29.1. "sys" --- System-specific parameters and functions
********************************************************

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

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**.

   3.2 版新加入.

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.

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).

   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).

   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 giving 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.)

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 some other code.

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.

sys._debugmallocstats()

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

   If Python is configured --with-pydebug, it also performs some
   expensive internal consistency checks.

   3.3 版新加入.

   **CPython implementation detail:** 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. Availability:
   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

   3.2 版更變: 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.excepthook(type, value, traceback)

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

   When an exception 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".

sys.__displayhook__
sys.__excepthook__

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

sys.exc_info()

   This function returns a tuple of three values that give information
   about the exception that is currently being handled.  The
   information returned is specific both to the current thread and to
   the current stack frame.  If the current stack frame is not
   handling an exception, the information is taken from the calling
   stack frame, or its caller, and so on until a stack frame is found
   that is handling an exception.  Here, "handling an exception" is
   defined as "executing an except clause."  For any stack frame, only
   information about the exception being currently handled is
   accessible.

   If no exception is being handled anywhere on the stack, a tuple
   containing three "None" values is returned.  Otherwise, the values
   returned are "(type, value, traceback)".  Their meaning is: *type*
   gets the type of the exception being handled (a subclass of
   "BaseException"); *value* gets the exception instance (an instance
   of the exception type); *traceback* gets a traceback object (see
   the Reference Manual) which encapsulates the call stack at the
   point where the exception originally occurred.

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])

   Exit from Python.  This is implemented by raising the "SystemExit"
   exception, so cleanup actions specified by finally clauses of "try"
   statements are honored, and it is possible to intercept the exit
   attempt at an outer level.

   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.

   3.6 版更變: 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 *struct sequence* *flags* exposes the status of command line
   flags. The attributes are read only.

   +-------------------------------+-------------------------------+
   | attribute                     | flag                          |
   |===============================|===============================|
   | "debug"                       | "-d"                          |
   +-------------------------------+-------------------------------+
   | "inspect"                     | "-i"                          |
   +-------------------------------+-------------------------------+
   | "interactive"                 | "-i"                          |
   +-------------------------------+-------------------------------+
   | "isolated"                    | "-I"                          |
   +-------------------------------+-------------------------------+
   | "optimize"                    | "-O" or "-OO"                 |
   +-------------------------------+-------------------------------+
   | "dont_write_bytecode"         | "-B"                          |
   +-------------------------------+-------------------------------+
   | "no_user_site"                | "-s"                          |
   +-------------------------------+-------------------------------+
   | "no_site"                     | "-S"                          |
   +-------------------------------+-------------------------------+
   | "ignore_environment"          | "-E"                          |
   +-------------------------------+-------------------------------+
   | "verbose"                     | "-v"                          |
   +-------------------------------+-------------------------------+
   | "bytes_warning"               | "-b"                          |
   +-------------------------------+-------------------------------+
   | "quiet"                       | "-q"                          |
   +-------------------------------+-------------------------------+
   | "hash_randomization"          | "-R"                          |
   +-------------------------------+-------------------------------+

   3.2 版更變: Added "quiet" attribute for the new "-q" flag.

   3.2.3 版新加入: The "hash_randomization" attribute.

   3.3 版更變: Removed obsolete "division_warning" attribute.

   3.4 版更變: Added "isolated" attribute for "-I" "isolated" flag.

sys.float_info

   A *struct sequence* 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.

   +-----------------------+------------------+----------------------------------------------------+
   | attribute             | float.h macro    | explanation                                        |
   |=======================|==================|====================================================|
   | "epsilon"             | DBL_EPSILON      | difference between 1 and the least value greater   |
   |                       |                  | than 1 that is representable as a float            |
   +-----------------------+------------------+----------------------------------------------------+
   | "dig"                 | DBL_DIG          | maximum number of decimal digits that can be       |
   |                       |                  | faithfully represented in a float;  see below      |
   +-----------------------+------------------+----------------------------------------------------+
   | "mant_dig"            | DBL_MANT_DIG     | float precision: the number of base-"radix" digits |
   |                       |                  | in the significand of a float                      |
   +-----------------------+------------------+----------------------------------------------------+
   | "max"                 | DBL_MAX          | maximum representable finite float                 |
   +-----------------------+------------------+----------------------------------------------------+
   | "max_exp"             | DBL_MAX_EXP      | maximum integer e such that "radix**(e-1)" is a    |
   |                       |                  | representable finite float                         |
   +-----------------------+------------------+----------------------------------------------------+
   | "max_10_exp"          | DBL_MAX_10_EXP   | maximum integer e such that "10**e" is in the      |
   |                       |                  | range of representable finite floats               |
   +-----------------------+------------------+----------------------------------------------------+
   | "min"                 | DBL_MIN          | minimum positive normalized float                  |
   +-----------------------+------------------+----------------------------------------------------+
   | "min_exp"             | DBL_MIN_EXP      | minimum integer e such that "radix**(e-1)" is a    |
   |                       |                  | normalized float                                   |
   +-----------------------+------------------+----------------------------------------------------+
   | "min_10_exp"          | DBL_MIN_10_EXP   | minimum integer e such that "10**e" is a           |
   |                       |                  | normalized float                                   |
   +-----------------------+------------------+----------------------------------------------------+
   | "radix"               | FLT_RADIX        | radix of exponent representation                   |
   +-----------------------+------------------+----------------------------------------------------+
   | "rounds"              | FLT_ROUNDS       | integer constant representing the rounding mode    |
   |                       |                  | used for arithmetic operations.  This reflects the |
   |                       |                  | value of the system FLT_ROUNDS macro at            |
   |                       |                  | interpreter startup time.  See section 5.2.4.2.2   |
   |                       |                  | of the C99 standard for an explanation of the      |
   |                       |                  | possible values and their meanings.                |
   +-----------------------+------------------+----------------------------------------------------+

   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.

   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.

   3.4 版新加入.

sys.getcheckinterval()

   Return the interpreter's "check interval"; see
   "setcheckinterval()".

   3.2 版後已棄用: Use "getswitchinterval()" instead.

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").  Availability:
   Unix.

sys.getfilesystemencoding()

   Return the name of the encoding used to convert between Unicode
   filenames and bytes filenames. 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.

   This encoding is always ASCII-compatible.

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

   * On Mac OS X, the encoding is "'utf-8'".

   * On Unix, the encoding is the locale encoding.

   * On Windows, the encoding may be "'utf-8'" or "'mbcs'",
     depending on user configuration.

   3.2 版更變: "getfilesystemencoding()" result cannot be "None"
   anymore.

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

sys.getfilesystemencodeerrors()

   Return the name of the error mode used to convert between Unicode
   filenames and bytes filenames. The encoding name is returned from
   "getfilesystemencoding()".

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

   3.6 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()".

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()".

   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.

   **CPython implementation detail:** 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 implementation detail:** 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 accurate 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.

   Availability: Windows.

   3.2 版更變: Changed to a named tuple and added
   *service_pack_minor*, *service_pack_major*, *suite_mask*, and
   *product_type*.

   3.6 版更變: Added *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 asychronous
   generator by an event loop.

   3.6 版新加入: See **PEP 525** for more details.

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

sys.get_coroutine_wrapper()

   Returns "None", or a wrapper set by "set_coroutine_wrapper()".

   3.5 版新加入: See **PEP 492** for more details.

   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 *struct sequence* giving parameters of the numeric hash
   implementation.  For more details about hashing of numeric types,
   see Hashing of numeric types.

   +-----------------------+----------------------------------------------------+
   | attribute             | explanation                                        |
   |=======================|====================================================|
   | "width"               | width in bits used for hash values                 |
   +-----------------------+----------------------------------------------------+
   | "modulus"             | prime modulus P used for numeric hash scheme       |
   +-----------------------+----------------------------------------------------+
   | "inf"                 | hash value returned for a positive infinity        |
   +-----------------------+----------------------------------------------------+
   | "nan"                 | hash value returned for a nan                      |
   +-----------------------+----------------------------------------------------+
   | "imag"                | multiplier used for the imaginary part of a        |
   |                       | complex number                                     |
   +-----------------------+----------------------------------------------------+
   | "algorithm"           | name of the algorithm for hashing of str, bytes,   |
   |                       | and memoryview                                     |
   +-----------------------+----------------------------------------------------+
   | "hash_bits"           | internal output size of the hash algorithm         |
   +-----------------------+----------------------------------------------------+
   | "seed_bits"           | size of the seed key of the hash algorithm         |
   +-----------------------+----------------------------------------------------+

   3.2 版新加入.

   3.4 版更變: Added *algorithm*, *hash_bits* and *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 *struct sequence*  "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 and ABI
   Versioning.

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.

   3.3 版新加入.

sys.int_info

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

   +---------------------------+------------------------------------------------+
   | Attribute                 | Explanation                                    |
   |===========================|================================================|
   | "bits_per_digit"          | number of bits held in each digit.  Python     |
   |                           | integers are stored internally in base         |
   |                           | "2**int_info.bits_per_digit"                   |
   +---------------------------+------------------------------------------------+
   | "sizeof_digit"            | size in bytes of the C type used to represent  |
   |                           | a digit                                        |
   +---------------------------+------------------------------------------------+

   3.1 版新加入.

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.

   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.

   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).

   3.3 版更變: 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. 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.

   3.4 版更變: *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.

sys.path

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

   As initialized upon program startup, the first item of this list,
   "path[0]", is the directory containing the script that was used to
   invoke the Python interpreter.  If the script directory is not
   available (e.g.  if the interpreter is invoked interactively or if
   the script is read from standard input), "path[0]" is the empty
   string, which directs Python to search modules in the current
   directory first.  Notice that the script directory is inserted
   *before* the entries inserted as a result of "PYTHONPATH".

   A program is free to modify this list for its own purposes.  Only
   strings and bytes 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**.

   3.3 版更變: "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, 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...

   For other systems, the values are:

   +------------------+-----------------------------+
   | System           | "platform" value            |
   |==================|=============================|
   | Linux            | "'linux'"                   |
   +------------------+-----------------------------+
   | Windows          | "'win32'"                   |
   +------------------+-----------------------------+
   | Windows/Cygwin   | "'cygwin'"                  |
   +------------------+-----------------------------+
   | Mac OS X         | "'darwin'"                  |
   +------------------+-----------------------------+

   3.3 版更變: 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.

   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.prefix

   A string giving the site-specific directory prefix where the
   platform independent Python files are installed; by default, this
   is the string "'/usr/local'".  This can be set at build time with
   the "--prefix" argument to the **configure** script.  The main
   collection of Python library modules is installed in the directory
   "*prefix*/lib/python*X.Y*" while the platform independent header
   files (all except "pyconfig.h") are stored in
   "*prefix*/include/python*X.Y*", 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_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.setcheckinterval(interval)

   Set the interpreter's "check interval".  This integer value
   determines how often the interpreter checks for periodic things
   such as thread switches and signal handlers.  The default is "100",
   meaning the check is performed every 100 Python virtual
   instructions. Setting it to a larger value may increase performance
   for programs using threads.  Setting it to a value "<=" 0 checks
   every virtual instruction, maximizing responsiveness as well as
   overhead.

   3.2 版後已棄用: This function doesn't have an effect anymore, as
   the internal logic for thread switching and asynchronous tasks has
   been rewritten.  Use "setswitchinterval()" instead.

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").

   Availability: Unix.

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".

   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.

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.

   3.5.1 版更變: 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.

   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 be
   registered using "settrace()" for each thread being debugged.

   Trace functions should have three arguments: *frame*, *event*, and
   *arg*. *frame* is the current stack frame.  *event* is a string:
   "'call'", "'line'", "'return'" or "'exception'".  *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 that scope, or "None" if the
   scope shouldn't be traced.

   The local trace function should return a reference to itself (or to
   another function for further tracing in that scope), or "None" to
   turn off tracing in that scope.

   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.

   "'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.

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

   For more information on code and frame objects, refer to The
   standard type hierarchy.

   **CPython implementation detail:** 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.

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.

   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_wrapper(wrapper)

   Allows intercepting creation of *coroutine* objects (only ones that
   are created by an "async def" function; generators decorated with
   "types.coroutine()" or "asyncio.coroutine()" will not be
   intercepted).

   The *wrapper* argument must be either:

   * a callable that accepts one argument (a coroutine object);

   * "None", to reset the wrapper.

   If called twice, the new wrapper replaces the previous one.  The
   function is thread-specific.

   The *wrapper* callable cannot define new coroutines directly or
   indirectly:

      def wrapper(coro):
          async def wrap(coro):
              return await coro
          return wrap(coro)
      sys.set_coroutine_wrapper(wrapper)

      async def foo():
          pass

      # The following line will fail with a RuntimeError, because
      # ``wrapper`` creates a ``wrap(coro)`` coroutine:
      foo()

   See also "get_coroutine_wrapper()".

   3.5 版新加入: See **PEP 492** for more details.

   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 default filesystem encoding and errors mode 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.

   Availability: Windows

   3.6 版新加入: See **PEP 529** for more details.

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 character encoding is platform-dependent.  Under Windows,
     if the stream is interactive (that is, if its "isatty()" method
     returns "True"), the console codepage is used, otherwise the ANSI
     code page.  Under other platforms, the locale encoding is used
     (see "locale.getpreferredencoding()").

     Under all platforms though, you can override this value by
     setting the "PYTHONIOENCODING" environment variable before
     starting Python.

   * When interactive, standard streams are line-buffered.
     Otherwise, they are block-buffered like regular text files.  You
     can override this value with the "-u" command-line option.

   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.thread_info

   A *struct sequence* holding information about the thread
   implementation.

   +--------------------+-----------------------------------------------------------+
   | Attribute          | Explanation                                               |
   |====================|===========================================================|
   | "name"             | Name of the thread implementation:  * "'nt'": Windows     |
   |                    | threads  * "'pthread'": POSIX threads  * "'solaris'":     |
   |                    | Solaris threads                                           |
   +--------------------+-----------------------------------------------------------+
   | "lock"             | Name of the lock implementation:  * "'semaphore'": a lock |
   |                    | uses a semaphore  * "'mutex+cond'": a lock uses a mutex   |
   |                    | and a condition variable  * "None" if this information is |
   |                    | unknown                                                   |
   +--------------------+-----------------------------------------------------------+
   | "version"          | Name and version of the thread library. It is a string,   |
   |                    | or "None" if these informations are unknown.              |
   +--------------------+-----------------------------------------------------------+

   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.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.

   3.1 版更變: Added named component attributes.

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 first three characters of "version".  It is
   provided in the "sys" module for informational purposes; modifying
   this value has no effect on the registry keys used by Python.
   Availability: 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 implementation detail:** This is a CPython-specific way
   of accessing options passed through "-X".  Other implementations
   may export them through other means, or not at all.

   3.2 版新加入.

-[ Citations ]-

[C99] ISO/IEC 9899:1999.  "Programming languages -- C."  A
      public draft of this standard is available at http://www.open-
      std.org/jtc1/sc22/wg14/www/docs/n1256.pdf.
