Variables and Expressions

Variables and expressions are the building blocks of data programming.

Expressions¶

An expression is a combination of values, variables, or even other expressions that produce a single value or result. From a science perspective, expressions are like research questions. We can calculate the answer to the following arithmetic expressions.

100 + 60
80 * 2
44 / 2
2 ** 3
3 * 4 + 5 * 6

The process of answering these questions is called evaluation. Python evaluates an expression (a question) to produce a value (an answer). With more complicated expressions, the order of operations might become unclear, so we can use parentheses to indicate precedence.

(12 - 2) / 5 * 3

Numbers and text¶

Every value has a type. Types represent specific forms of data with specific ways for using and combining them.

100 and 60 are examples of the int data type used to represent integers. We can add two int values using the + operator. The result of dividing two int values can be an int or a float (decimals, or real numbers).

100 + 60
100 / 60

What about text? The str type represents text enclosed by either ' (single quotes) or " (double quotes). Adding two str values with the + operator concatenates the strings.

"cse160"
"cse" + '160'
"100" + 60
"" # empty string

What happens if we try to add a str and an int?

"cse" + 160

Variables¶

A variable is a container for storing values. We assign values to variables using assignment statements.

x = 5
y = 34
z = 100 + 60

In Python, variable names follow some rules.

• Variable names must start with a letter, not a digit or special character.

• Variable names are case sensitive, so Total is different from total.

• Variable names should follow our standard Python style guidelines.

An assignment statement is executed through a two-step process:

1. Evaluating the expression on the right-hand side to a value

2. Storing that value in the variable shown on the left-hand side

Expressions can also refer to previously-assigned variables. In the following code snippet, what is stored in x, y, and z?

x = 5
y = 10 + 10
z = x + 1

Python Tutor¶

In the previous example, it wasn’t too hard to keep track of all three variables. But the more complicated programs we’ll run into in the future will require a more careful analysis of how Python evaluates programs. What are the values of first and second at the end of the following snippet?

first = 8
second = 19
first = first + second
second = first - second
first = first - second

After running the snippet, copy-paste the code in tutor.py into your Python console and run it to load Python Tutor, a free online tool for visualizing program evaluation.

Let’s try another example that will teach us a bit more about how variables and assignment statements work. What are the values of a, b, and c at the end of the following snippet?

a = 5
b = 10
c = b
a = a + 1
b = b - 1
c = c + a