Chapter 8. Assignment, Expressions, and Print

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:

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.

Table 8-1 summarizes Python's statement set. Part III deals with entries in the table through break and continue. You've informally been introduced to a few of the statements in Table 8-1. Part III will fill in details that were skipped earlier, introduce the rest of Python's procedural statement set, and cover the overall syntax model.

Table 8-1. Python statements

Statement	Role	Example
Assignment	Creating references	curly, moe, larry = 'good', 'bad', 'ugly'
Calls	Running 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	Sequence iteration	for x in mylist: print x
while/else	General loops	while 1: print 'hello'
pass	Empty placeholder	while 1: pass
break, continue	Loop jumps	while 1: if not line: break
try/except/ finally	Catching exceptions	try: action( ) except: print 'action error'
raise	Triggering exception	raise endSearch, location
import, from	Module access	import sys; from sys import stdin
def, return, yield	Building functions	def f(a, b, c=1, *d): return a+b+c+d[0]def gen(n): for i in n, yield i*2
class	Building objects	class subclass: staticData = [ ]
global	Namespaces	def function( ): global x, y; x = 'new'
del	Deleting references	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

Statements that have to do with larger program units—functions, classes, modules, and exceptions—lead to larger programming ideas, so they will each have a section of their own. More exotic statements like exec (which compiles and executes code constructed as strings) are covered later in the book, or in Python standard documentation.