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Grace Murray Hopper at the UNIVAC keyboard
Figure 2.1 credit: modification of work "Grace Hopper at Univac I console", courtesy of the Computer History Museum

A computer program is a sequence of statements that run one after the other. In Python, many statements consist of one or more expressions. An expression represents a single value to be computed. Ex: The expression 3*x - 5 evaluates to 7 when x is 4. Learning to recognize expressions opens the door for programming all kinds of interesting calculations.

Expressions are often a combination of literals, variables, and operators. In the previous example, 3 and 5 are literals, x is a variable, and * and - are operators. Expressions can be arbitrarily long, consisting of many calculations. Expressions can also be as short as one value. Ex: In the assignment statement x = 5, the literal 5 is an expression.

The Statements chapter introduced simple expressions like 1 * 2 and "Hi " + "there". This chapter explores other kinds of expressions for working with numbers and strings. The first section shows a great way to experiment with expressions using a Python shell. Later sections present more details about integers and floating-point numbers, explain how to import and use the math module, and show how to make long lines of code easier to read.

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