site — Site-specific configuration hook

Source code: Lib/site.py

This module is automatically imported during initialization. The automatic import can be suppressed using the interpreter’s -S option.

Importing this module normally appends site-specific paths to the module search path and adds callables, including help() to the built-in namespace. However, Python startup option -S blocks this, and this module can be safely imported with no automatic modifications to the module search path or additions to the builtins. To explicitly trigger the usual site-specific additions, call the main() function.

Changed in version 3.3: Importing the module used to trigger paths manipulation even when using -S.

It starts by constructing up to four directories from a head and a tail part. For the head part, it uses sys.prefix and sys.exec_prefix; empty heads are skipped. For the tail part, it uses the empty string and then lib/site-packages (on Windows) or lib/pythonX.Y[t]/site-packages (on Unix and macOS). (The optional suffix “t” indicates the free-threaded build, and is appended if "t" is present in the sys.abiflags constant.) For each of the distinct head-tail combinations, it sees if it refers to an existing directory, and if so, adds it to sys.path and also inspects the newly added path for configuration files.

Changed in version 3.5: Support for the “site-python” directory has been removed.

Changed in version 3.13: On Unix, Free threading Python installations are identified by the “t” suffix in the version-specific directory name, such as lib/python3.13t/.

Changed in version 3.14: site is no longer responsible for updating sys.prefix and sys.exec_prefix on Virtual Environments. This is now done during the path initialization. As a result, under Virtual Environments, sys.prefix and sys.exec_prefix no longer depend on the site initialization, and are therefore unaffected by -S.

When running under a virtual environment, the pyvenv.cfg file in sys.prefix is checked for site-specific configurations. If the include-system-site-packages key exists and is set to true (case-insensitive), the system-level prefixes will be searched for site-packages, otherwise they won’t. If the system-level prefixes are not searched then the user site prefixes are also implicitly not searched for site-packages.

The site module recognizes two startup configuration files of the form name.pth for path configurations, and name.start for pre-first-line code execution. Both files can exist in one of the four directories mentioned above. Within each directory, these files are sorted alphabetically by filename, then parsed in sorted order.

Path extensions (.pth files)

name.pth contains additional items (one per line) to be appended to sys.path. Items that name non-existing directories are never added to sys.path, and no check is made that the item refers to a directory rather than a file. No item is added to sys.path more than once. Blank lines and lines beginning with # are skipped.

For backward compatibility, lines starting with import (followed by space or tab) are executed with exec().

Changed in version 3.15: import lines in name.pth are ignored when a matching name.start file exists.

Deprecated since version 3.15, will be removed in version 3.20: import lines in name.pth files are deprecated and will be silently ignored in Python 3.18 and 3.19. In Python 3.20 a warning will be produced for import lines in name.pth files.

Startup entry points (.start files)

Added in version 3.15.

A startup entry point file is a file whose name has the form name.start and exists in one of the site-packages directories described above. Each file specifies entry points to be called during interpreter startup, using the pkg.mod:callable syntax understood by pkgutil.resolve_name().

Each non-blank line that does not begin with # must contain an entry point reference in the form pkg.mod:callable. The colon and callable portion are mandatory. Each callable is invoked with no arguments, and any return value is discarded.

.start files are processed after all .pth path extensions have been applied to sys.path, ensuring that paths are available before any startup code runs.

Unlike sys.path extensions from .pth files, duplicate entry points are not de-duplicated — if an entry point appears more than once, it will be called more than once.

If an exception occurs during resolution or invocation of an entry point, a traceback is printed to sys.stderr and processing continues with the remaining entry points.

.start files must be encoded in UTF-8.

PEP 829 defined the original specification for these features.

Note

If a name.start file exists alongside a name.pth file with the same base name, any import lines in the .pth file are ignored in favor of the entry points in the .start file.

Note

Executable lines (i.e. import lines in a name.pth file or name.start entry points) are always run at Python startup (unless -S is given to disable the site.py module entirely), regardless of whether a particular module is actually going to be used.

Note

name.start files invoke pkgutil.resolve_name() with strict=True, which requires the full pkg.mod:callable form.

Changed in version 3.13: The .pth files are now decoded by UTF-8 at first and then by the locale encoding if it fails.

Deprecated since version 3.15, will be removed in version 3.20: Decoding name.pth files in any encoding other than utf-8-sig is deprecated in Python 3.15, and support for decoding from the locale encoding will be removed in Python 3.20.

Changed in version 3.15: .pth file lines starting with import are deprecated. During the deprecation period, such lines are still executed, but a diagnostic message is emitted when the -v flag is given. If a name.start file with the same base name exists, import lines in name.pth files are silently ignored. See Startup entry points (.start files) and PEP 829.

Errors on individual lines no longer abort processing of the rest of the file. Each error is reported and the remaining lines continue to be processed.

Startup file examples

For example, suppose sys.prefix and sys.exec_prefix are set to /usr/local. The Python X.Y library is then installed in /usr/local/lib/pythonX.Y. Suppose this has a subdirectory /usr/local/lib/pythonX.Y/site-packages with three sub-subdirectories, foo, bar and spam, and two path configuration files, foo.pth and bar.pth. Assume foo.pth contains the following:

# foo package configuration

foo
bar
bletch

and bar.pth contains:

# bar package configuration

bar

Then the following version-specific directories are added to sys.path, in this order:

/usr/local/lib/pythonX.Y/site-packages/bar
/usr/local/lib/pythonX.Y/site-packages/foo

Note that bletch is omitted because it doesn’t exist; the bar directory precedes the foo directory because bar.pth comes alphabetically before foo.pth; and spam is omitted because it is not mentioned in either path configuration file.

Let’s say that there is also a foo.start file containing the following:

# foo package startup code

foo.submod:initialize()

Now, after sys.path has been extended as above, and before Python turns control over to user code, the foo.submod module is imported and the initialize() function from that module is called.

Migrating from import lines in .pth files to .start files

If your package currently ships a name.pth file, you can keep all sys.path extension lines unchanged. Only import lines need to be migrated.

To migrate, create a callable (taking zero arguments) within an importable module in your package. Reference it as a pkg.mod:callable entry point in a matching name.start file. Move everything on your import line after the first semi-colon into the callable() function.

If your package must straddle older Pythons that do not support PEP 829 and newer Pythons that do, change the import lines in your name.pth to use the following form:

import pkg.mod; pkg.mod.callable()

Older Pythons will execute these import lines, while newer Pythons will ignore them in favor of the name.start file. After the straddling period, remove all import lines from your .pth files.

sitecustomize

After these path manipulations, an attempt is made to import a module named sitecustomize, which can perform arbitrary site-specific customizations. It is typically created by a system administrator in the site-packages directory. If this import fails with an ImportError or its subclass exception, and the exception’s name attribute equals 'sitecustomize', it is silently ignored. If Python is started without output streams available, as with pythonw.exe on Windows (which is used by default to start IDLE), attempted output from sitecustomize is ignored. Any other exception causes a silent and perhaps mysterious failure of the process.

usercustomize

After this, an attempt is made to import a module named usercustomize, which can perform arbitrary user-specific customizations, if ENABLE_USER_SITE is true. This file is intended to be created in the user site-packages directory (see below), which is part of sys.path unless disabled by -s. If this import fails with an ImportError or its subclass exception, and the exception’s name attribute equals 'usercustomize', it is silently ignored.

Note that for some non-Unix systems, sys.prefix and sys.exec_prefix are empty, and the path manipulations are skipped; however the import of sitecustomize and usercustomize is still attempted.

Readline configuration

On systems that support readline, this module will also import and configure the rlcompleter module, if Python is started in interactive mode and without the -S option. The default behavior is to enable tab completion and to use ~/.python_history as the history save file. To disable it, delete (or override) the sys.__interactivehook__ attribute in your sitecustomize or usercustomize module or your PYTHONSTARTUP file.

Changed in version 3.4: Activation of rlcompleter and history was made automatic.

Module contents

site.PREFIXES

A list of prefixes for site-packages directories.

site.ENABLE_USER_SITE

Flag showing the status of the user site-packages directory. True means that it is enabled and was added to sys.path. False means that it was disabled by user request (with -s or PYTHONNOUSERSITE). None means it was disabled for security reasons (mismatch between user or group id and effective id) or by an administrator.

site.USER_SITE

Path to the user site-packages for the running Python. Can be None if getusersitepackages() hasn’t been called yet. Default value is ~/.local/lib/pythonX.Y[t]/site-packages for UNIX and non-framework macOS builds, ~/Library/Python/X.Y/lib/python/site-packages for macOS framework builds, and %APPDATA%\Python\PythonXY\site-packages on Windows. The optional “t” indicates the free-threaded build. This directory is a site directory, which means that .pth files in it will be processed.

site.USER_BASE

Path to the base directory for the user site-packages. Can be None if getuserbase() hasn’t been called yet. Default value is ~/.local for UNIX and macOS non-framework builds, ~/Library/Python/X.Y for macOS framework builds, and %APPDATA%\Python for Windows. This value is used to compute the installation directories for scripts, data files, Python modules, etc. for the user installation scheme. See also PYTHONUSERBASE.

site.main()

Adds all the standard site-specific directories to the module search path. This function is called automatically when this module is imported, unless the Python interpreter was started with the -S flag.

Changed in version 3.3: This function used to be called unconditionally.

site.addsitedir(sitedir, known_paths=None)

Add a directory to sys.path and process its .pth and .start files. Typically used in sitecustomize or usercustomize (see above).

The known_paths argument is an optional set of case-normalized paths used to prevent duplicate sys.path entries. When None (the default), the set is built from the current sys.path.

Changed in version 3.15: Also processes .start files. See Startup entry points (.start files). All .pth and .start files are now read and accumulated before any path extensions, import line execution, or entry point invocations take place.

site.getsitepackages()

Return a list containing all global site-packages directories.

Added in version 3.2.

site.getuserbase()

Return the path of the user base directory, USER_BASE. If it is not initialized yet, this function will also set it, respecting PYTHONUSERBASE.

Added in version 3.2.

site.getusersitepackages()

Return the path of the user-specific site-packages directory, USER_SITE. If it is not initialized yet, this function will also set it, respecting USER_BASE. To determine if the user-specific site-packages was added to sys.path ENABLE_USER_SITE should be used.

Added in version 3.2.

Command-line interface

The site module also provides a way to get the user directories from the command line:

$ python -m site --user-site
/home/user/.local/lib/python3.11/site-packages

If it is called without arguments, it will print the contents of sys.path on the standard output, followed by the value of USER_BASE and whether the directory exists, then the same thing for USER_SITE, and finally the value of ENABLE_USER_SITE.

--user-base

Print the path to the user base directory.

--user-site

Print the path to the user site-packages directory.

If both options are given, user base and user site will be printed (always in this order), separated by os.pathsep.

If any option is given, the script will exit with one of these values: 0 if the user site-packages directory is enabled, 1 if it was disabled by the user, 2 if it is disabled for security reasons or by an administrator, and a value greater than 2 if there is an error.

See also