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Sometimes when you are importing from a module, you would like to know whether
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a modules function is being used as an import, or if you are using the original
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.py file of that module. In this case we can use the:
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      if __name__ == "__main__":
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line to determine this. For example:
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When your script is run by passing it as a command to the Python interpreter:
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    python myscript.py
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all of the code that is at indentation level 0 gets executed. Functions and
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classes that are defined are, well, defined, but none of their code gets ran.
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Unlike other languages, there's no main() function that gets run automatically
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- the main() function is implicitly all the code at the top level.
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In this case, the top-level code is an if block.  __name__ is a built-in variable
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 which evaluate to the name of the current module. However, if a module is being
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 run directly (as in myscript.py above), then __name__ instead is set to the
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 string "__main__". Thus, you can test whether your script is being run directly
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  or being imported by something else by testing
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    if __name__ == "__main__":
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        ...
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If that code is being imported into another module, the various function and
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class definitions will be imported, but the main() code won't get run. As a
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basic example, consider the following two scripts:
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    # file one.py
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    def func():
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        print("func() in one.py")
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    print("top-level in one.py")
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    if __name__ == "__main__":
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        print("one.py is being run directly")
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    else:
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        print("one.py is being imported into another module")
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and then:
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    # file two.py
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    import one
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    print("top-level in two.py")
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    one.func()
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    if __name__ == "__main__":
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        print("two.py is being run directly")
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    else:
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        print("two.py is being imported into another module")
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Now, if you invoke the interpreter as
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    python one.py
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The output will be
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    top-level in one.py
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one.py is being run directly
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If you run two.py instead:
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    python two.py
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You get
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  top-level in one.py
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  one.py is being imported into another module
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  top-level in two.py
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  func() in one.py
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  two.py is being run directly
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Thus, when module one gets loaded, its __name__ equals "one" instead of __main__.
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