"sys" --- 系統特定的參數與函式
******************************

======================================================================

This module provides access to some variables used or maintained by
the interpreter and to functions that interact strongly with the
interpreter. It is always available.

sys.abiflags

   On POSIX systems where Python was built with the standard
   "configure" script, this contains the ABI flags as specified by
   **PEP 3149**.

   在 3.2 版新加入.

   在 3.8 版的變更: Default flags became an empty string ("m" flag for
   pymalloc has been removed).

sys.addaudithook(hook)

   Append the callable *hook* to the list of active auditing hooks for
   the current (sub)interpreter.

   When an auditing event is raised through the "sys.audit()"
   function, each hook will be called in the order it was added with
   the event name and the tuple of arguments. Native hooks added by
   "PySys_AddAuditHook()" are called first, followed by hooks added in
   the current (sub)interpreter.  Hooks can then log the event, raise
   an exception to abort the operation, or terminate the process
   entirely.

   Note that audit hooks are primarily for collecting information
   about internal or otherwise unobservable actions, whether by Python
   or libraries written in Python. They are not suitable for
   implementing a "sandbox". In particular, malicious code can
   trivially disable or bypass hooks added using this function. At a
   minimum, any security-sensitive hooks must be added using the C API
   "PySys_AddAuditHook()" before initialising the runtime, and any
   modules allowing arbitrary memory modification (such as "ctypes")
   should be completely removed or closely monitored.

   Calling "sys.addaudithook()" will itself raise an auditing event
   named "sys.addaudithook" with no arguments. If any existing hooks
   raise an exception derived from "RuntimeError", the new hook will
   not be added and the exception suppressed. As a result, callers
   cannot assume that their hook has been added unless they control
   all existing hooks.

   所有會被 CPython 所引發的事件請參考稽核事件總表、設計相關討論請見
   **PEP 578**。

   在 3.8 版新加入.

   在 3.8.1 版的變更: Exceptions derived from "Exception" but not
   "RuntimeError" are no longer suppressed.

   **CPython 實作細節：** When tracing is enabled (see "settrace()"),
   Python hooks are only traced if the callable has a "__cantrace__"
   member that is set to a true value. Otherwise, trace functions will
   skip the hook.

sys.argv

   The list of command line arguments passed to a Python script.
   "argv[0]" is the script name (it is operating system dependent
   whether this is a full pathname or not).  If the command was
   executed using the "-c" command line option to the interpreter,
   "argv[0]" is set to the string "'-c'".  If no script name was
   passed to the Python interpreter, "argv[0]" is the empty string.

   To loop over the standard input, or the list of files given on the
   command line, see the "fileinput" module.

   另請參閱 "sys.orig_argv"。

   備註:

     On Unix, command line arguments are passed by bytes from OS.
     Python decodes them with filesystem encoding and
     "surrogateescape" error handler. When you need original bytes,
     you can get it by "[os.fsencode(arg) for arg in sys.argv]".

sys.audit(event, *args)

   Raise an auditing event and trigger any active auditing hooks.
   *event* is a string identifying the event, and *args* may contain
   optional arguments with more information about the event.  The
   number and types of arguments for a given event are considered a
   public and stable API and should not be modified between releases.

   舉例來說，一個名為 "os.chdir" 的稽核事件擁有一個引數 *path*，其內容
   為所要求的新工作目錄。

   "sys.audit()" will call the existing auditing hooks, passing the
   event name and arguments, and will re-raise the first exception
   from any hook. In general, if an exception is raised, it should not
   be handled and the process should be terminated as quickly as
   possible. This allows hook implementations to decide how to respond
   to particular events: they can merely log the event or abort the
   operation by raising an exception.

   Hooks are added using the "sys.addaudithook()" or
   "PySys_AddAuditHook()" functions.

   The native equivalent of this function is "PySys_Audit()". Using
   the native function is preferred when possible.

   所有會被 CPython 所引發的事件請參考稽核事件總表。

   在 3.8 版新加入.

sys.base_exec_prefix

   Set during Python startup, before "site.py" is run, to the same
   value as "exec_prefix". If not running in a virtual environment,
   the values will stay the same; if "site.py" finds that a virtual
   environment is in use, the values of "prefix" and "exec_prefix"
   will be changed to point to the virtual environment, whereas
   "base_prefix" and "base_exec_prefix" will remain pointing to the
   base Python installation (the one which the virtual environment was
   created from).

   在 3.3 版新加入.

sys.base_prefix

   Set during Python startup, before "site.py" is run, to the same
   value as "prefix". If not running in a virtual environment, the
   values will stay the same; if "site.py" finds that a virtual
   environment is in use, the values of "prefix" and "exec_prefix"
   will be changed to point to the virtual environment, whereas
   "base_prefix" and "base_exec_prefix" will remain pointing to the
   base Python installation (the one which the virtual environment was
   created from).

   在 3.3 版新加入.

sys.byteorder

   An indicator of the native byte order.  This will have the value
   "'big'" on big-endian (most-significant byte first) platforms, and
   "'little'" on little-endian (least-significant byte first)
   platforms.

sys.builtin_module_names

   A tuple of strings containing the names of all modules that are
   compiled into this Python interpreter.  (This information is not
   available in any other way --- "modules.keys()" only lists the
   imported modules.)

   See also the "sys.stdlib_module_names" list.

sys.call_tracing(func, args)

   Call "func(*args)", while tracing is enabled.  The tracing state is
   saved, and restored afterwards.  This is intended to be called from
   a debugger from a checkpoint, to recursively debug or profile some
   other code.

   Tracing is suspended while calling a tracing function set by
   "settrace()" or "setprofile()" to avoid infinite recursion.
   "call_tracing()" enables explicit recursion of the tracing
   function.

sys.copyright

   A string containing the copyright pertaining to the Python
   interpreter.

sys._clear_type_cache()

   Clear the internal type cache. The type cache is used to speed up
   attribute and method lookups. Use the function *only* to drop
   unnecessary references during reference leak debugging.

   This function should be used for internal and specialized purposes
   only.

sys._current_frames()

   Return a dictionary mapping each thread's identifier to the topmost
   stack frame currently active in that thread at the time the
   function is called. Note that functions in the "traceback" module
   can build the call stack given such a frame.

   This is most useful for debugging deadlock:  this function does not
   require the deadlocked threads' cooperation, and such threads' call
   stacks are frozen for as long as they remain deadlocked.  The frame
   returned for a non-deadlocked thread may bear no relationship to
   that thread's current activity by the time calling code examines
   the frame.

   This function should be used for internal and specialized purposes
   only.

   引發一個不附帶引數的稽核事件 "sys._current_frames"。

sys._current_exceptions()

   Return a dictionary mapping each thread's identifier to the topmost
   exception currently active in that thread at the time the function
   is called. If a thread is not currently handling an exception, it
   is not included in the result dictionary.

   This is most useful for statistical profiling.

   This function should be used for internal and specialized purposes
   only.

   引發一個不附帶引數的稽核事件 "sys._current_exceptions"。

sys.breakpointhook()

   This hook function is called by built-in "breakpoint()".  By
   default, it drops you into the "pdb" debugger, but it can be set to
   any other function so that you can choose which debugger gets used.

   The signature of this function is dependent on what it calls.  For
   example, the default binding (e.g. "pdb.set_trace()") expects no
   arguments, but you might bind it to a function that expects
   additional arguments (positional and/or keyword).  The built-in
   "breakpoint()" function passes its "*args" and "**kws" straight
   through.  Whatever "breakpointhooks()" returns is returned from
   "breakpoint()".

   The default implementation first consults the environment variable
   "PYTHONBREAKPOINT".  If that is set to ""0"" then this function
   returns immediately; i.e. it is a no-op.  If the environment
   variable is not set, or is set to the empty string,
   "pdb.set_trace()" is called. Otherwise this variable should name a
   function to run, using Python's dotted-import nomenclature, e.g.
   "package.subpackage.module.function". In this case,
   "package.subpackage.module" would be imported and the resulting
   module must have a callable named "function()".  This is run,
   passing in "*args" and "**kws", and whatever "function()" returns,
   "sys.breakpointhook()" returns to the built-in "breakpoint()"
   function.

   Note that if anything goes wrong while importing the callable named
   by "PYTHONBREAKPOINT", a "RuntimeWarning" is reported and the
   breakpoint is ignored.

   Also note that if "sys.breakpointhook()" is overridden
   programmatically, "PYTHONBREAKPOINT" is *not* consulted.

   在 3.7 版新加入.

sys._debugmallocstats()

   Print low-level information to stderr about the state of CPython's
   memory allocator.

   If Python is built in debug mode ("configure --with-pydebug
   option"), it also performs some expensive internal consistency
   checks.

   在 3.3 版新加入.

   **CPython 實作細節：** This function is specific to CPython.  The
   exact output format is not defined here, and may change.

sys.dllhandle

   Integer specifying the handle of the Python DLL.

   適用：Windows。

sys.displayhook(value)

   If *value* is not "None", this function prints "repr(value)" to
   "sys.stdout", and saves *value* in "builtins._". If "repr(value)"
   is not encodable to "sys.stdout.encoding" with "sys.stdout.errors"
   error handler (which is probably "'strict'"), encode it to
   "sys.stdout.encoding" with "'backslashreplace'" error handler.

   "sys.displayhook" is called on the result of evaluating an
   *expression* entered in an interactive Python session.  The display
   of these values can be customized by assigning another one-argument
   function to "sys.displayhook".

   Pseudo-code:

      def displayhook(value):
          if value is None:
              return
          # Set '_' to None to avoid recursion
          builtins._ = None
          text = repr(value)
          try:
              sys.stdout.write(text)
          except UnicodeEncodeError:
              bytes = text.encode(sys.stdout.encoding, 'backslashreplace')
              if hasattr(sys.stdout, 'buffer'):
                  sys.stdout.buffer.write(bytes)
              else:
                  text = bytes.decode(sys.stdout.encoding, 'strict')
                  sys.stdout.write(text)
          sys.stdout.write("\n")
          builtins._ = value

   在 3.2 版的變更: Use "'backslashreplace'" error handler on
   "UnicodeEncodeError".

sys.dont_write_bytecode

   If this is true, Python won't try to write ".pyc" files on the
   import of source modules.  This value is initially set to "True" or
   "False" depending on the "-B" command line option and the
   "PYTHONDONTWRITEBYTECODE" environment variable, but you can set it
   yourself to control bytecode file generation.

sys._emscripten_info

   A *named tuple* holding information about the environment on the
   *wasm32-emscripten* platform. The named tuple is provisional and
   may change in the future.

   _emscripten_info.emscripten_version

      Emscripten version as tuple of ints (major, minor, micro), e.g.
      "(3, 1, 8)".

   _emscripten_info.runtime

      運行環境字串，例如瀏覽器使用者代理 (browser user agent)
      "'Node.js v14.18.2'" 或 "'UNKNOWN'"。

   _emscripten_info.pthreads

      "True" if Python is compiled with Emscripten pthreads support.

   _emscripten_info.shared_memory

      "True" if Python is compiled with shared memory support.

   適用：Emscripten。

   在 3.11 版新加入.

sys.pycache_prefix

   If this is set (not "None"), Python will write bytecode-cache
   ".pyc" files to (and read them from) a parallel directory tree
   rooted at this directory, rather than from "__pycache__"
   directories in the source code tree. Any "__pycache__" directories
   in the source code tree will be ignored and new ".pyc" files
   written within the pycache prefix. Thus if you use "compileall" as
   a pre-build step, you must ensure you run it with the same pycache
   prefix (if any) that you will use at runtime.

   A relative path is interpreted relative to the current working
   directory.

   This value is initially set based on the value of the "-X"
   "pycache_prefix=PATH" command-line option or the
   "PYTHONPYCACHEPREFIX" environment variable (command-line takes
   precedence). If neither are set, it is "None".

   在 3.8 版新加入.

sys.excepthook(type, value, traceback)

   This function prints out a given traceback and exception to
   "sys.stderr".

   When an exception other than "SystemExit" is raised and uncaught,
   the interpreter calls "sys.excepthook" with three arguments, the
   exception class, exception instance, and a traceback object.  In an
   interactive session this happens just before control is returned to
   the prompt; in a Python program this happens just before the
   program exits.  The handling of such top-level exceptions can be
   customized by assigning another three-argument function to
   "sys.excepthook".

   引發一個附帶引數 "hook"、"type"、"value"、"traceback" 的稽核事件
   "sys.excepthook"。

   也參考:

     The "sys.unraisablehook()" function handles unraisable exceptions
     and the "threading.excepthook()" function handles exception
     raised by "threading.Thread.run()".

sys.__breakpointhook__
sys.__displayhook__
sys.__excepthook__
sys.__unraisablehook__

   These objects contain the original values of "breakpointhook",
   "displayhook", "excepthook", and "unraisablehook" at the start of
   the program.  They are saved so that "breakpointhook",
   "displayhook" and "excepthook", "unraisablehook" can be restored in
   case they happen to get replaced with broken or alternative
   objects.

   在 3.7 版新加入: __breakpointhook__

   在 3.8 版新加入: __unraisablehook__

sys.exception()

   This function, when called while an exception handler is executing
   (such as an "except" or "except*" clause), returns the exception
   instance that was caught by this handler. When exception handlers
   are nested within one another, only the exception handled by the
   innermost handler is accessible.

   If no exception handler is executing, this function returns "None".

   在 3.11 版新加入.

sys.exc_info()

   This function returns the old-style representation of the handled
   exception. If an exception "e" is currently handled (so
   "exception()" would return "e"), "exc_info()" returns the tuple
   "(type(e), e, e.__traceback__)". That is, a tuple containing the
   type of the exception (a subclass of "BaseException"), the
   exception itself, and a traceback object which typically
   encapsulates the call stack at the point where the exception last
   occurred.

   If no exception is being handled anywhere on the stack, this
   function return a tuple containing three "None" values.

   在 3.11 版的變更: The "type" and "traceback" fields are now derived
   from the "value" (the exception instance), so when an exception is
   modified while it is being handled, the changes are reflected in
   the results of subsequent calls to "exc_info()".

sys.exec_prefix

   A string giving the site-specific directory prefix where the
   platform-dependent Python files are installed; by default, this is
   also "'/usr/local'".  This can be set at build time with the "--
   exec-prefix" argument to the **configure** script.  Specifically,
   all configuration files (e.g. the "pyconfig.h" header file) are
   installed in the directory "*exec_prefix*/lib/python*X.Y*/config",
   and shared library modules are installed in
   "*exec_prefix*/lib/python*X.Y*/lib-dynload", where *X.Y* is the
   version number of Python, for example "3.2".

   備註:

     If a virtual environment is in effect, this value will be changed
     in "site.py" to point to the virtual environment. The value for
     the Python installation will still be available, via
     "base_exec_prefix".

sys.executable

   A string giving the absolute path of the executable binary for the
   Python interpreter, on systems where this makes sense. If Python is
   unable to retrieve the real path to its executable,
   "sys.executable" will be an empty string or "None".

sys.exit([arg])

   Raise a "SystemExit" exception, signaling an intention to exit the
   interpreter.

   The optional argument *arg* can be an integer giving the exit
   status (defaulting to zero), or another type of object.  If it is
   an integer, zero is considered "successful termination" and any
   nonzero value is considered "abnormal termination" by shells and
   the like.  Most systems require it to be in the range 0--127, and
   produce undefined results otherwise.  Some systems have a
   convention for assigning specific meanings to specific exit codes,
   but these are generally underdeveloped; Unix programs generally use
   2 for command line syntax errors and 1 for all other kind of
   errors.  If another type of object is passed, "None" is equivalent
   to passing zero, and any other object is printed to "stderr" and
   results in an exit code of 1.  In particular, "sys.exit("some error
   message")" is a quick way to exit a program when an error occurs.

   Since "exit()" ultimately "only" raises an exception, it will only
   exit the process when called from the main thread, and the
   exception is not intercepted. Cleanup actions specified by finally
   clauses of "try" statements are honored, and it is possible to
   intercept the exit attempt at an outer level.

   在 3.6 版的變更: If an error occurs in the cleanup after the Python
   interpreter has caught "SystemExit" (such as an error flushing
   buffered data in the standard streams), the exit status is changed
   to 120.

sys.flags

   The *named tuple* *flags* exposes the status of command line flags.
   The attributes are read only.

   +----------------------------------------------------+----------------------------------------------------+
   | flags.debug                                        | "-d"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.inspect                                      | "-i"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.interactive                                  | "-i"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.isolated                                     | "-I"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.optimize                                     | "-O" 或 "-OO"                                      |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.dont_write_bytecode                          | "-B"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.no_user_site                                 | "-s"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.no_site                                      | "-S"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.ignore_environment                           | "-E"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.verbose                                      | "-v"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.bytes_warning                                | "-b"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.quiet                                        | "-q"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.hash_randomization                           | "-R"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.dev_mode                                     | "-X dev" （Python 開發模式）                       |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.utf8_mode                                    | "-X utf8"                                          |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.safe_path                                    | "-P"                                               |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.int_max_str_digits                           | "-X int_max_str_digits" (integer string conversion |
   |                                                    | length limitation)                                 |
   +----------------------------------------------------+----------------------------------------------------+
   | flags.warn_default_encoding                        | "-X warn_default_encoding"                         |
   +----------------------------------------------------+----------------------------------------------------+

   在 3.2 版的變更: 新增 "quiet" 屬性，用於新的 "-q" 旗標。

   在 3.2.3 版新加入: "hash_randomization" 屬性。

   在 3.3 版的變更: 移除過時的 "division_warning" 屬性。

   在 3.4 版的變更: 新增 "isolated" 屬性，用於 "-I" "isolated" 旗標。

   在 3.7 版的變更: Added the "dev_mode" attribute for the new Python
   Development Mode and the "utf8_mode" attribute for the new  "-X"
   "utf8" flag.

   在 3.10 版的變更: Added "warn_default_encoding" attribute for "-X"
   "warn_default_encoding" flag.

   在 3.11 版的變更: 新增 "safe_path" 屬性，用於 "-P" 選項。

   在 3.11 版的變更: 新增 "int_max_str_digits" 屬性。

sys.float_info

   A *named tuple* holding information about the float type. It
   contains low level information about the precision and internal
   representation.  The values correspond to the various floating-
   point constants defined in the standard header file "float.h" for
   the 'C' programming language; see section 5.2.4.2.2 of the 1999
   ISO/IEC C standard [C99], 'Characteristics of floating types', for
   details.


   Attributes of the "float_info" *named tuple*
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

   +-----------------------------------+-----------------------------------+-----------------------------------+
   | 屬性                              | float.h macro                     | 解釋                              |
   |===================================|===================================|===================================|
   | float_info.epsilon                | "DBL_EPSILON"                     | difference between 1.0 and the    |
   |                                   |                                   | least value greater than 1.0 that |
   |                                   |                                   | is representable as a float.  另  |
   |                                   |                                   | 請參閱 "math.ulp()"。             |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | float_info.dig                    | "DBL_DIG"                         | The maximum number of decimal     |
   |                                   |                                   | digits that can be faithfully     |
   |                                   |                                   | represented in a float; see       |
   |                                   |                                   | below.                            |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | float_info.mant_dig               | "DBL_MANT_DIG"                    | Float precision: the number of    |
   |                                   |                                   | base-"radix" digits in the        |
   |                                   |                                   | significand of a float.           |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | float_info.max                    | "DBL_MAX"                         | The maximum representable         |
   |                                   |                                   | positive finite float.            |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | float_info.max_exp                | "DBL_MAX_EXP"                     | The maximum integer *e* such that |
   |                                   |                                   | "radix**(e-1)" is a representable |
   |                                   |                                   | finite float.                     |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | float_info.max_10_exp             | "DBL_MAX_10_EXP"                  | The maximum integer *e* such that |
   |                                   |                                   | "10**e" is in the range of        |
   |                                   |                                   | representable finite floats.      |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | float_info.min                    | "DBL_MIN"                         | The minimum representable         |
   |                                   |                                   | positive *normalized* float.  Use |
   |                                   |                                   | "math.ulp(0.0)" to get the        |
   |                                   |                                   | smallest positive *denormalized*  |
   |                                   |                                   | representable float.              |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | float_info.min_exp                | "DBL_MIN_EXP"                     | The minimum integer *e* such that |
   |                                   |                                   | "radix**(e-1)" is a normalized    |
   |                                   |                                   | float.                            |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | float_info.min_10_exp             | "DBL_MIN_10_EXP"                  | The minimum integer *e* such that |
   |                                   |                                   | "10**e" is a normalized float.    |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | float_info.radix                  | "FLT_RADIX"                       | The radix of exponent             |
   |                                   |                                   | representation.                   |
   +-----------------------------------+-----------------------------------+-----------------------------------+
   | float_info.rounds                 | "FLT_ROUNDS"                      | An integer representing the       |
   |                                   |                                   | rounding mode for floating-point  |
   |                                   |                                   | arithmetic. This reflects the     |
   |                                   |                                   | value of the system "FLT_ROUNDS"  |
   |                                   |                                   | macro at interpreter startup      |
   |                                   |                                   | time:  * "-1": indeterminable  *  |
   |                                   |                                   | "0": toward zero  * "1": to       |
   |                                   |                                   | nearest  * "2": toward positive   |
   |                                   |                                   | infinity  * "3": toward negative  |
   |                                   |                                   | infinity  All other values for    |
   |                                   |                                   | "FLT_ROUNDS" characterize         |
   |                                   |                                   | implementation- defined rounding  |
   |                                   |                                   | behavior.                         |
   +-----------------------------------+-----------------------------------+-----------------------------------+

   The attribute "sys.float_info.dig" needs further explanation.  If
   "s" is any string representing a decimal number with at most
   "sys.float_info.dig" significant digits, then converting "s" to a
   float and back again will recover a string representing the same
   decimal value:

      >>> import sys
      >>> sys.float_info.dig
      15
      >>> s = '3.14159265358979'    # decimal string with 15 significant digits
      >>> format(float(s), '.15g')  # convert to float and back -> same value
      '3.14159265358979'

   But for strings with more than "sys.float_info.dig" significant
   digits, this isn't always true:

      >>> s = '9876543211234567'    # 16 significant digits is too many!
      >>> format(float(s), '.16g')  # conversion changes value
      '9876543211234568'

sys.float_repr_style

   A string indicating how the "repr()" function behaves for floats.
   If the string has value "'short'" then for a finite float "x",
   "repr(x)" aims to produce a short string with the property that
   "float(repr(x)) == x".  This is the usual behaviour in Python 3.1
   and later.  Otherwise, "float_repr_style" has value "'legacy'" and
   "repr(x)" behaves in the same way as it did in versions of Python
   prior to 3.1.

   在 3.1 版新加入.

sys.getallocatedblocks()

   Return the number of memory blocks currently allocated by the
   interpreter, regardless of their size.  This function is mainly
   useful for tracking and debugging memory leaks.  Because of the
   interpreter's internal caches, the result can vary from call to
   call; you may have to call "_clear_type_cache()" and "gc.collect()"
   to get more predictable results.

   If a Python build or implementation cannot reasonably compute this
   information, "getallocatedblocks()" is allowed to return 0 instead.

   在 3.4 版新加入.

sys.getandroidapilevel()

   Return the build time API version of Android as an integer.

   適用：Android。

   在 3.7 版新加入.

sys.getdefaultencoding()

   Return the name of the current default string encoding used by the
   Unicode implementation.

sys.getdlopenflags()

   Return the current value of the flags that are used for "dlopen()"
   calls.  Symbolic names for the flag values can be found in the "os"
   module ("RTLD_*xxx*" constants, e.g. "os.RTLD_LAZY").

   適用：Unix。

sys.getfilesystemencoding()

   Get the *filesystem encoding*: the encoding used with the
   *filesystem error handler* to convert between Unicode filenames and
   bytes filenames. The filesystem error handler is returned from
   "getfilesystemencodeerrors()".

   For best compatibility, str should be used for filenames in all
   cases, although representing filenames as bytes is also supported.
   Functions accepting or returning filenames should support either
   str or bytes and internally convert to the system's preferred
   representation.

   "os.fsencode()" and "os.fsdecode()" should be used to ensure that
   the correct encoding and errors mode are used.

   The *filesystem encoding and error handler* are configured at
   Python startup by the "PyConfig_Read()" function: see
   "filesystem_encoding" and "filesystem_errors" members of
   "PyConfig".

   在 3.2 版的變更: "getfilesystemencoding()" 的結果不再為 "None"。

   在 3.6 版的變更: Windows is no longer guaranteed to return
   "'mbcs'". See **PEP 529** and "_enablelegacywindowsfsencoding()"
   for more information.

   在 3.7 版的變更: Return "'utf-8'" if the Python UTF-8 Mode is
   enabled.

sys.getfilesystemencodeerrors()

   Get the *filesystem error handler*: the error handler used with the
   *filesystem encoding* to convert between Unicode filenames and
   bytes filenames. The filesystem encoding is returned from
   "getfilesystemencoding()".

   "os.fsencode()" and "os.fsdecode()" should be used to ensure that
   the correct encoding and errors mode are used.

   The *filesystem encoding and error handler* are configured at
   Python startup by the "PyConfig_Read()" function: see
   "filesystem_encoding" and "filesystem_errors" members of
   "PyConfig".

   在 3.6 版新加入.

sys.get_int_max_str_digits()

   Returns the current value for the integer string conversion length
   limitation. See also "set_int_max_str_digits()".

   在 3.11 版新加入.

sys.getrefcount(object)

   Return the reference count of the *object*.  The count returned is
   generally one higher than you might expect, because it includes the
   (temporary) reference as an argument to "getrefcount()".

   Note that the returned value may not actually reflect how many
   references to the object are actually held.  Consequently, do not
   rely on the returned value to be accurate, other than a value of 0
   or 1.

sys.getrecursionlimit()

   Return the current value of the recursion limit, the maximum depth
   of the Python interpreter stack.  This limit prevents infinite
   recursion from causing an overflow of the C stack and crashing
   Python.  It can be set by "setrecursionlimit()".

sys.getsizeof(object[, default])

   Return the size of an object in bytes. The object can be any type
   of object. All built-in objects will return correct results, but
   this does not have to hold true for third-party extensions as it is
   implementation specific.

   Only the memory consumption directly attributed to the object is
   accounted for, not the memory consumption of objects it refers to.

   If given, *default* will be returned if the object does not provide
   means to retrieve the size.  Otherwise a "TypeError" will be
   raised.

   "getsizeof()" calls the object's "__sizeof__" method and adds an
   additional garbage collector overhead if the object is managed by
   the garbage collector.

   See recursive sizeof recipe for an example of using "getsizeof()"
   recursively to find the size of containers and all their contents.

sys.getswitchinterval()

   Return the interpreter's "thread switch interval"; see
   "setswitchinterval()".

   在 3.2 版新加入.

sys._getframe([depth])

   Return a frame object from the call stack.  If optional integer
   *depth* is given, return the frame object that many calls below the
   top of the stack.  If that is deeper than the call stack,
   "ValueError" is raised.  The default for *depth* is zero, returning
   the frame at the top of the call stack.

   引發一個附帶引數 "frame" 的稽核事件 "sys._getframe"。

   **CPython 實作細節：** This function should be used for internal
   and specialized purposes only. It is not guaranteed to exist in all
   implementations of Python.

sys.getprofile()

   Get the profiler function as set by "setprofile()".

sys.gettrace()

   Get the trace function as set by "settrace()".

   **CPython 實作細節：** The "gettrace()" function is intended only
   for implementing debuggers, profilers, coverage tools and the like.
   Its behavior is part of the implementation platform, rather than
   part of the language definition, and thus may not be available in
   all Python implementations.

sys.getwindowsversion()

   Return a named tuple describing the Windows version currently
   running.  The named elements are *major*, *minor*, *build*,
   *platform*, *service_pack*, *service_pack_minor*,
   *service_pack_major*, *suite_mask*, *product_type* and
   *platform_version*. *service_pack* contains a string,
   *platform_version* a 3-tuple and all other values are integers. The
   components can also be accessed by name, so
   "sys.getwindowsversion()[0]" is equivalent to
   "sys.getwindowsversion().major". For compatibility with prior
   versions, only the first 5 elements are retrievable by indexing.

   *platform* will be "2" (VER_PLATFORM_WIN32_NT).

   *product_type* may be one of the following values:

   +-----------------------------------------+-----------------------------------+
   | Constant                                | Meaning                           |
   |=========================================|===================================|
   | "1" (VER_NT_WORKSTATION)                | The system is a workstation.      |
   +-----------------------------------------+-----------------------------------+
   | "2" (VER_NT_DOMAIN_CONTROLLER)          | The system is a domain            |
   |                                         | controller.                       |
   +-----------------------------------------+-----------------------------------+
   | "3" (VER_NT_SERVER)                     | The system is a server, but not a |
   |                                         | domain controller.                |
   +-----------------------------------------+-----------------------------------+

   This function wraps the Win32 "GetVersionEx()" function; see the
   Microsoft documentation on "OSVERSIONINFOEX()" for more information
   about these fields.

   *platform_version* returns the major version, minor version and
   build number of the current operating system, rather than the
   version that is being emulated for the process. It is intended for
   use in logging rather than for feature detection.

   備註:

     *platform_version* derives the version from kernel32.dll which
     can be of a different version than the OS version. Please use
     "platform" module for achieving accurate OS version.

   適用：Windows。

   在 3.2 版的變更: Changed to a named tuple and added
   *service_pack_minor*, *service_pack_major*, *suite_mask*, and
   *product_type*.

   在 3.6 版的變更: 新增 *platform_version*

sys.get_asyncgen_hooks()

   Returns an *asyncgen_hooks* object, which is similar to a
   "namedtuple" of the form "(firstiter, finalizer)", where
   *firstiter* and *finalizer* are expected to be either "None" or
   functions which take an *asynchronous generator iterator* as an
   argument, and are used to schedule finalization of an asynchronous
   generator by an event loop.

   在 3.6 版新加入: 更多細節請見 **PEP 525**。

   備註:

     This function has been added on a provisional basis (see **PEP
     411** for details.)

sys.get_coroutine_origin_tracking_depth()

   Get the current coroutine origin tracking depth, as set by
   "set_coroutine_origin_tracking_depth()".

   在 3.7 版新加入.

   備註:

     This function has been added on a provisional basis (see **PEP
     411** for details.)  Use it only for debugging purposes.

sys.hash_info

   A *named tuple* giving parameters of the numeric hash
   implementation.  For more details about hashing of numeric types,
   see Hashing of numeric types.

   hash_info.width

      The width in bits used for hash values

   hash_info.modulus

      The prime modulus P used for numeric hash scheme

   hash_info.inf

      The hash value returned for a positive infinity

   hash_info.nan

      (This attribute is no longer used)

   hash_info.imag

      The multiplier used for the imaginary part of a complex number

   hash_info.algorithm

      The name of the algorithm for hashing of str, bytes, and
      memoryview

   hash_info.hash_bits

      The internal output size of the hash algorithm

   hash_info.seed_bits

      The size of the seed key of the hash algorithm

   在 3.2 版新加入.

   在 3.4 版的變更: 新增 *algorithm*、*hash_bits* 與 *seed_bits*

sys.hexversion

   The version number encoded as a single integer.  This is guaranteed
   to increase with each version, including proper support for non-
   production releases.  For example, to test that the Python
   interpreter is at least version 1.5.2, use:

      if sys.hexversion >= 0x010502F0:
          # use some advanced feature
          ...
      else:
          # use an alternative implementation or warn the user
          ...

   This is called "hexversion" since it only really looks meaningful
   when viewed as the result of passing it to the built-in "hex()"
   function.  The *named tuple*  "sys.version_info" may be used for a
   more human-friendly encoding of the same information.

   More details of "hexversion" can be found at API 和 ABI 版本管理.

sys.implementation

   An object containing information about the implementation of the
   currently running Python interpreter.  The following attributes are
   required to exist in all Python implementations.

   *name* is the implementation's identifier, e.g. "'cpython'".  The
   actual string is defined by the Python implementation, but it is
   guaranteed to be lower case.

   *version* is a named tuple, in the same format as
   "sys.version_info".  It represents the version of the Python
   *implementation*.  This has a distinct meaning from the specific
   version of the Python *language* to which the currently running
   interpreter conforms, which "sys.version_info" represents.  For
   example, for PyPy 1.8 "sys.implementation.version" might be
   "sys.version_info(1, 8, 0, 'final', 0)", whereas "sys.version_info"
   would be "sys.version_info(2, 7, 2, 'final', 0)".  For CPython they
   are the same value, since it is the reference implementation.

   *hexversion* is the implementation version in hexadecimal format,
   like "sys.hexversion".

   *cache_tag* is the tag used by the import machinery in the
   filenames of cached modules.  By convention, it would be a
   composite of the implementation's name and version, like
   "'cpython-33'".  However, a Python implementation may use some
   other value if appropriate.  If "cache_tag" is set to "None", it
   indicates that module caching should be disabled.

   "sys.implementation" may contain additional attributes specific to
   the Python implementation.  These non-standard attributes must
   start with an underscore, and are not described here.  Regardless
   of its contents, "sys.implementation" will not change during a run
   of the interpreter, nor between implementation versions.  (It may
   change between Python language versions, however.)  See **PEP 421**
   for more information.

   在 3.3 版新加入.

   備註:

     The addition of new required attributes must go through the
     normal PEP process. See **PEP 421** for more information.

sys.int_info

   A *named tuple* that holds information about Python's internal
   representation of integers.  The attributes are read only.

   int_info.bits_per_digit

      The number of bits held in each digit. Python integers are
      stored internally in base "2**int_info.bits_per_digit".

   int_info.sizeof_digit

      The size in bytes of the C type used to represent a digit.

   int_info.default_max_str_digits

      The default value for "sys.get_int_max_str_digits()" when it is
      not otherwise explicitly configured.

   int_info.str_digits_check_threshold

      The minimum non-zero value for "sys.set_int_max_str_digits()",
      "PYTHONINTMAXSTRDIGITS", or "-X int_max_str_digits".

   在 3.1 版新加入.

   在 3.11 版的變更: Added "default_max_str_digits" and
   "str_digits_check_threshold".

sys.__interactivehook__

   When this attribute exists, its value is automatically called (with
   no arguments) when the interpreter is launched in interactive mode.
   This is done after the "PYTHONSTARTUP" file is read, so that you
   can set this hook there.  The "site" module sets this.

   引發一個附帶引數 "hook" 的稽核事件 "cpython.run_interactivehook"。

   在 3.4 版新加入.

sys.intern(string)

   Enter *string* in the table of "interned" strings and return the
   interned string -- which is *string* itself or a copy. Interning
   strings is useful to gain a little performance on dictionary lookup
   -- if the keys in a dictionary are interned, and the lookup key is
   interned, the key comparisons (after hashing) can be done by a
   pointer compare instead of a string compare.  Normally, the names
   used in Python programs are automatically interned, and the
   dictionaries used to hold module, class or instance attributes have
   interned keys.

   Interned strings are not immortal; you must keep a reference to the
   return value of "intern()" around to benefit from it.

sys.is_finalizing()

   Return "True" if the Python interpreter is *shutting down*, "False"
   otherwise.

   在 3.5 版新加入.

sys.last_type
sys.last_value
sys.last_traceback

   These three variables are not always defined; they are set when an
   exception is not handled and the interpreter prints an error
   message and a stack traceback. Their intended use is to allow an
   interactive user to import a debugger module and engage in post-
   mortem debugging without having to re-execute the command that
   caused the error.  (Typical use is "import pdb; pdb.pm()" to enter
   the post-mortem debugger; see "pdb" module for more information.)

   The meaning of the variables is the same as that of the return
   values from "exc_info()" above.

sys.maxsize

   An integer giving the maximum value a variable of type "Py_ssize_t"
   can take.  It's usually "2**31 - 1" on a 32-bit platform and "2**63
   - 1" on a 64-bit platform.

sys.maxunicode

   An integer giving the value of the largest Unicode code point, i.e.
   "1114111" ("0x10FFFF" in hexadecimal).

   在 3.3 版的變更: Before **PEP 393**, "sys.maxunicode" used to be
   either "0xFFFF" or "0x10FFFF", depending on the configuration
   option that specified whether Unicode characters were stored as
   UCS-2 or UCS-4.

sys.meta_path

   A list of *meta path finder* objects that have their "find_spec()"
   methods called to see if one of the objects can find the module to
   be imported. By default, it holds entries that implement Python's
   default import semantics. The "find_spec()" method is called with
   at least the absolute name of the module being imported. If the
   module to be imported is contained in a package, then the parent
   package's "__path__" attribute is passed in as a second argument.
   The method returns a *module spec*, or "None" if the module cannot
   be found.

   也參考:

     "importlib.abc.MetaPathFinder"
        The abstract base class defining the interface of finder
        objects on "meta_path".

     "importlib.machinery.ModuleSpec"
        The concrete class which "find_spec()" should return instances
        of.

   在 3.4 版的變更: *Module specs* were introduced in Python 3.4, by
   **PEP 451**. Earlier versions of Python looked for a method called
   "find_module()". This is still called as a fallback if a
   "meta_path" entry doesn't have a "find_spec()" method.

sys.modules

   This is a dictionary that maps module names to modules which have
   already been loaded.  This can be manipulated to force reloading of
   modules and other tricks. However, replacing the dictionary will
   not necessarily work as expected and deleting essential items from
   the dictionary may cause Python to fail.  If you want to iterate
   over this global dictionary always use "sys.modules.copy()" or
   "tuple(sys.modules)" to avoid exceptions as its size may change
   during iteration as a side effect of code or activity in other
   threads.

sys.orig_argv

   The list of the original command line arguments passed to the
   Python executable.

   The elements of "sys.orig_argv" are the arguments to the Python
   interpreter, while the elements of "sys.argv" are the arguments to
   the user's program. Arguments consumed by the interpreter itself
   will be present in "sys.orig_argv" and missing from "sys.argv".

   在 3.10 版新加入.

sys.path

   A list of strings that specifies the search path for modules.
   Initialized from the environment variable "PYTHONPATH", plus an
   installation-dependent default.

   By default, as initialized upon program startup, a potentially
   unsafe path is prepended to "sys.path" (*before* the entries
   inserted as a result of "PYTHONPATH"):

   * "python -m module" command line: prepend the current working
     directory.

   * "python script.py" command line: prepend the script's directory.
     If it's a symbolic link, resolve symbolic links.

   * "python -c code" and "python" (REPL) command lines: prepend an
     empty string, which means the current working directory.

   To not prepend this potentially unsafe path, use the "-P" command
   line option or the "PYTHONSAFEPATH" environment variable.

   A program is free to modify this list for its own purposes.  Only
   strings should be added to "sys.path"; all other data types are
   ignored during import.

   也參考:

     * Module "site" This describes how to use .pth files to extend
       "sys.path".

sys.path_hooks

   A list of callables that take a path argument to try to create a
   *finder* for the path. If a finder can be created, it is to be
   returned by the callable, else raise "ImportError".

   Originally specified in **PEP 302**.

sys.path_importer_cache

   A dictionary acting as a cache for *finder* objects. The keys are
   paths that have been passed to "sys.path_hooks" and the values are
   the finders that are found. If a path is a valid file system path
   but no finder is found on "sys.path_hooks" then "None" is stored.

   Originally specified in **PEP 302**.

   在 3.3 版的變更: "None" is stored instead of "imp.NullImporter"
   when no finder is found.

sys.platform

   This string contains a platform identifier that can be used to
   append platform-specific components to "sys.path", for instance.

   For Unix systems, except on Linux and AIX, this is the lowercased
   OS name as returned by "uname -s" with the first part of the
   version as returned by "uname -r" appended, e.g. "'sunos5'" or
   "'freebsd8'", *at the time when Python was built*.  Unless you want
   to test for a specific system version, it is therefore recommended
   to use the following idiom:

      if sys.platform.startswith('freebsd'):
          # FreeBSD-specific code here...
      elif sys.platform.startswith('linux'):
          # Linux-specific code here...
      elif sys.platform.startswith('aix'):
          # AIX-specific code here...

   For other systems, the values are:

   +------------------+-----------------------------+
   | System           | "platform" value            |
   |==================|=============================|
   | AIX              | "'aix'"                     |
   +------------------+-----------------------------+
   | Emscripten       | "'emscripten'"              |
   +------------------+-----------------------------+
   | Linux            | "'linux'"                   |
   +------------------+-----------------------------+
   | WASI             | "'wasi'"                    |
   +------------------+-----------------------------+
   | Windows          | "'win32'"                   |
   +------------------+-----------------------------+
   | Windows/Cygwin   | "'cygwin'"                  |
   +------------------+-----------------------------+
   | macOS            | "'darwin'"                  |
   +------------------+-----------------------------+

   在 3.3 版的變更: On Linux, "sys.platform" doesn't contain the major
   version anymore. It is always "'linux'", instead of "'linux2'" or
   "'linux3'".  Since older Python versions include the version
   number, it is recommended to always use the "startswith" idiom
   presented above.

   在 3.8 版的變更: On AIX, "sys.platform" doesn't contain the major
   version anymore. It is always "'aix'", instead of "'aix5'" or
   "'aix7'".  Since older Python versions include the version number,
   it is recommended to always use the "startswith" idiom presented
   above.

   也參考:

     "os.name" has a coarser granularity.  "os.uname()" gives system-
     dependent version information.

     The "platform" module provides detailed checks for the system's
     identity.

sys.platlibdir

   Name of the platform-specific library directory. It is used to
   build the path of standard library and the paths of installed
   extension modules.

   It is equal to ""lib"" on most platforms. On Fedora and SuSE, it is
   equal to ""lib64"" on 64-bit platforms which gives the following
   "sys.path" paths (where "X.Y" is the Python "major.minor" version):

   * "/usr/lib64/pythonX.Y/": Standard library (like "os.py" of the
     "os" module)

   * "/usr/lib64/pythonX.Y/lib-dynload/": C extension modules of the
     standard library (like the "errno" module, the exact filename is
     platform specific)

   * "/usr/lib/pythonX.Y/site-packages/" (always use "lib", not
     "sys.platlibdir"): Third-party modules

   * "/usr/lib64/pythonX.Y/site-packages/": C extension modules of
     third-party packages

   在 3.9 版新加入.

sys.prefix

   A string giving the site-specific directory prefix where the
   platform independent Python files are installed; on Unix, the
   default is "/usr/local". This can be set at build time with the "--
   prefix" argument to the **configure** script.  See Installation
   paths for derived paths.

   備註:

     If a virtual environment is in effect, this value will be changed
     in "site.py" to point to the virtual environment. The value for
     the Python installation will still be available, via
     "base_prefix".

sys.ps1
sys.ps2

   Strings specifying the primary and secondary prompt of the
   interpreter.  These are only defined if the interpreter is in
   interactive mode.  Their initial values in this case are "'>>> '"
   and "'... '".  If a non-string object is assigned to either
   variable, its "str()" is re-evaluated each time the interpreter
   prepares to read a new interactive command; this can be used to
   implement a dynamic prompt.

sys.setdlopenflags(n)

   Set the flags used by the interpreter for "dlopen()" calls, such as
   when the interpreter loads extension modules.  Among other things,
   this will enable a lazy resolving of symbols when importing a
   module, if called as "sys.setdlopenflags(0)".  To share symbols
   across extension modules, call as
   "sys.setdlopenflags(os.RTLD_GLOBAL)".  Symbolic names for the flag
   values can be found in the "os" module ("RTLD_*xxx*" constants,
   e.g. "os.RTLD_LAZY").

   適用：Unix。

sys.set_int_max_str_digits(maxdigits)

   Set the integer string conversion length limitation used by this
   interpreter. See also "get_int_max_str_digits()".

   在 3.11 版新加入.

sys.setprofile(profilefunc)

   Set the system's profile function, which allows you to implement a
   Python source code profiler in Python.  See chapter The Python
   Profilers for more information on the Python profiler.  The
   system's profile function is called similarly to the system's trace
   function (see "settrace()"), but it is called with different
   events, for example it isn't called for each executed line of code
   (only on call and return, but the return event is reported even
   when an exception has been set). The function is thread-specific,
   but there is no way for the profiler to know about context switches
   between threads, so it does not make sense to use this in the
   presence of multiple threads. Also, its return value is not used,
   so it can simply return "None".  Error in the profile function will
   cause itself unset.

   備註:

     The same tracing mechanism is used for "setprofile()" as
     "settrace()". To trace calls with "setprofile()" inside a tracing
     function (e.g. in a debugger breakpoint), see "call_tracing()".

   Profile functions should have three arguments: *frame*, *event*,
   and *arg*. *frame* is the current stack frame.  *event* is a
   string: "'call'", "'return'", "'c_call'", "'c_return'", or
   "'c_exception'". *arg* depends on the event type.

   The events have the following meaning:

   "'call'"
      A function is called (or some other code block entered).  The
      profile function is called; *arg* is "None".

   "'return'"
      A function (or other code block) is about to return.  The
      profile function is called; *arg* is the value that will be
      returned, or "None" if the event is caused by an exception being
      raised.

   "'c_call'"
      A C function is about to be called.  This may be an extension
      function or a built-in.  *arg* is the C function object.

   "'c_return'"
      A C function has returned. *arg* is the C function object.

   "'c_exception'"
      A C function has raised an exception.  *arg* is the C function
      object.

   引發一個不附帶引數的稽核事件 "sys.setprofile"。

sys.setrecursionlimit(limit)

   Set the maximum depth of the Python interpreter stack to *limit*.
   This limit prevents infinite recursion from causing an overflow of
   the C stack and crashing Python.

   The highest possible limit is platform-dependent.  A user may need
   to set the limit higher when they have a program that requires deep
   recursion and a platform that supports a higher limit.  This should
   be done with care, because a too-high limit can lead to a crash.

   If the new limit is too low at the current recursion depth, a
   "RecursionError" exception is raised.

   在 3.5.1 版的變更: A "RecursionError" exception is now raised if
   the new limit is too low at the current recursion depth.

sys.setswitchinterval(interval)

   Set the interpreter's thread switch interval (in seconds).  This
   floating-point value determines the ideal duration of the
   "timeslices" allocated to concurrently running Python threads.
   Please note that the actual value can be higher, especially if
   long-running internal functions or methods are used.  Also, which
   thread becomes scheduled at the end of the interval is the
   operating system's decision.  The interpreter doesn't have its own
   scheduler.

   在 3.2 版新加入.

sys.settrace(tracefunc)

   Set the system's trace function, which allows you to implement a
   Python source code debugger in Python.  The function is thread-
   specific; for a debugger to support multiple threads, it must
   register a trace function using "settrace()" for each thread being
   debugged or use "threading.settrace()".

   Trace functions should have three arguments: *frame*, *event*, and
   *arg*. *frame* is the current stack frame.  *event* is a string:
   "'call'", "'line'", "'return'", "'exception'" or "'opcode'".  *arg*
   depends on the event type.

   The trace function is invoked (with *event* set to "'call'")
   whenever a new local scope is entered; it should return a reference
   to a local trace function to be used for the new scope, or "None"
   if the scope shouldn't be traced.

   The local trace function should return a reference to itself, or to
   another function which would then be used as the local trace
   function for the scope.

   If there is any error occurred in the trace function, it will be
   unset, just like "settrace(None)" is called.

   備註:

     Tracing is disabled while calling the trace function (e.g. a
     function set by "settrace()"). For recursive tracing see
     "call_tracing()".

   The events have the following meaning:

   "'call'"
      A function is called (or some other code block entered).  The
      global trace function is called; *arg* is "None"; the return
      value specifies the local trace function.

   "'line'"
      The interpreter is about to execute a new line of code or re-
      execute the condition of a loop.  The local trace function is
      called; *arg* is "None"; the return value specifies the new
      local trace function.  See "Objects/lnotab_notes.txt" for a
      detailed explanation of how this works. Per-line events may be
      disabled for a frame by setting "f_trace_lines" to "False" on
      that frame.

   "'return'"
      A function (or other code block) is about to return.  The local
      trace function is called; *arg* is the value that will be
      returned, or "None" if the event is caused by an exception being
      raised.  The trace function's return value is ignored.

   "'exception'"
      An exception has occurred.  The local trace function is called;
      *arg* is a tuple "(exception, value, traceback)"; the return
      value specifies the new local trace function.

   "'opcode'"
      The interpreter is about to execute a new opcode (see "dis" for
      opcode details).  The local trace function is called; *arg* is
      "None"; the return value specifies the new local trace function.
      Per-opcode events are not emitted by default: they must be
      explicitly requested by setting "f_trace_opcodes" to "True" on
      the frame.

   Note that as an exception is propagated down the chain of callers,
   an "'exception'" event is generated at each level.

   For more fine-grained usage, it's possible to set a trace function
   by assigning "frame.f_trace = tracefunc" explicitly, rather than
   relying on it being set indirectly via the return value from an
   already installed trace function. This is also required for
   activating the trace function on the current frame, which
   "settrace()" doesn't do. Note that in order for this to work, a
   global tracing function must have been installed with "settrace()"
   in order to enable the runtime tracing machinery, but it doesn't
   need to be the same tracing function (e.g. it could be a low
   overhead tracing function that simply returns "None" to disable
   itself immediately on each frame).

   For more information on code and frame objects, refer to 標準型別階
   層.

   Raises an auditing event "sys.settrace" with no arguments.

   **CPython 實作細節：** The "settrace()" function is intended only
   for implementing debuggers, profilers, coverage tools and the like.
   Its behavior is part of the implementation platform, rather than
   part of the language definition, and thus may not be available in
   all Python implementations.

   在 3.7 版的變更: "'opcode'" event type added; "f_trace_lines" and
   "f_trace_opcodes" attributes added to frames

sys.set_asyncgen_hooks([firstiter] [, finalizer])

   Accepts two optional keyword arguments which are callables that
   accept an *asynchronous generator iterator* as an argument. The
   *firstiter* callable will be called when an asynchronous generator
   is iterated for the first time. The *finalizer* will be called when
   an asynchronous generator is about to be garbage collected.

   引發一個不附帶引數的稽核事件 "sys.set_asyncgen_hooks_firstiter"。

   引發一個不附帶引數的稽核事件 "sys.set_asyncgen_hooks_finalizer"。

   Two auditing events are raised because the underlying API consists
   of two calls, each of which must raise its own event.

   在 3.6 版新加入: See **PEP 525** for more details, and for a
   reference example of a *finalizer* method see the implementation of
   "asyncio.Loop.shutdown_asyncgens" in Lib/asyncio/base_events.py

   備註:

     This function has been added on a provisional basis (see **PEP
     411** for details.)

sys.set_coroutine_origin_tracking_depth(depth)

   Allows enabling or disabling coroutine origin tracking. When
   enabled, the "cr_origin" attribute on coroutine objects will
   contain a tuple of (filename, line number, function name) tuples
   describing the traceback where the coroutine object was created,
   with the most recent call first. When disabled, "cr_origin" will be
   None.

   To enable, pass a *depth* value greater than zero; this sets the
   number of frames whose information will be captured. To disable,
   pass set *depth* to zero.

   This setting is thread-specific.

   在 3.7 版新加入.

   備註:

     This function has been added on a provisional basis (see **PEP
     411** for details.)  Use it only for debugging purposes.

sys._enablelegacywindowsfsencoding()

   Changes the *filesystem encoding and error handler* to 'mbcs' and
   'replace' respectively, for consistency with versions of Python
   prior to 3.6.

   This is equivalent to defining the "PYTHONLEGACYWINDOWSFSENCODING"
   environment variable before launching Python.

   See also "sys.getfilesystemencoding()" and
   "sys.getfilesystemencodeerrors()".

   適用：Windows。

   在 3.6 版新加入: 更多細節請見 **PEP 529**。

sys.stdin
sys.stdout
sys.stderr

   *File objects* used by the interpreter for standard input, output
   and errors:

   * "stdin" is used for all interactive input (including calls to
     "input()");

   * "stdout" is used for the output of "print()" and *expression*
     statements and for the prompts of "input()";

   * The interpreter's own prompts and its error messages go to
     "stderr".

   These streams are regular *text files* like those returned by the
   "open()" function.  Their parameters are chosen as follows:

   * The encoding and error handling are is initialized from
     "PyConfig.stdio_encoding" and "PyConfig.stdio_errors".

     On Windows, UTF-8 is used for the console device.  Non-character
     devices such as disk files and pipes use the system locale
     encoding (i.e. the ANSI codepage).  Non-console character devices
     such as NUL (i.e. where "isatty()" returns "True") use the value
     of the console input and output codepages at startup,
     respectively for stdin and stdout/stderr. This defaults to the
     system *locale encoding* if the process is not initially attached
     to a console.

     The special behaviour of the console can be overridden by setting
     the environment variable PYTHONLEGACYWINDOWSSTDIO before starting
     Python. In that case, the console codepages are used as for any
     other character device.

     Under all platforms, you can override the character encoding by
     setting the "PYTHONIOENCODING" environment variable before
     starting Python or by using the new "-X" "utf8" command line
     option and "PYTHONUTF8" environment variable.  However, for the
     Windows console, this only applies when
     "PYTHONLEGACYWINDOWSSTDIO" is also set.

   * When interactive, the "stdout" stream is line-buffered.
     Otherwise, it is block-buffered like regular text files.  The
     "stderr" stream is line-buffered in both cases.  You can make
     both streams unbuffered by passing the "-u" command-line option
     or setting the "PYTHONUNBUFFERED" environment variable.

   在 3.9 版的變更: Non-interactive "stderr" is now line-buffered
   instead of fully buffered.

   備註:

     To write or read binary data from/to the standard streams, use
     the underlying binary "buffer" object.  For example, to write
     bytes to "stdout", use "sys.stdout.buffer.write(b'abc')".However,
     if you are writing a library (and do not control in which context
     its code will be executed), be aware that the standard streams
     may be replaced with file-like objects like "io.StringIO" which
     do not support the "buffer" attribute.

sys.__stdin__
sys.__stdout__
sys.__stderr__

   These objects contain the original values of "stdin", "stderr" and
   "stdout" at the start of the program.  They are used during
   finalization, and could be useful to print to the actual standard
   stream no matter if the "sys.std*" object has been redirected.

   It can also be used to restore the actual files to known working
   file objects in case they have been overwritten with a broken
   object.  However, the preferred way to do this is to explicitly
   save the previous stream before replacing it, and restore the saved
   object.

   備註:

     Under some conditions "stdin", "stdout" and "stderr" as well as
     the original values "__stdin__", "__stdout__" and "__stderr__"
     can be "None". It is usually the case for Windows GUI apps that
     aren't connected to a console and Python apps started with
     **pythonw**.

sys.stdlib_module_names

   A frozenset of strings containing the names of standard library
   modules.

   It is the same on all platforms. Modules which are not available on
   some platforms and modules disabled at Python build are also
   listed. All module kinds are listed: pure Python, built-in, frozen
   and extension modules. Test modules are excluded.

   For packages, only the main package is listed: sub-packages and
   sub-modules are not listed. For example, the "email" package is
   listed, but the "email.mime" sub-package and the "email.message"
   sub-module are not listed.

   See also the "sys.builtin_module_names" list.

   在 3.10 版新加入.

sys.thread_info

   A *named tuple* holding information about the thread
   implementation.

   thread_info.name

      The name of the thread implementation:

      * ""nt"": Windows threads

      * ""pthread"": POSIX threads

      * ""pthread-stubs"": stub POSIX threads (on WebAssembly
        platforms without threading support)

      * ""solaris"": Solaris threads

   thread_info.lock

      The name of the lock implementation:

      * ""semaphore"": a lock uses a semaphore

      * ""mutex+cond"": a lock uses a mutex and a condition variable

      * 為 "None" 表示此資訊未知

   thread_info.version

      The name and version of the thread library. It is a string, or
      "None" if this information is unknown.

   在 3.3 版新加入.

sys.tracebacklimit

   When this variable is set to an integer value, it determines the
   maximum number of levels of traceback information printed when an
   unhandled exception occurs. The default is "1000".  When set to "0"
   or less, all traceback information is suppressed and only the
   exception type and value are printed.

sys.unraisablehook(unraisable, /)

   處理一個不可被引發的例外。

   Called when an exception has occurred but there is no way for
   Python to handle it. For example, when a destructor raises an
   exception or during garbage collection ("gc.collect()").

   The *unraisable* argument has the following attributes:

   * "exc_type": Exception type.

   * "exc_value": Exception value, can be "None".

   * "exc_traceback": Exception traceback, can be "None".

   * "err_msg": Error message, can be "None".

   * "object": Object causing the exception, can be "None".

   The default hook formats "err_msg" and "object" as: "f'{err_msg}:
   {object!r}'"; use "Exception ignored in" error message if "err_msg"
   is "None".

   "sys.unraisablehook()" can be overridden to control how unraisable
   exceptions are handled.

   也參考: "excepthook()" which handles uncaught exceptions.

   警告:

     Storing "exc_value" using a custom hook can create a reference
     cycle. It should be cleared explicitly to break the reference
     cycle when the exception is no longer needed.Storing "object"
     using a custom hook can resurrect it if it is set to an object
     which is being finalized. Avoid storing "object" after the custom
     hook completes to avoid resurrecting objects.

   Raise an auditing event "sys.unraisablehook" with arguments *hook*,
   *unraisable* when an exception that cannot be handled occurs. The
   *unraisable* object is the same as what will be passed to the hook.
   If no hook has been set, *hook* may be "None".

   在 3.8 版新加入.

sys.version

   A string containing the version number of the Python interpreter
   plus additional information on the build number and compiler used.
   This string is displayed when the interactive interpreter is
   started.  Do not extract version information out of it, rather, use
   "version_info" and the functions provided by the "platform" module.

sys.api_version

   The C API version for this interpreter.  Programmers may find this
   useful when debugging version conflicts between Python and
   extension modules.

sys.version_info

   A tuple containing the five components of the version number:
   *major*, *minor*, *micro*, *releaselevel*, and *serial*.  All
   values except *releaselevel* are integers; the release level is
   "'alpha'", "'beta'", "'candidate'", or "'final'".  The
   "version_info" value corresponding to the Python version 2.0 is
   "(2, 0, 0, 'final', 0)".  The components can also be accessed by
   name, so "sys.version_info[0]" is equivalent to
   "sys.version_info.major" and so on.

   在 3.1 版的變更: 新增了附名的元件屬性。

sys.warnoptions

   This is an implementation detail of the warnings framework; do not
   modify this value.  Refer to the "warnings" module for more
   information on the warnings framework.

sys.winver

   The version number used to form registry keys on Windows platforms.
   This is stored as string resource 1000 in the Python DLL.  The
   value is normally the major and minor versions of the running
   Python interpreter.  It is provided in the "sys" module for
   informational purposes; modifying this value has no effect on the
   registry keys used by Python.

   適用：Windows。

sys._xoptions

   A dictionary of the various implementation-specific flags passed
   through the "-X" command-line option.  Option names are either
   mapped to their values, if given explicitly, or to "True".
   Example:

      $ ./python -Xa=b -Xc
      Python 3.2a3+ (py3k, Oct 16 2010, 20:14:50)
      [GCC 4.4.3] on linux2
      Type "help", "copyright", "credits" or "license" for more information.
      >>> import sys
      >>> sys._xoptions
      {'a': 'b', 'c': True}

   **CPython 實作細節：** This is a CPython-specific way of accessing
   options passed through "-X".  Other implementations may export them
   through other means, or not at all.

   在 3.2 版新加入.

-[ 引用 ]-

[C99] ISO/IEC 9899:1999.  "Programming languages -- C."  公開草案可在
      以下網址取得 https://www.open-
      std.org/jtc1/sc22/wg14/www/docs/n1256.pdf。
