Names Point at Values

A value you cannot refer to again is not much use. If a program works out a total, it needs a way to hold on to that total and use it on the next line. A variable is a name you give to a value so you can use it later. You make one with a single equals sign: price = 12 gives the name price to the value 12.

The best picture of what that line does is a luggage tag. The value 12 is the suitcase. The name price is a tag tied to it. You can read the tag to find the suitcase, move the tag to another suitcase, or tie a second tag to the same one. That picture, a label rather than a box, is how Python really works, and one surprise in Chapter 6 makes sense only with it.

Assignment

The single equals sign is called assignment, and it does not mean "is equal to". It means: work out the value on the right, then tie the name on the left to it. The right side always goes first. After price = 12, any line that mentions price gets 12.

Changing a Value

A name can be tied to a new value at any time. The line price = price + 1 looks impossible as mathematics, but it is an instruction, not an equation. Read it right to left: take the value price points at now, add one, and tie the name price to the result. In the session below, printing price before and after shows 12 and then 13.

>>> price = 12
>>> print(price)
12
>>> price = price + 1
>>> print(price)
13

Good Names

Names follow a few firm rules. They may contain letters, digits and underscores; they may not start with a digit; and they may not contain spaces. Break a rule and Python stops before running anything. In the session below, a name starting with the digit 2 produces a SyntaxError about an invalid decimal literal: Python saw a digit and expected a number, not a name.

>>> 2nd_price = 5
  File "<python-input-4>", line 1
    2nd_price = 5
    ^
SyntaxError: invalid decimal literal

Beyond the rules there is a convention that almost all Python code follows. Names are written in lower case with underscores between words, such as monthly_total or first_name. The style is called snake_case. And a name should say what the value means. The two versions below do exactly the same thing. Only one of them can be understood a week later.

a = 48.5
b = 4
c = a / b

bill_total = 48.5
people = 4
share_each = bill_total / people

Short names do not make a program faster, and long names do not make it slower. Names cost nothing to run and everything to read.

Labels, Not Boxes

Because a name is a label, two names can point at the same value. After a = 12 and b = a, both names are tied to the same 12. With numbers and text that never causes a problem, because those values cannot be changed in place. With lists, which arrive in Chapter 4, it causes one of the classic surprises, and Chapter 6 comes back to it with this exact picture.