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.
备注
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 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 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/python*majorversion*.*minorversion*/os.py
(lib/python3.11/os.py
). On Windows prefix
and exec_prefix
are the same, however on other platforms 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 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.
备注
Certain command line options may further affect path calculations. See -E, -I, -s and -S for further details.
Virtual environments
If Python is run in a virtual environment (as described at 虚拟环境和包) 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 deducingprefix
andexec_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.
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 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 site module when import site
has been specified.
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. Alternatively the older Py_SetPath() can be used to bypass the initialization of the module search path.
参见
查找模块 for detailed Windows notes.
在类Unix环境下使用Python for Unix details.