What Does if __name__ == "__main__" Do in Python?

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November 30, 2024
What Does if __name__ == "__main__" Do in Python?
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The if __name__ == "__main__" idiom is a Python construct that helps control code execution in scripts. It’s a conditional statement that allows you to define code that runs only when the file is executed as a script, not when it’s imported as a module.

When you run a Python script, the interpreter assigns the value "__main__" to the __name__ variable. If Python imports the code as a module, then it sets __name__ to the module’s name instead. By encapsulating code within if __name__ == "__main__", you can ensure that it only runs in the intended context.

By the end of this tutorial, you’ll understand that:

  • Python’s if __name__ == "__main__" idiom allows code to run only when the script is executed, not when it’s imported.
  • The idiom checks if the __name__ variable equals "__main__", confirming that the script is the top-level module.
  • Using this idiom helps prevent unintended code execution during module imports.
  • It’s useful for adding script-specific logic, such as user input or test cases, without affecting module imports.
  • Best practices suggest using this idiom minimally and placing it at the bottom of the script for clarity.

You’ve likely encountered Python’s if __name__ == "__main__" idiom when reading other people’s code. No wonder—it’s widespread! Understanding Python’s if __name__ == "__main__" idiom will help you to manage script execution and module imports effectively. In this tutorial you’ll explore its mechanics, appropriate usage, and best practices.

Take the Quiz: Test your knowledge with our interactive “Python Name-Main Idiom” quiz. You’ll receive a score upon completion to help you track your learning progress:

What Does if __name__ == "__main__" Do in Python?

Interactive Quiz

Python Name-Main Idiom

Test your knowledge of Python's if __name__ == "__main__" idiom by answering a series of questions! You've probably encountered the name-main idiom and might have even used it in your own scripts. But did you use it correctly?

In Short: It Allows You to Execute Code When the File Runs as a Script, but Not When It’s Imported as a Module

For most practical purposes, you can think of the conditional block that you open with if __name__ == "__main__" as a way to store code that should only run when your file is executed as a script.

You’ll see what that means in a moment. For now, say you have the following file:

Python echo.py 
 1def echo(text: str, repetitions: int = 3) -> str:
 2    """Imitate a real-world echo."""
 3    echoes = [text[-i:].lower() for i in range(repetitions, 0, -1)]
 4    return "\n".join(echoes + ["."])
 5
 6if __name__ == "__main__":
 7    text = input("Yell something at a mountain: ")
 8    print(echo(text))

In this example, you define a function, echo(), that mimics a real-world echo by gradually printing fewer and fewer of the final letters of the input text.

Below that, in lines 6 to 8, you use the if __name__ == "__main__" idiom. This code starts with the conditional statement if __name__ == "__main__" in line 6. In the indented lines, 7 and 8, you then collect user input and call echo() with that input. These two lines will execute when you run echo.py as a script from your command line:

Shell 
$ python echo.py
Yell something at a mountain: HELLOOOO ECHOOOOOOOOOO
ooo
oo
o
.

When you run the file as a script by passing the file object to your Python interpreter, the expression __name__ == "__main__" returns True. The code block under if then runs, so Python collects user input and calls echo().

Try it out yourself! You can download all the code files that you’ll use in this tutorial from the link below:

At the same time, if you import echo() in another module or a console session, then the nested code won’t run:

Python 
>>> from echo import echo
>>> print(echo("Please help me I'm stuck on a mountain"))
ain
in
n
.

In this case, you want to use echo() in the context of another script or interpreter session, so you won’t need to collect user input. Running input() would mess with your code by producing a side effect when importing echo.

When you nest the code that’s specific to the script usage of your file under the if __name__ == "__main__" idiom, then you avoid running code that’s irrelevant for imported modules.

Nesting code under if __name__ == "__main__" allows you to cater to different use cases:

  • Script: When run as a script, your code prompts the user for input, calls echo(), and prints the result.
  • Module: When you import echo as a module, then echo() gets defined, but no code executes. You provide echo() to the main code session without any side effects.

By implementing the if __name__ == "__main__" idiom in your code, you set up an additional entry point that allows you to use echo() right from the command line.

There you go! You’ve now covered the most important information about this topic. Still, there’s more to find out, and there are some subtleties that can help you build a deeper understanding of this code specifically and Python more generally.

Read the full article at https://realpython.com/if-name-main-python/ »

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November 30, 2024 at 07:30PM
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