3.2. Names

Names are given to values in Python using an assignment statement. In an assignment, a name is followed by =, which is followed by any expression. The value of the expression to the right of = is assigned to the name. Once a name has a value assigned to it, the value will be substituted for that name in future expressions.

a = 10
b = 20
a + b

30

A previously assigned name can be used in the expression to the right of =.

quarter = 1/4
half = 2 * quarter
half

0.5

However, only the current value of an expression is assigned to a name. If that value changes later, names that were defined in terms of that value will not change automatically.

quarter = 4
half

0.5

Names must start with a letter, but can contain both letters and numbers. A name cannot contain a space; instead, it is common to use an underscore character _ to replace each space. Names are only as useful as you make them; it’s up to the programmer to choose names that are easy to interpret. Typically, more meaningful names can be invented than a and b. For example, to describe the sales tax on a $5 purchase in Berkeley, CA, the following names clarify the meaning of the various quantities involved.

purchase_price = 5
state_tax_rate = 0.075
county_tax_rate = 0.02
city_tax_rate = 0
sales_tax_rate = state_tax_rate + county_tax_rate + city_tax_rate
sales_tax = purchase_price * sales_tax_rate
sales_tax

0.475