Variables — Python's version of a named cell

What's a variable?

A variable is a name that points to a value. In Excel, you might put 100 in cell B2 and refer to it as =B2 from then on. In Python you write:

price = 100

From now on, anywhere you write price, Python substitutes in 100.

price = 100
tax = price * 0.2
print(price + tax)   # 120

The single equals sign means "store this"

This trips people up at first: = in Python is not the same as = in algebra. It doesn't mean "equals." It means "take what's on the right and store it under the name on the left."

x = 5      # store 5 in x
x = x + 1  # take whatever x is (5), add 1, store back in x
print(x)   # 6

The naming rules

• Start with a letter or underscore. Can't start with a number.
• Made of letters, numbers, underscores. No spaces, no dashes, no punctuation.
• Case-sensitive: Price, price, and PRICE are three different variables.
• Don't use Python's reserved words (if, for, def, class, True, None…). Your editor will warn you.

Style: snake_case

Python convention is lowercase with underscores: monthly_total, customer_name, tax_rate. Not monthlyTotal (that's Java) or MonthlyTotal (that's a class). Stick to snake_case and your code will look professional.

Walkthrough: build a tiny invoice

customer_name = "Acme Co."
quantity = 3
unit_price = 19.99
tax_rate = 0.08

subtotal = quantity * unit_price
tax = subtotal * tax_rate
total = subtotal + tax

print("Customer:", customer_name)
print("Subtotal:", subtotal)
print("Tax:     ", tax)
print("Total:   ", total)

Customer: Acme Co.
Subtotal: 59.97
Tax:      4.7976
Total:    64.7676