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
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
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.
PYTHONPATH will affect all installed Python versions/environments.
Be wary of setting this in your shell profile or global environment variables.
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
.pyd files on Windows and
.so files on other platforms. The
directory with the platform-independent Python modules is called
The directory with the extension modules is called
PYTHONHOME environment variable may be used to set the
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 is determined, the
prefix directory is found by first looking
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
Python on Unix will look for
lib/python3.11/os.py). On Windows
exec_prefix are the same,
however on other platforms
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,
exec_prefix are available at
site module is processed and
are added to the module search path. A common way to customize the search path is
usercustomize modules as described in
site module documentation.
Certain command line options may further affect path calculations.
-S for further details.
If Python is run in a virtual environment (as described at Virtual Environments and Packages)
exec_prefix are specific to the virtual environment.
pyvenv.cfg file is found alongside the main executable, or in the
directory one level above the executable, the following variations apply:
homeis an absolute path and
PYTHONHOMEis not set, this path is used instead of the path to the main executable when deducing
To completely override
sys.path create a
._pth file with the same
name as the shared library or executable (
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.
When the file exists, all registry and environment variables are ignored,
isolated mode is enabled, and
site is not imported unless one line in the
import site. Blank paths and lines starting with
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.
.pth files (without leading underscore) will be processed normally
site module when
import site has been specified.
If Python is embedded within another application
PyConfig structure can be used to initialize Python. The path specific
details are described at Python Path Configuration. Alternatively the older
can be used to bypass the initialization of the module search path.
Finding modules for detailed Windows notes.
Using Python on Unix platforms for Unix details.