The initialization of the sys.path module search path

A module search path is initialized when Python starts. This module search path may be accessed at sys.path.

The first entry in the module search path is the directory that contains the input script, if there is one. Otherwise, the first entry is the current directory, which is the case when executing the interactive shell, a -c command, or -m module.

The PYTHONPATH environment variable is often used to add directories to the search path. If this environment variable is found then the contents are added to the module search path.

注釈

PYTHONPATH will affect all installed Python versions/environments. Be wary of setting this in your shell profile or global environment variables. The site module offers more nuanced techniques as mentioned below.

The next items added are the directories containing standard Python modules as well as any extension modules that these modules depend on. Extension modules are .pyd files on Windows and .so files on other platforms. The directory with the platform-independent Python modules is called prefix. The directory with the extension modules is called exec_prefix.

The PYTHONHOME environment variable may be used to set the prefix and exec_prefix locations. Otherwise these directories are found by using the Python executable as a starting point and then looking for various 'landmark' files and directories. Note that any symbolic links are followed so the real Python executable location is used as the search starting point. The Python executable location is called home.

Once home is determined, the prefix directory is found by first looking for pythonmajorversionminorversion.zip (python311.zip). On Windows the zip archive is searched for in home and on Unix the archive is expected to be in lib. Note that the expected zip archive location is added to the module search path even if the archive does not exist. If no archive was found, Python on Windows will continue the search for prefix by looking for Lib\os.py. Python on Unix will look for lib/pythonmajorversion.minorversion/os.py (lib/python3.11/os.py). On Windows prefix and exec_prefix are the same, however on other platforms lib/pythonmajorversion.minorversion/lib-dynload (lib/python3.11/lib-dynload) is searched for and used as an anchor for exec_prefix. On some platforms lib may be lib64 or another value, see sys.platlibdir and PYTHONPLATLIBDIR.

Once found, prefix and exec_prefix are available at sys.prefix and sys.exec_prefix respectively.

Finally, the site module is processed and site-packages directories are added to the module search path. A common way to customize the search path is to create sitecustomize or usercustomize modules as described in the site module documentation.

注釈

Certain command line options may further affect path calculations. See -E, -I, -s and -S for further details.

仮想環境 (Virtual environments)

If Python is run in a virtual environment (as described at 仮想環境とパッケージ) then prefix and exec_prefix are specific to the virtual environment.

If a pyvenv.cfg file is found alongside the main executable, or in the directory one level above the executable, the following variations apply:

  • If home is an absolute path and PYTHONHOME is not set, this path is used instead of the path to the main executable when deducing prefix and exec_prefix.

_pth files

To completely override sys.path create a ._pth file with the same name as the shared library or executable (python._pth or python311._pth). The shared library path is always known on Windows, however it may not be available on other platforms. In the ._pth file specify one line for each path to add to sys.path. The file based on the shared library name overrides the one based on the executable, which allows paths to be restricted for any program loading the runtime if desired.

ファイルが存在したときは、全てのレジストリと環境変数は無視され、隔離モードになり、そのファイルに import site と指定していない限りは site がインポートできなくなります。 空行と # で始まる行は無視されます。 それぞれのパスはファイルの場所を指す絶対パスあるいは相対パスです。 site 以外のインポート文は許可されておらず、任意のコードも書けません。

import site を指定したときは、(アンダースコアが前に付かない) .pth ファイルは site モジュールにより通常通り処理されることに注意してください。

Embedded Python

If Python is embedded within another application Py_InitializeFromConfig() and the PyConfig structure can be used to initialize Python. The path specific details are described at Python Path Configuration.

参考