10. Variables

A variable is a name used to refer to a memory location where a value is stored.
It can be thought of as a box that stores data.
In Python, the same variable can be reused to store values of any type.
e.g In rectangle_length = 2.3, the variable names is rectangle_length, the type is float (decimal) and the value is 2.3.
e.g In user_name = Annie, the variable names is user_name, the type is str (string) and the value is Annie.

Rules for Python variable names:

A variable name must start with a letter or the underscore character
A variable name cannot start with a number
A variable name can only contain alpha-numeric characters (A-z, 0-9) and underscores ( _ )
Variable names are case-sensitive (age, Age and AGE are three different variables)

10.1. Python Reserved Words

Reserved Words are keywords that have a set meaning and can’t be used for other purposes such as for variable names. Reserved words are case-sensitive and must be used exactly as shown. They are all entirely lowercase, except for False, None, and True.

The list of reserved words is:

False, None, True, and, as, assert, async, await, break, class, continue, def, del, elif, else, except, finally, for, from, global, if, import, in, is, lambda, nonlocal, not, or, pass, raise, return, try, while, with, yield

A list of keywords can be shown by using print(help("keywords")).

A second method for listing keywords uses the keyword library. (see: https://docs.python.org/3/library/keyword.html)

import keyword

print(keyword.kwlist)

10.2. Python conventions

It is helpful to use meaningful variable names that indicate what the variable is. e.g. height instead of x for the height of an object.

10.3. Snake case

Snake case is used for variables and functions.

Snake case uses underscores between words. e.g. player_count, player_1_score, player_2_score

10.4. ALL_CAPS

ALL_CAPS are used for constants, such as PI = 3.14, GRAVITY = 9.8, MILES_TO_METRES = 1609.

10.5. CapWords

CapWords, such as AnimalFeatures, are used for Class names in python.

10.6. camelCase

camelCase variables, such as playerScore, are not recommended in python.

10.7. KebabCase

KebabCase variables, such as player-score, are not recommended in python.

10.8. Sample code

The print statement below can be generalised using a variable for the team name and a variable for the number of premierships.

This is good practice since it separates the data from the output.

print('Federer has won 20 Grand Slams.')

Generalised to:

player_name = 'Federer'
wins = '20'
print(player_name + ' has won ' + wins + ' Grand Slams.')

10.9. Tasks

Questions

Write AGE in snake case.
Write MyName in snake case.
Write MyFirstNameLastName in snake case.
Write rectangleArea in snake case.
Write cm_in_an_inch = 2.14 as ALL_CAPS.
Write lbs_in_a_kg = 2.2 as ALL_CAPS.
A program asks for a person’s age and stores it. What would be a good variable to use: x, variable1, AGE, age, Years_Old?
A program uses a person’s first name and last name. What would be a good variable to use for their last name: x, variable1, SURNAME, last_name, Name?
A program calculates the area of a rectangle. What would be two good variables to use for the length and width of the rectangle: x, y, LENGTH, length, Width, width?