different variables
No reserved words	❌ if, for, class, return

Good names (descriptive):

user_age = 25
total_price = 49.99
is_logged_in = True

Bad names (avoid):

a = 25      # what is 'a'?
x1 = 49.99  # meaningless
t = True    # unclear

Python Variable Naming Convention

Python uses snake_case for variables and functions:

first_name = "Alice"     ✅
firstName = "Alice"      # camelCase — works but not Pythonic

Checking a Variable's Type

Use type() to see what type a value is:

name = "Alice"
age = 25
height = 1.75
is_active = True

print(type(name))      # <class 'str'>
print(type(age))       # <class 'int'>
print(type(height))    # <class 'float'>
print(type(is_active)) # <class 'bool'>

Casting (Type Conversion)

Casting means converting a value from one type to another.

Convert to integer

x = int("42")     # string "42" → integer 42
y = int(3.9)      # float 3.9 → integer 3 (truncated, not rounded)
z = int(True)     # True → 1

print(x, y, z)    # 42 3 1

Convert to float

a = float("3.14")   # string → float
b = float(10)       # integer → float

print(a, b)         # 3.14 10.0

Convert to string

age = 25
text = str(age)     # integer → string

print("Age: " + text)   # Age: 25  (can now concatenate)

Convert to boolean

print(bool(1))      # True
print(bool(0))      # False
print(bool(""))     # False
print(bool("hi"))   # True

A Common Beginner Pattern

When you get input from the user, it always comes as a string. You need to cast it:

user_input = "42"          # pretend this came from input()
number = int(user_input)   # convert to int to do math
result = number * 2
print(result)              # 84

Common Mistakes

Mistake	Example	Fix
Adding string and int	"Age: " + 25	Cast: "Age: " + str(25)
Casting invalid string	int("hello")	ValueError — only numbers in strings
Forgetting case sensitivity	Name vs name	Use consistent names
Starting name with number	1value = 5	SyntaxError — start with letter

Quick Reference

# Create
name = "Alice"
age = 25
height = 1.75

# Multiple
a, b, c = 1, 2, 3

# Check type
type(age)        # <class 'int'>

# Cast to int
int("42")        # 42
int(3.9)         # 3

# Cast to float
float("3.14")    # 3.14
float(10)        # 10.0

# Cast to string
str(100)         # "100"

# Cast to bool
bool(0)          # False
bool("hello")    # True

Practice Exercises

1. Store your info: Create variables for your name, age, and city. Print them all with an f-string.
2. Type check: Create 4 variables of different types and print type() for each.
3. Casting: Write a program that takes "100" as a string and prints "100" + 50 = 150 by casting correctly.
4. Swap: Assign a = 5 and b = 10, then swap their values and print both.

What's Next

→ Lesson 4: Comments and Docstrings