sys.path モジュール検索パスの初期化¶
モジュール検索パスは Python の開始時に初期化されます。このモジュール検索パスは、 sys.path でアクセス可能です。
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.base_prefix and sys.base_exec_prefix respectively.
If PYTHONHOME is not set, and a pyvenv.cfg file is found alongside
the main executable, or in its parent directory, sys.prefix and
sys.exec_prefix get set to the directory containing pyvenv.cfg,
otherwise they are set to the same value as sys.base_prefix and
sys.base_exec_prefix, respectively.
This is used by Virtual Environments.
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.
バージョン 3.14 で変更: sys.prefix and sys.exec_prefix are now set to the
pyvenv.cfg directory during the path initialization. This was previously
done by site, therefore affected by -S.
Virtual Environments¶
Virtual environments place a pyvenv.cfg file in their prefix, which causes
sys.prefix and sys.exec_prefix to point to them, instead of the
base installation.
The prefix and exec_prefix values of the base installation are available
at sys.base_prefix and sys.base_exec_prefix.
As well as being used as a marker to identify virtual environments,
pyvenv.cfg may also be used to configure the site initialization.
Please refer to site's
virtual environments documentation.
注釈
PYTHONHOME overrides the pyvenv.cfg detection.
_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。
埋め込みの Python¶
Python が他のアプリケーションに埋め込まれている場合は、 Py_InitializeFromConfig() と PyConfig 構造体が Python の初期化に使用可能です。パス固有の詳細は、 Python Path Configuration で説明されています。
参考
Windows の詳細についてのメモは、 モジュールの検索 を参照。
Unix の詳細は、 Unix プラットフォームで Python を使う を参照。