Now that we've seen Python's core built-in object types, this chapter explores its fundamental statement forms. In simple terms, statements are the things you write to tell Python what your programs should do. If programs do things with stuff, statements are the way you specify what sort of things a program does. Python is a procedural, statement-based language; by combining statements, you specify a procedure that Python performs to satisfy a program's goals.
Another way to understand the role of statements is to revisit the concept hierarchy introduced in Chapter 4, which talked about built-in objects and the expressions used to manipulate them. This chapter climbs the hierarchy to the next level:
Programs are composed of modules.
Modules contain statements.
Statements contain expressions.
Expressions create and process objects.
At its core, Python syntax is composed of statements and expressions. Expressions process objects, and are embedded in statements. Statements code the larger logic of a program's operation—they use and direct expressions to process the objects we've already seen. Moreover, statements are where objects spring into existence (e.g., in expressions within assignment statements), and some statements create entirely new kinds of objects (functions, classes, and so on). Statements always exist in modules, which themselves are managed with statements.