2.6. Testing Modules

Python modules are objects and have several useful attributes. You can use this to easily test your modules as you write them. Here's an example that uses the if __name__ trick.

if __name__ == "__main__":

Some quick observations before you get to the good stuff. First, parentheses are not required around the if expression. Second, the if statement ends with a colon, and is followed by indented code.

Like C, Python uses == for comparison and = for assignment. Unlike C, Python does not support in-line assignment, so there's no chance of accidentally assigning the value you thought you were comparing.

So why is this particular if statement a trick? Modules are objects, and all modules have a built-in attribute __name__. A module's __name__ depends on how you're using the module. If you import the module, then __name__ is the module's filename, without a directory path or file extension. But you can also run the module directly as a standalone program, in which case __name__ will be a special default value, __main__.

>>> import odbchelper
>>> odbchelper.__name__

Knowing this, you can design a test suite for your module within the module itself by putting it in this if statement. When you run the module directly, __name__ is __main__, so the test suite executes. When you import the module, __name__ is something else, so the test suite is ignored. This makes it easier to develop and debug new modules before integrating them into a larger program.

On MacPython, there is an additional step to make the if __name__ trick work. Pop up the module's options menu by clicking the black triangle in the upper-right corner of the window, and make sure Run as __main__ is checked.

Further Reading on Importing Modules