How does Python\'s `sys.path` get its initial values?

Where Does Python's sys.path Get Its Initial Values?

The initialization of Python's sys.path involves a complex process that seeks to intelligently determine the most relevant paths for importing modules. Here's a detailed breakdown of the steps:

1. Python's Location Determination

Python ascertains its physical location by resolving any symbolic links and leveraging information from the operating system. This value is assigned to sys.executable.

2. Initialization of sys.exec_prefix and sys.prefix

If a pyvenv.cfg file exists in the directory containing sys.executable or one level up, its contents are examined.

• Linux and Mac: If the home configuration option is specified, it supersedes the sys.executable directory as the initial value for sys.prefix.
• Windows: The PYTHONHOME environment variable takes precedence. If set, it sets sys.prefix and sys.exec_prefix. However, if the pyvenv.cfg file contains a home setting, it overrides the PYTHONHOME value.

Otherwise, sys.exec_prefix and sys.prefix are determined by traversing backward from the location of sys.executable or the pyvenv.cfg directory. Specific landmark files used in this process are lib/python/dyn-load and lib/python/os.py.

3. Initial Population of sys.path

• The directory of the script being executed is included in sys.path.
• The content of the PYTHONPATH environment variable is added, except in specific scenarios (e.g., on Windows with applocal=true).
• The zip file path, either lib/python35.zip on Linux/Mac or os.path.join(os.dirname(sys.executable), "python.zip") on Windows, is added.

Additional Platform-Specific and Iterative Steps:

• Windows (non-applocal): Registry keys related to PythonPath are examined and added to sys.path.
• Windows (with unknown sys.prefix): A search is conducted through directories in sys.path for landmark files to dynamically determine sys.prefix, if possible.
• Mac and Linux: The value of sys.exec_prefix is appended to sys.path.
• Windows: The compilation-time defined paths in PYTHONPATH are added relative to sys.prefix (if found dynamically).
• All Platforms: Python loads the site module, which dynamically adds further paths to sys.path based on a comprehensive search process.

This detailed algorithm ensures that Python meticulously combines system-defined paths and user-provided environment variables to provide an intelligent and flexible search path for importing modules.

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