Beginning Python - From Novice to Professional

CHAPTER 10 ■ BATTERIES INCLUDED 205
If each module could be imported several times, you would end up with a problem here. The module
clientdb would import billing, which again imports clientdb, which . . . you get the picture. You get an
endless loop of imports (endless recursion, remember?). However, because nothing happens the second time
you import the module, the loop is broken.
If you insist on reloading your module, you can use the built-in function reload. It takes a single argument
(the module you want to reload) and returns the reloaded module. This may be useful if you have made
changes to your module and want those changes reflected in your program while it is running. To reload the
simple hello module (containing only a print statement), you would use the following:
>>> hello = reload(hello)
Hello, world!
Here I assume that hello has already been imported (once). By assigning the result of reload to hello,
I have replaced the previous version with the reloaded one. As you can see from the printed greeting, I am
really importing the module here.
Note that if you’ve created an object using a given module and you then reload that module, your object
will not be re-created. If you want your object to be based on the reloaded module, you will have to create it anew.
Modules Are Used to Define Things
So modules are executed the first time they are imported into your program. That seems sort of
useful—but not very. What makes them worthwhile is that they (just like classes) keep their
scope around afterward. That means that any class or function you define, and any variable
you assign a value to, become attributes of the module. This may seem complicated, but in
practice it is very simple. Let’s say you have written a module like the one in Listing 10-2 and
stored it in a file called hello2.py. Also assume that you’ve put it in a place where the Python
interpreter can find it, either using the sys.path trick from the previous section, or the more
conventional methods from the section “Making Your Modules Available,” which follows.
Listing 10-2. A Simple Module Containing a Function
# hello2.py
def hello():
print "Hello, world!"
You can then import it like this:
>>> import hello2
The module is then executed, which means that the function hello is defined in the scope
of the module, which means that you can access the function like this:
>>> hello2.hello()
Hello, world!
Any name defined in the global scope of the module will be available in the same manner.