How is Python\'s `sys.path` Initialized?

Where is Python's sys.path Initialized From?

Python meticulously crafts sys.path to provide a robust search path for modules and packages. The initialization process involves various steps and considerations.

Steps to Determine sys.path:

1. Determine sys.executable: Python identifies its absolute path from the operating system. This path becomes sys.executable.

2. Establish sys.exec_prefix and sys.prefix:

   • Check for pyvenv.cfg in the same or parent directory of sys.executable.
   • Use the configured home value or the directory of sys.executable as sys.prefix.
   • Examine the PYTHONHOME environment variable.
   • Search for landmark files (e.g., lib/python/os.py) to determine sys.exec_prefix and sys.prefix.

3. Configure sys.path:

   • Add the script directory to sys.path (on Linux/Mac).
   • Include contents of PYTHONPATH.
   • Add the Python zip file.
   • Include paths from relevant registry keys (Windows only, unless applocal = true in pyvenv.cfg).

4. Compile-time and Dynamic Values:

   • Add paths specified in the PYTHONPATH macro (relative to sys.prefix).
   • Include sys.exec_prefix (in Linux/Mac) or sys.prefix directory (in Windows).

5. Fallback Prefix Determination:

   • If sys.prefix could not be determined, search for it dynamically within sys.path.

6. Site Package Import:

   • Import the site module, which further populates sys.path based on sys.prefix and sys.exec_prefix.

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