What is if __name__ == "__main___", and how do I use it.

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When a python module is called it is assigned the __name__ of __main__ otherwise if it's imported it will be assigned the __name__ of the module.

Concrete example

Let's create a module to play with __name__ a bit. We will call this module nodes.py . It is a module that we may want to run by it'self or import and use in other modules.

#!python
# nodes.py

if __name__ == "nodes":
    import sys
    import __main__

    print(f"you have imported me {__name__} from {sys.modules['__main__'].__file__}")

if __name__ == "__main__":
    print("you are running me as main")

I have set this module up to execute one of two if statements based on whether the module it'self is being ran or if the module is being imported.

Note it is not common to have a if __name__ == "nodes": block, this is just for demnonstration purposes.

running python nodes.py

Running a python script with the command python <filename.py> will execute your script top to bottom.

python nodes.py

This will print out you are running me as main

If you don't already have python installed try using miniconda or replit.com

running ./nodes.py

You can also simply execute the script from bash if you first set the module to be executable.

chmod +x nodes.py
./nodes.py

Note once you have set the file to be executable, it will remain executable chmod +x nodes.py is only needed one time, even if you edit the file.

pipeline.py

Let's create a second module pipeline.py and import the first module nodes and see what happens.

#!python
# pipeline.py
import nodes

Just like nodes we can run pipeline either way if it's executable

python pipeline.py
# must run chmod +x pipeline.py first.
./pipeline.py

Either way it will print out you have imported me nodes from ./pipeline.py

REPL

If we were to import nodes from the repl we would see an error in this case, due to the fact that there is no __main__ file since it's a repl session.

Use Cases

The main use case for if __name__ == "__main__": is flexibility. Simply importing a module should not execute any code, print anything to the screen, change your filesystem, or generally have any side effects in most cases. It is something that most python users would not expect. We can use this block to make it such that the module can be both imported and executed.

rich

The rich library uses it to make examples of each module print to the screen if it's executed. I personally think this is a fantastic idea.

etl

In my world of data analysis we often setup a script of functions that will behave as an etl pipeline of sorts. Since we may want to reuse some of these functions in other scripts it's common to hide the actual execution of these functions in a if __name__ == "__main__": block so that we don't start making changes to the data simply by importing the module.

cli

Most cli applications will leverage if __name__ == "__main__": to run something when called as a script instead of being imported. This allows us dt do things such as testing much easier.

Check out the example on the first page of the click framework's docs

Recap

if __name__ == "__main__": is not so cryptic or scary, it's just looking to see if this module was called as a script or imported from somewhere else, and executing some different behavior based on how it was called.

if __name__ == "__main__":
    print("you are running me as main")


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