Variables and Types

Variables are used to store data in Python programs. They act as containers for values that can be used and manipulated.

What is a Variable?

• A variable is a name that refers to a value.
• You can think of it as a label attached to a value.

Example:

name = "Alice"
age = 25
height = 5.6

In this example:

• name stores the string "Alice".
• age stores the integer 25.
• height stores the floating-point number 5.6.

Variable Naming Rules

1. Names can contain letters, numbers, and underscores (_).
2. Names must start with a letter or an underscore, not a number.
3. Names cannot be the same as Python’s reserved keywords (e.g., if, for, class).

Valid and Invalid Names:

# Valid
first_name = "Alice"
_age = 30
user2 = "Bob"

# Invalid
2name = "Error"  # Starts with a number
first-name = "Error"  # Contains a hyphen
class = "Error"  # Reserved keyword

Data Types in Python

Python is dynamically typed, which means you don’t need to declare the type of a variable. The type is determined by the value assigned.

Common Data Types:

Checking the Type of a Variable

Use the type() function to check a variable’s type:

x = 42
print(type(x))  # Output: <class 'int'>

Reassigning Variables

Variables can be reassigned to values of different types:

x = 10  # x is an integer
x = "Python"  # x is now a string

Multiple Assignments

You can assign multiple variables in a single line:

a, b, c = 1, 2, 3

Or assign the same value to multiple variables:

x = y = z = 0

Constants

Constants are variables meant to remain unchanged. By convention, their names are written in uppercase:

PI = 3.14159

Practice Exercises

1. Create variables for:
   • Your name.
   • Your age.
   • Your favorite number.
2. Print the type of each variable using type().
3. Reassign one variable to a new value of a different type and print the result.

With this knowledge, you’re ready to start working with data in Python!