02. if __name__ == "__main__"¶

Overview¶

The following document explains the mechanism behind the special code snippet that commonly appears in Python scripts.

if __name__ == "__main__":

  # The following code block runs only when this script is executed directly, when the condition is match
  pass

Explanation¶

The snippet is related to the concept of the top-level code environment, which is the scope in which the interpreter executes the script directly.

This included 2 parts in the condition: __name__ and special literal of "__main__"

(1) The __name__ variable

The __name__ variable is a built-in variable that holds the name of the current module. When a module is imported, the interpreter sets the __name__ variable to the name of the module.

There are two ways to set the __name__ variable:

1. When a module is imported, the interpreter sets the __name__ variable to the name of the module.

Note: This is usually the name of the Python file itself without the .py extension.

If the file is part of a package, __name__ will also include the parent package’s path.

1. When a module is executed in the top-level code environment, the interpreter sets the __name__ variable to "__main__".

(2) The literal string of "__main__"

For the description, __main__ is the name of the environment where top-level code is run.

Top-level code” is the first user-specified Python module that starts running.

It’s "top-level" because it imports all other modules that the program needs.

Sometimes "top-level code" is called an entry point to the application.

The top-level code environment is created in one of following ways:

1. The scope of an interactive prompt

2. The Python module passed to the Python interpreter as a file argument

3. The Python module or package passed to the Python interpreter with the -m argument

4. Python code read by the Python interpreter from standard input

5. Python code passed to the Python interpreter with the -c argument.

For the example, reference to __main__.html detail from Python

Based on that, the idiom checks if the __name__ variable equals "__main__", confirming that the script is the top-level module.

This idiom has two main use cases:

1. Preventing unintended code execution during module imports: By checking if the current module is the top-level module, you can ensure that certain code snippets are only executed when the module is run directly (i.e., not when it's imported as a module by another script).

2. Adding script-specific logic: By using this idiom, you can include code that's specific to the script and won't affect module imports.

Please see the detail example more at Realpython if-name-main-python document

References¶

• Python main official