How does Python initialize the sys.path list?

Where is Python's sys.path Initialized From?

Python's sys.path, which contains the directories where Python searches for modules to import, undergoes a complex initialization process to determine its initial values. Here's a breakdown of how it's set up:

sys.executable Determination

• Python locates its physical location on the filesystem and uses it as the value for sys.executable.

sys.exec_prefix and sys.prefix Initialization

• Python reads the pyvenv.cfg file in the same directory as sys.executable or one directory up.

• If the file exists, it extracts the following information:

  • On Windows: If "applocal = true," sys.prefix is set to the directory containing sys.executable. Otherwise, it uses the "home = " directive.
  • On Linux/Mac: Sys.exec_prefix is set to the "home = " directive. Sys.prefix is also set to this value unless the PYTHONHOME environment variable exists.
• If pyvenv.cfg does not exist or does not contain the necessary information, Python attempts to determine sys.exec_prefix and sys.prefix by walking backward from sys.executable, looking for specific landmark files.

sys.path Initialization

1. Python adds the directory of the script being executed to sys.path.
2. On Linux/Mac, it adds the PYTHONPATH environment variable, if set.
3. It includes the zip file path (/lib/python35.zip or path/to/python.zip).
4. On Windows and without "applocal = true" in pyvenv.cfg, it adds subkeys from "HK_CURRENT_USERSoftwarePythonPythonCorePythonPath."
5. On Windows and without "applocal = true" in pyvenv.cfg, and if sys.prefix was not found, it adds the "core contents" of "HK_CURRENT_USERSoftwarePythonPythonCorePythonPath."
6. On Windows and without "applocal = true" in pyvenv.cfg, it adds subkeys from "HK_LOCAL_MACHINESoftwarePythonPythonCorePythonPath."
7. On Windows and without "applocal = true" in pyvenv.cfg, and if sys.prefix was not found, it adds the "core contents" of "HK_LOCAL_MACHINESoftwarePythonPythonCorePythonPath."
8. On Windows and with PYTHONPATH not set, prefix not found, and no registry keys present, it adds the relative compile-time value of PYTHONPATH.
9. It incorporates paths in the PYTHONPATH compile-time macro relative to the dynamically-found sys.prefix.
10. On Linux/Mac, it adds sys.exec_prefix. On Windows, it adds the directory used to search for sys.prefix.
11. On Windows and without "applocal = true" in pyvenv.cfg, it attempts to find sys.prefix by locating landmark files in sys.path. If unsuccessful, it leaves sys.prefix blank.

Additional Modification by the Site Module

• The site module further modifies sys.path by adding directories based on sys.prefix, sys.exec_prefix, and platform-specific paths.
• It inspects these new paths for configuration files, potentially adding more paths.

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