Chapter 3. Basic Statements

Now that we've seen Python's fundamental built-in object types, we're going to move on in this chapter to explore its basic statement types. 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. By and large, Python is a procedural, statement-based language; by combining statements, you specify a procedure Python performs to satisfy a program's goals.

Another way to understand the role of statements is to revisit the concept hierarchy we introduced in Chapter 2. In that chapter we talked about built-in objects; now we climb the hierarchy to the next level:

1. Programs are composed of modules.

2. Modules contain statements.

3. Statements create and process objects.

Statements process the objects we've already seen. Moreover, statements are where objects spring into existence (e.g., in assignment statement expressions), and some statements create entirely new kinds of objects (functions, classes, and so on). And although we won't discuss this in detail until Chapter 5, statements always exist in modules, which themselves are managed with statements.

Table 3.1 summarizes Python's statement set. We've introduced a few of these already; for instance, in Chapter 2, we saw that the del statement deletes data structure components, the assignment statement creates references to objects, and so on. In this chapter, we fill in details that were skipped and introduce the rest of Python's basic procedural statements. We stop short when statements that have to do with larger program units—functions, classes, modules, and exceptions—are reached. Since these statements lead to more sophisticated programming ideas, we'll give them each a chapter of their own. More exotic statements like exec (which compiles and executes code we create as strings) and assert are covered later in the book.

Table?.1. Python Statements

Statement	Role	Examples
Assignment	references	curly, moe, larry = 'good', 'bad', 'ugly'
Calls	functions	stdout.write("spam, ham, toast\n")
Print	Printing objects	print 'The Killer', joke
If/elif/else	Selecting actions	if "python" in text: print text
For/else	iteration	for x in mylist: print x
While/else	General loops	while 1: print 'hello'
Pass	placeholder	while 1: pass
Continue	Loop jumps	while 1: if not line: break
Try/except/finally	exceptions	try: action() except: print 'action error'
Raise	exception	raise endSearch, location
Import, From	Module access	import sys; from sys import stdin
Def, Return	functions	def f(a, b, c=1, *d): return a+b+c+d[0]
Class	Building objects	class subclass: staticData = []
Global	Namespaces	def function(): global x, y; x = 'new'
Del	Deleting things	del data[k]; del data[i:j]; del obj.attr
Exec	Running code strings	exec "import " + modName in gdict, ldict
Assert	Debugging checks	assert X > Y