Beginning Python - From Novice to Professional

CHAPTER 10 ■ BATTERIES INCLUDED 211
After the first statement (1), the contents of the __init__ module in drawing would be
available; the drawing and colors modules, however, would not be. After the second statement
(2), the colors module would be available, but only under its full name, drawing.colors. After
the third statement (3), the shapes module would be available, under its short name (that is,
simply shapes). Note that these statements are just examples. There is no need, for example, to
import the package itself before importing one of its modules as I have done here. The second
statement could very well be executed on its own, as could the third. You may nest packages
inside each other.
Exploring Modules
Before I tackle some of the standard library modules, I’ll show you how to explore modules on
your own. This is a valuable skill because you will encounter lots of useful modules in your
career as a Python programmer, and I couldn’t possibly cover all of them here. The current
standard library is large enough to warrant books all by itself (and such books have been
written)—and it’s growing. New modules are added with each release, and often some of the
modules undergo slight changes and improvements. Also, you will most certainly find several
useful modules on the Web, and being able to grok them quickly and easily will make your
programming much more enjoyable. 1
What’s in a Module?
The most direct way of probing a module is to investigate it in the Python interpreter. The first
thing you need to do is to import it, of course. Let’s say you’ve heard rumors about a standard
module called copy:
>>> import copy
No exceptions are raised—so it exists. But what does it do? And what does it contain?
Using dir
To find out what a module contains, you can use the dir function, which lists all the attributes
of an object (and therefore all functions, classes, variables, and so on of a module). If you try to
print out dir(copy), you get a long list of names. (Go ahead, try it.) Several of these names begin
with an underscore—a hint (by convention) that they aren’t meant to be used outside the module.
So let’s filter them out with a little list comprehension (check the section on list comprehension
in Chapter 5 if you don’t remember how this works):
>>> [name for name in dir(copy) if name[0] != '_']
['Error', 'PyStringMap', 'copy', 'deepcopy', 'error']
The list comprehension is the list consisting of all the names from dir(copy) that don’t
have an underscore as their first letter. This list is much less confusing than the full listing.
1. The term “grok” is hackerspeak, meaning “to understand fully,” taken from Robert A. Heinlein’s novel
Stranger in a Strange Land (Ace Books, reissue 1995).