Variables

A variable is a name bound to a value with = (assignment). Read x = 5 as "let the name x refer to 5," not "x equals 5".

>>> price = 20
>>> quantity = 3
>>> total = price * quantity
>>> print(total)
60

A variable can be rebound, that is, it can be made to point to a different value, potentially of a different type.

>>> n = 10
>>> n = n + 1
>>> print(n)
11
>>> n = "now text"
>>> print(n)
now text

Right side first

In n = n + 1, Python evaluates n + 1 first, using the current value of n then makes n refer to the result. That is, the right hand side of an assignment statement is always evaluated first.

The value cannot be on the left-hand side of an assignment statement

The following is illegal in Python:

10 = n

The left-hand side of an assignment statement must always be an identifier, a.k.a variable name.

Naming rules & style

use letters, digits, and underscores
a variable name cannot start with a digit
variable names are case-sensitive (total and Total are different names)
style: lowercase with_underscores
style: descriptive. Prefer tax_rate over tr

Assignment shortcuts

Multiple assignments on one line:

>>> x, y = 1, 2
>>> a = b = 0

Update and rebind:

>>> count = 0
>>> count += 5
>>> count -= 2
>>> print(count)
3