How __main.py__ Can Be Used To Execute Directory With Python

Pavol Kutaj

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Jan 26, 2022

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1. NOTES

• python lets you specify the main entry point which is run when a directory is executed by python
• the main entry point must be called __main__.py
• for example, my python app is located in the support-kb folder
• without __main__.py, when I call the folder from one-folder-up

~\Documents\ ▶ python support-kb
can't find '__main__' module in 'support-kb

• if I put the file called __main__.py in the support-kb folder, python will execute it as an entry point to the app
• you can create that file and once you call the folder, whatever is in __main__.py will get executed

~\Documents\support-kb ▶ echo "print('executing __main.py__ of support-kb')" > __main__.py
~\Documents\support-kb ▶ cd ..
~\Documents\ ▶ python support-kb
executing __main.py__ of support-kb

• what happens is that a directory that is being “executed” is first added to sys.path
• this way, __main__.py can easily import any other modules with which it shares a directory
• in this way, the directory can be conceptualized as a program
• and this helps to organize a program in better ways:
• use separate modules for the logically distinct parts of your program

2. CONSEQUENCES/CONSTRAINTS

• there are consequences to this approach!
• the first is that your folder name has to be compliant with module names: all-lowercase, separated exclusivelly with underscore
• this means that support-kb is an incorrect module name and I can't use it in import statements
• my workaround is to start the __main__.py by appending "" to sys.path and change the CWD to the name of the actual directory

# __main.py__
sys.path.append("")
os.chdir("./support-kb")

3. EXAMPLE

• the __main__.py could contain the main menu for running different scripts
• i’ve suggested as much in How to Create a Menu in Python that runs specific scripts when selected — Stack Overflow

4. LINKS

• https://docs.python.org/3/library/__main__.html#main-py-in-python-packages
• How to Create a Menu in Python that runs specific scripts when selected — Stack Overflow
• Core Python: Organizing Larger Programs @ Pluralsight