The Object of This Chapter

In this chapter, you learned the following:

• An operating system is a program that manages your computer’s hardware on behalf of other programs. An interpreter or virtual machine is a program that sits on top of the operating system and runs your programs for you. The Python shell is an interpreter, translating your Python statements into language the operating system understands and translating the results back so you can see and use them.

• Programs are made up of statements, or instructions. These can be simple expressions like 3 + 4 and assignment statements like celsius = 20 (which create new variables or change the values of existing ones). There are many other kinds of statements in Python, and we’ll introduce them throughout the book.

• Every value in Python has a specific type, which determines what operations can be applied to it. The two types used to represent numbers are int and float. Floating-point numbers are approximations to real numbers.

• Python evaluates an expression by applying higher-precedence operators before lower-precedence operators. You can change that order by putting parentheses around subexpressions.

• Python stores every value in computer memory. A memory location containing a value is called an object.

• Variables are created by executing assignment statements. If a variable already exists because of a previous assignment statement, Python will use that one instead of creating a new one.

• Variables contain memory addresses of values. We say that variables refer to values.

• Variables must be assigned values before they can be used in expressions.