In python, unlike in C++/C that always has void main() to run the program,
it is usually stated as: if __name__ == “__main__” :
This post is just a note from stack overflow, so basically I just repost the same question in this link: http://stackoverflow.com/questions/419163/what-does-if-name-main-do
When the Python interpreter reads a source file, it executes all of the code found in it. Before executing the code, it will define a few special variables. For example, if the python interpreter is running that module (the source file) as the main program, it sets the special __name__ variable to have a value “__main__”. If this file is being imported from another module, __name__ will be set to the module’s name.
When your script is run by passing it as a command to the Python interpreter,
python myscript.pyall of the code that is at indentation level 0 gets executed. Functions and classes that are defined are, well, defined, but none of their code gets ran. Unlike other languages, there’s no main() function that gets run automatically – the main() function is implicitly all the code at the top level.
In this case, the top-level code is an if block. __name__ is a built-in variable which evaluate to the name of the current module. However, if a module is being run directly (as in myscript.py above), then __name__ instead is set to the string "__main__". Thus, you can test whether your script is being run directly or being imported by something else by testing
if __name__ == "__main__":
...If that code is being imported into another module, the various function and class definitions will be imported, but the main() code won’t get run.
As a basic example, consider the following two scripts:
# file one.py
def func():
print("func() in one.py")
print("top-level in one.py")
if __name__ == "__main__":
print("one.py is being run directly")
else:
print("one.py is being imported into another module")
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# file two.py
import one
print("top-level in two.py")
one.func()
if __name__ == "__main__":
print("two.py is being run directly")
else:
print("two.py is being imported into another module")Now, if you invoke the interpreter as
python one.pyThe output will be
top-level in one.py
one.py is being run directlyIf you run two.py instead:
python two.pyYou get
top-level in one.py
one.py is being imported into another module
top-level in two.py
func() in one.py
two.py is being run directlyThus, when module one gets loaded, its __name__ equals "one" instead of __main__.
Credits: Adam Rosenfeld