The initialization of the :data:`sys.path` module search path
A module search path is initialized when Python starts. This module search path may be accessed at :data:`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 :option:`-c` command, or :option:`-m` module.
The :envvar:`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.
Note
:envvar:`PYTHONPATH` will affect all installed Python versions/environments. Be wary of setting this in your shell profile or global environment variables. The :mod:`site` module offers more nuanced techniques as mentioned below.
The next items added are the directories containing standard Python modules as
well as any :term:`extension module`s 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 :envvar:`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 :file:`python{majorversion}{minorversion}.zip` (python311.zip). On Windows
the zip archive is searched for in home and on Unix the archive is expected
to be in :file:`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 :file:`Lib\\os.py`.
Python on Unix will look for :file:`lib/python{majorversion}.{minorversion}/os.py`
(lib/python3.11/os.py). On Windows prefix and exec_prefix are the same,
however on other platforms :file:`lib/python{majorversion}.{minorversion}/lib-dynload`
(lib/python3.11/lib-dynload) is searched for and used as an anchor for
exec_prefix. On some platforms :file:`lib` may be :file:`lib64` or another value,
see :data:`sys.platlibdir` and :envvar:`PYTHONPLATLIBDIR`.
Once found, prefix and exec_prefix are available at
:data:`sys.base_prefix` and :data:`sys.base_exec_prefix` respectively.
If :envvar:`PYTHONHOME` is not set, and a pyvenv.cfg file is found alongside
the main executable, or in its parent directory, :data:`sys.prefix` and
:data:`sys.exec_prefix` get set to the directory containing pyvenv.cfg,
otherwise they are set to the same value as :data:`sys.base_prefix` and
:data:`sys.base_exec_prefix`, respectively.
This is used by :ref:`sys-path-init-virtual-environments`.
Finally, the :mod:`site` module is processed and :file:`site-packages` directories are added to the module search path. A common way to customize the search path is to create :mod:`sitecustomize` or :mod:`usercustomize` modules as described in the :mod:`site` module documentation.
Note
Certain command line options may further affect path calculations. See :option:`-E`, :option:`-I`, :option:`-s` and :option:`-S` for further details.
.. versionchanged:: 3.14 :data:`sys.prefix` and :data:`sys.exec_prefix` are now set to the ``pyvenv.cfg`` directory during the path initialization. This was previously done by :mod:`site`, therefore affected by :option:`-S`.
Virtual environments place a pyvenv.cfg file in their prefix, which causes
:data:`sys.prefix` and :data:`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 :data:`sys.base_prefix` and :data:`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 :mod:`site` initialization.
Please refer to :mod:`site`'s
:ref:`virtual environments documentation <site-virtual-environments-configuration>`.
Note
:envvar:`PYTHONHOME` overrides the pyvenv.cfg detection.
Note
There are other ways how "virtual environments" could be implemented, this
documentation refers implementations based on the pyvenv.cfg mechanism,
such as :mod:`venv`. Most virtual environment implementations follow the
model set by :mod:`venv`, but there may be exotic implementations that
diverge from it.
To completely override :data:`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 :data:`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.
When the file exists, all registry and environment variables are ignored,
isolated mode is enabled, and :mod:`site` is not imported unless one line in the
file specifies import site. Blank paths and lines starting with # are
ignored. Each path may be absolute or relative to the location of the file.
Import statements other than to site are not permitted, and arbitrary code
cannot be specified.
Note that .pth files (without leading underscore) will be processed normally
by the :mod:`site` module when import site has been specified.
If Python is embedded within another application :c:func:`Py_InitializeFromConfig` and the :c:type:`PyConfig` structure can be used to initialize Python. The path specific details are described at :ref:`init-path-config`.
.. seealso:: * :ref:`windows_finding_modules` for detailed Windows notes. * :ref:`using-on-unix` for Unix details.