Waylon WalkerWhen 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 itself 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 itself is being run 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
https://waylonwalker.com/install-miniconda/
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.pyis 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 its executed. I personally think this is a fantastic idea.
etl
In my world of data analysis, we often set up 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 its common to hide the actual execution of these functions in an 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 to 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")
Related Links
- example from rich.live click framework's docs try it yourself in your browser with replit.com StackOverflow: What does if name == “main”: do?
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