What are the Differences in Invocation when Executing Python Code with -m?

Understanding Invocation Differences in Python Code Execution with -m

The Python interpreter provides the -m option, which allows you to execute library modules as scripts within the __main__ module. This approach differs from running scripts directly without the -m option, leading to certain distinctions and implications.

Practical Differences

When using -m, Python manages packages differently. If you attempt to execute a package without -m, the interpreter will treat it as a regular script. In contrast, with -m, packages can be imported and relative imports will correctly consider the package as the starting point.

For example, consider the following code in a.py:

<code class="py">if __name__ == "__main__":
    print(__package__)
    print(__name__)</code>

Running python a.py results in:

None
__main__

Whereas, using python -m a yields:

""
__main__

Here, the __package__ variable is empty in the first case because the file is executed as a script. With -m, __package__ is set to an empty string since the module is not within a package.

Explanation of package and name

For __package__:

• For scripts run without -m, __package__ is set to None as they cannot be packages.
• For modules run with -m that are within packages, __package__ is set to the name of the package.

For __name__:

• It references the main module created when running scripts.
• For packages run with -m, it refers to the global namespace stored in sys.modules['__main__']. The main module in a package is executed when running the package with -m.

Implications for Package Execution

Running a package as a script with -m requires a __main__.py module within the package. This module acts as the entry point when Python executes the package using the -m switch.

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