"site" --- Site-specific configuration hook
*******************************************

**原始碼：**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.

在 3.3 版的變更: 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/python*X.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.

在 3.5 版的變更: Support for the "site-python" directory has been
removed.

在 3.13 版的變更: On Unix, *Free threading* Python installations are
identified by the "t" suffix in the version-specific directory name,
such as "lib/python3.13t/".

在 3.14 版的變更: "site" is no longer responsible for updating
"sys.prefix" and "sys.exec_prefix" on 虛擬環境. This is now done
during the path initialization. As a result, under 虛擬環境,
"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()".

在 3.13 版的變更: The ".pth" files are now decoded by UTF-8 at first
and then by the *locale encoding* if it fails.

在 3.15 版的變更: ".pth" file lines starting with "import" are
deprecated.  During the deprecation period, such lines are still
executed (except in the case below), but a diagnostic message is
emitted only when the "-v" flag is given."import" lines in
"*name*.pth" are silently ignored when a matching "*name*.start" file
exists.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.

自從版本 3.15 後不推薦使用，將會自版本 3.20 中移除。: 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."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)
=====================================

在 3.15 版被加入.

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.

備註:

  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.

備註:

  Executable lines ("import" lines in "*name*.pth" files and
  "*name*.start" file 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.

備註:

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


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/python*X.Y*".  Suppose this has a subdirectory
"/usr/local/lib/python*X.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.

在 3.4 版的變更: Activation of rlcompleter and history was made
automatic.


模組內容
========

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/python*X.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\Python*XY*\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.

   在 3.3 版的變更: This function used to be called unconditionally.

site.addsitedir(sitedir, known_paths=None, *, defer_processing_start_files=False)

   Add a directory to sys.path and parse the ".pth" and ".start" files
   found in that directory.  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".

   While ".pth" and ".start" files are always parsed, set
   *defer_processing_start_files* to "True" to prevent processing the
   startup data found in those files, so that you can process them
   explicitly (this is typically used by the "main()" function).

   在 3.15 版的變更: 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.

   在 3.2 版被加入.

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".

   在 3.2 版被加入.

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.

   在 3.2 版被加入.


命令列介面
==========

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.

也參考:

  * **PEP 370** -- Per user site-packages directory

  * **PEP 829** -- Startup entry points and the deprecation of import
    lines in ".pth" files

  * sys.path 模組搜尋路徑的初始化 -- "sys.path" 的初始化。
