What are the purposes and benefits of using the `-m` switch in Python?

What is the Purpose of the -m Switch?

Background: The Problem

Consider the following two code snippets:

python -m mymod1 mymod2.py args

python mymod1.py mymod2.py args

In both cases, mymod1.py is called and sys.argv is set to ['mymod1.py', 'mymod2.py', 'args']. This raises the question: what is the purpose of the -m switch?

The Answer: The Role of -m

The -m switch serves several important purposes:

1. Execute Python Code from the Command Line: It allows you to execute Python code from the command line by specifying the module name instead of the filename. This is useful when you know the module name but not the filename, particularly for standard library modules or third-party modules.
2. Add a Directory to sys.path: Python determines module filenames based on its sys.path. The -m switch adds the current directory to sys.path, allowing you to import modules from the current directory.
3. Execute Code with Relative Imports: When executing modules with relative imports from the command line, the -m switch sets the __package__ variable to the parent module of the specified module name. This enables relative imports to work correctly.

Use Cases

Two notable use cases for the -m switch are:

1. Running Standard Library or Third-Party Modules: If you know the module name but not the filename, you can use -m to run it from the command line, e.g., python -m http.server.
2. Executing Local Packages with Imports: You can execute a local package containing absolute or relative imports using -m, without having to install it. This is similar to using pip install -e . in development mode.

Detailed Comparisons

To illustrate the differences between module execution methods, here are detailed comparisons:

Execution via Import Statement:

• sys.path is unmodified.
• __name__ is set to the absolute module name.
• __package__ is set to the immediate parent package.
• __init__.py is evaluated for all packages and the code module.

Execution via Command Line with Filename:

• sys.path is modified to include the filename's parent directory.
• __name__ is set to '__main__'.
• __package__ is set to None.
• __init__.py is not evaluated for any package.

Execution via Command Line with Module Name:

• sys.path is modified to include the current directory.
• __name__ is set to '__main__'.
• __package__ is set to the immediate parent package of the module name.
• __init__.py is evaluated for all packages and the code module.

Conclusion

The -m switch is a versatile tool that enhances Python's execution capabilities. It enables convenient module execution, incorporates features of importing and command line execution, and allows local package execution with relative imports.

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