What Is the __main_

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∟What Is the __main__ Module

This section provides a quick introduction on the __main__ module, which is a special 'module' object created by the Python system as the first module to start executing user specified Python code.

What Is the __main__ Module? The __main__ module is a special "module" object created by the Python system as the first module to start executing user specified Python code. The __main__ module is referred as the entry point of the Python application execution.

The __main__ module can be identified by module's "__name__" attribute: __name__ == '__main__'.

If you start a Python interactive shell, you will be given an empty __main__ module:

herong$ python

# name of the current module
>>> __name__
'__main__'

# built-in attributes in the module
>>> dir()
['__annotations__', '__builtins__', '__doc__', '__loader__', 
 '__name__', '__package__', '__spec__']

>>> __annotations__
{}

>>> __builtins__
<module 'builtins' (built-in)>

>>> __doc__

>>> __loader__
<class '_frozen_importlib.BuiltinImporter'>

>>> __name__
'__main__'

>>> __package__

>>> __spec__

If you start Python with a given Python code file, the code file will be loaded as the __main__ module and executed immediately.

To test this behavior, I cloned another copy of module_test.py to print the current module __name__ value:

#  module_test_3.py
#- Copyright 2011 (c) HerongYang.com. All Rights Reserved.
#
print("Hi there! - from 'module_test' module")

version = "1.00 - from 'version' attribute"

def help():
   print("How can I help you? - from 'help()' function")

class first:
   print("Welcome on board! - from 'first' class")
   count = 0
   def rise(self):
      first.count += 1
      print(f"count = {first.count} - from 'rise()' method")

print(f"Current module __name__ = {__name__} - from module_test.py")

Now start Python with module_test_3.py:

herong$ python module_test_3.py

Hi there! - from 'module_test' module
Welcome on board! - from 'first' class

current module __name__ = __main__ - from module_test.py

Since a Python code file can also be executed and imported as a module file, we can design it to have different execution branches for two execution mode:

1. Executed as a standalone application - The code file is loaded by the "python" command line program, and executed under the __main__ module. If the code file is designed to offer functions and classes to other applications, you can use this mode to test those functions and classes.

2. Executed as a supporting module to other applications - The code file is loaded by an "import" statement in other applications, and executed under the module named after its file name. If the code file is designed only as a standalone application, you can use this mode to raise an error or terminate the execution.

The code file below shows you how to write different branches for different execution mode.

#  module_test_4.py
#- Copyright 2011 (c) HerongYang.com. All Rights Reserved.

version = "1.00 - from 'version' attribute"

def help():
   print("How can I help you? - from 'help()' function")

class first:
   print("Welcome on board! - from 'first' class")
   count = 0
   def rise(self):
      first.count += 1
      print(f"count = {first.count} - from 'rise()' method")

print(f"__name__ = {__name__}")

# standalone application code branch
if __name__ == '__main__':
   print(f"Running in standalone mode: do real work or test only")

# supporting module code branch
elif __name__ == 'module_test_4':
   print(f"Running in import mode: be quiet or stop it")

else:
   print(f"Should never reach here: stop it")

If you run the above code file with the "python" command, you will get:

herong$ python module_test_4.py

Welcome on board! - from 'first' class
__name__ = __main__
Running in standalone mode: do real work or test only

If you import the above code file in another Python application, you will get:

herong$ python

>>> import module_test_4 as m
Welcome on board! - from 'first' class
__name__ = module_test_4
Running in import mode: be quiet or stop it