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206 CHAPTER 10 ■ BATTERIES INCLUDED
WHY BOTHER?
Why would you want to do this, you may wonder. Why not just define everything in your main program? The
primary reason is code reuse. If you put your code in a module, you can use it in more than one of your programs,
which means that if you write a good client database and put it in a module called clientdb, you can use it
both when billing, when sending out spam (though I hope you won’t), and in any program that needs access
to your client data. If you hadn’t put this in a separate module, you would have to rewrite the code in each one
of these programs. So, remember: To make your code reusable, make it modular! (And, yes, this is definitely
related to abstraction.)
if __name__ == '__main__'
Modules are used to define things such as functions and classes, but every once in a while
(quite often, actually), it is useful to add some test code to a module that checks whether things
work as they should. For example, if you wanted to make sure that the hello function worked,
you might rewrite the module hello2 into a new one, hello3, defined in Listing 10-3.
Listing 10-3. A Simple Module with Some Problematic Test Code
# hello3.py
def hello():
print "Hello, world!"
# A test:
hello()
This seems reasonable—if you run this as a normal program, you will see that it works.
However, if you import it as a module, to use the hello function in another program, the test
code is executed, as in the first hello module in this chapter:
>>> import hello3
Hello, world!
>>> hello3.hello()
Hello, world!
This is not what you want. The key to avoiding it is “telling” the module whether it’s being
run as a program on its own, or being imported into another program. To do that, you need the
variable __name__:
>>> __name__
'__main__'
>>> hello3.__name__
'hello3'
As you can see, in the “main program” (including the interactive prompt of the interpreter),
the variable __name__ has the value '__main__', while in an imported module, it is set to the
name of that module. Therefore, you can make your module’s test code more well behaved by
putting in an if statement, as shown in Listing 10-4.