Variables

A variable is a name attached to a value. Python creates the variable the first time you assign to it, and you never declare its type.

Creating variables

age = 21
name = "Rohan"
height = 1.75
is_student = True

print(name, "is", age, "years old")

The = sign is assignment, not equality. It means "make age refer to 21".

Variables can change type

Because the type belongs to the value, not the name, a variable can point at a different kind of value later. This is legal but usually confusing, so avoid it in real code.

x = 10
print(x, type(x))
x = "ten"
print(x, type(x))

Naming rules

A variable name:

• must start with a letter or an underscore,
• can contain letters, digits and underscores,
• is case sensitive (total and Total are different),
• cannot be a keyword like if or class.

user_name = "ok"
_private = "ok"
score2 = "ok"
# 2score = "not ok"   starts with a digit
# user-name = "not ok"   hyphen is subtraction

Naming conventions

The Python style guide (called PEP 8) recommends:

• snake_case for variables and functions: total_price, send_email
• PascalCase for classes: BankAccount
• UPPER_CASE for constants that should never change: MAX_RETRIES = 3

Choose names that say what the value is. n is fine in a two line loop; in a 200 line script it is not.

Assigning several variables at once

a, b, c = 1, 2, 3
print(a, b, c)

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

Swapping two variables needs no temporary variable:

a, b = 5, 9
a, b = b, a
print(a, b)

Unpacking a collection

If the right hand side is a list or tuple, Python spreads its items across the names on the left. The counts must match.

fruits = ["apple", "banana", "cherry"]
first, second, third = fruits
print(second)

Printing variables

print() accepts several values and separates them with a space. For more control, f-strings (covered in the strings lessons) let you embed variables directly in text:

city = "Jaipur"
temp = 34
print(f"It is {temp} degrees in {city} today")

Practice

1. Create variables for your name, your city and the year you started college, then print one sentence that uses all three.
2. Store the numbers 3 and 8 in two variables, swap them in one line, and print the result.