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Python 3: An interactive deep dive

Running Scripts

Understand how to run Python scripts directly and test modules by using the special __name__ attribute. Learn to execute code conditionally within a script to control program behavior and get started with your first Python program.

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Python modules are objects and have several useful attributes. You can use this to easily test your modules as you write them, by including a special block of code that executes when you run the Python file on the command line. Take the last few lines of humansize.py:

Python 3.5
from humansize import approximate_size
if __name__ == '__main__':
    print(approximate_size(1000000000000, False))
    print(approximate_size(1000000000000))

Everything in Python is an object. 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 what makes this if statement special? Well, 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.

Python 3.5
import humansize
print(humansize.__name__)
#humansize

But you can also run the module directly as a standalone program, in which case __name__ will be a special default value, __main__. Python will evaluate this if statement, find a true expression, and execute the if code block. In this case, to print two values.

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And that’s your first Python program!