28.1. "sys" — 系统相关的参数和函数
**********************************

该模块提供了一些变量和函数。这些变量可能被解释器使用，也可能由解释器提
供。这些函数会影响解释器。本模块总是可用的。

sys.argv

   一个列表，其中包含了被传递给 Python 脚本的命令行参数。 "argv[0]" 为
   脚本的名称（是否是完整的路径名取决于操作系统）。如果是通过 Python
   解释器的命令行参数 "-c" 来执行的， "argv[0]" 会被设置成字符串
   "'-c'" 。如果没有脚本名被传递给 Python 解释器， "argv[0]" 为空字符
   串。

   为了遍历标准输入，或者通过命令行传递的文件列表，参照 "fileinput" 模
   块

sys.byteorder

   本地字节顺序的指示符。在大端序（最高有效位优先）操作系统上值为
   "'big'" ，在小端序（最低有效位优先）操作系统上为 "'little'" 。

   2.0 新版功能.

sys.builtin_module_names

   一个元素为字符串的元组。包含了所有的被编译进 Python 解释器的模块。
   （这个信息无法通过其他的办法获取， "modules.keys()" 只包括被导入过
   的模块。）

sys.call_tracing(func, args)

   在启用跟踪时调用 "func(*args)"  来保存跟踪状态，然后恢复跟踪状态。
   这将从检查点的调试器调用，以便递归地调试其他的一些代码。

sys.copyright

   一个字符串，包含了 Python 解释器有关的版权信息

sys._clear_type_cache()

   清除内部的类型缓存。类型缓存是为了加速查找方法和属性的。在调试引用
   泄漏的时候调用这个函数 *只会* 清除不必要的引用。

   这个函数应该只在内部为了一些特定的目的使用。

   2.6 新版功能.

sys._current_frames()

   返回一个字典，将每个线程的标识符映射到调用函数时该线程中当前活动的
   最顶层堆栈帧。注意 "traceback" 模块中的函数可以在给定帧的情况下构建
   调用堆栈。

   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.

   这个函数应该只在内部为了一些特定的目的使用。

   2.5 新版功能.

sys.dllhandle

   Integer specifying the handle of the Python DLL. Availability:
   Windows.

sys.displayhook(value)

   If *value* is not "None", this function prints it to "sys.stdout",
   and saves it in "__builtin__._".

   "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".

sys.dont_write_bytecode

   If this is true, Python won’t try to write ".pyc" or ".pyo" 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.

   2.6 新版功能.

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 or having executed an except clause.”  For
   any stack frame, only information about the most recently handled
   exception 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 exception type of the exception being handled (a class
   object); *value* gets the exception parameter (its *associated
   value* or the second argument to "raise", which is always a class
   instance if the exception type is a class object); *traceback* gets
   a traceback object (see the Reference Manual) which encapsulates
   the call stack at the point where the exception originally
   occurred.

   If "exc_clear()" is called, this function will return three "None"
   values until either another exception is raised in the current
   thread or the execution stack returns to a frame where another
   exception is being handled.

   警告: Assigning the *traceback* return value to a local variable
     in a function that is handling an exception will cause a circular
     reference.  This will prevent anything referenced by a local
     variable in the same function or by the traceback from being
     garbage collected.  Since most functions don’t need access to the
     traceback, the best solution is to use something like "exctype,
     value = sys.exc_info()[:2]" to extract only the exception type
     and value.  If you do need the traceback, make sure to delete it
     after use (best done with a "try" … "finally" statement) or to
     call "exc_info()" in a function that does not itself handle an
     exception.

   注解: Beginning with Python 2.2, such cycles are automatically
     reclaimed when garbage collection is enabled and they become
     unreachable, but it remains more efficient to avoid creating
     cycles.

sys.exc_clear()

   This function clears all information relating to the current or
   last exception that occurred in the current thread.  After calling
   this function, "exc_info()" will return three "None" values until
   another exception is raised in the current thread or the execution
   stack returns to a frame where another exception is being handled.

   This function is only needed in only a few obscure situations.
   These include logging and error handling systems that report
   information on the last or current exception.  This function can
   also be used to try to free resources and trigger object
   finalization, though no guarantee is made as to what objects will
   be freed, if any.

   2.3 新版功能.

sys.exc_type
sys.exc_value
sys.exc_traceback

   1.5 版后已移除: Use "exc_info()" instead.

   Since they are global variables, they are not specific to the
   current thread, so their use is not safe in a multi-threaded
   program.  When no exception is being handled, "exc_type" is set to
   "None" and the other two are undefined.

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 "2.7".

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.

sys.exitfunc

   This value is not actually defined by the module, but can be set by
   the user (or by a program) to specify a clean-up action at program
   exit.  When set, it should be a parameterless function.  This
   function will be called when the interpreter exits.  Only one
   function may be installed in this way; to allow multiple functions
   which will be called at termination, use the "atexit" module.

   注解: The exit function is not called when the program is killed
     by a signal, when a Python fatal internal error is detected, or
     when "os._exit()" is called.

   2.4 版后已移除: Use "atexit" instead.

sys.flags

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

   +-------------------------------+-------------------------------------+
   | 属性                          | flag                                |
   |===============================|=====================================|
   | "debug"                       | "-d"                                |
   +-------------------------------+-------------------------------------+
   | "py3k_warning"                | "-3"                                |
   +-------------------------------+-------------------------------------+
   | "division_warning"            | "-Q"                                |
   +-------------------------------+-------------------------------------+
   | "division_new"                | "-Qnew"                             |
   +-------------------------------+-------------------------------------+
   | "inspect"                     | "-i"                                |
   +-------------------------------+-------------------------------------+
   | "interactive"                 | "-i"                                |
   +-------------------------------+-------------------------------------+
   | "optimize"                    | "-O" or "-OO"                       |
   +-------------------------------+-------------------------------------+
   | "dont_write_bytecode"         | "-B"                                |
   +-------------------------------+-------------------------------------+
   | "no_user_site"                | "-s"                                |
   +-------------------------------+-------------------------------------+
   | "no_site"                     | "-S"                                |
   +-------------------------------+-------------------------------------+
   | "ignore_environment"          | "-E"                                |
   +-------------------------------+-------------------------------------+
   | "tabcheck"                    | "-t" or "-tt"                       |
   +-------------------------------+-------------------------------------+
   | "verbose"                     | "-v"                                |
   +-------------------------------+-------------------------------------+
   | "unicode"                     | "-U"                                |
   +-------------------------------+-------------------------------------+
   | "bytes_warning"               | "-b"                                |
   +-------------------------------+-------------------------------------+
   | "hash_randomization"          | "-R"                                |
   +-------------------------------+-------------------------------------+

   2.6 新版功能.

   2.7.3 新版功能: The "hash_randomization" attribute.

sys.float_info

   A structseq 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.

   +-----------------------+------------------+----------------------------------------------------+
   | 属性                  | 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'

   2.6 新版功能.

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 2.7
   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 2.7.

   2.7 新版功能.

sys.getcheckinterval()

   Return the interpreter’s “check interval”; see
   "setcheckinterval()".

   2.3 新版功能.

sys.getdefaultencoding()

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

   2.0 新版功能.

sys.getdlopenflags()

   Return the current value of the flags that are used for "dlopen()"
   calls. The flag constants are defined in the "dl" and "DLFCN"
   modules. Availability: Unix.

   2.2 新版功能.

sys.getfilesystemencoding()

   Return the name of the encoding used to convert Unicode filenames
   into system file names, or "None" if the system default encoding is
   used. The result value depends on the operating system:

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

   * On Unix, the encoding is the user’s preference according to the
     result of nl_langinfo(CODESET), or "None" if the
     "nl_langinfo(CODESET)" failed.

   * On Windows NT+, file names are Unicode natively, so no
     conversion is performed. "getfilesystemencoding()" still returns
     "'mbcs'", as this is the encoding that applications should use
     when they explicitly want to convert Unicode strings to byte
     strings that are equivalent when used as file names.

   * On Windows 9x, the encoding is "'mbcs'".

   2.3 新版功能.

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.

   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.

   2.6 新版功能.

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()".

   2.6 新版功能.

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.

   2.6 新版功能.

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*, and *product_type*.
   *service_pack* contains a string while 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* may be one of the following values:

   +-------------------------------------------+---------------------------+
   | 常数                                      | Platform                  |
   |===========================================|===========================|
   | "0 (VER_PLATFORM_WIN32s)"                 | Win32s on Windows 3.1     |
   +-------------------------------------------+---------------------------+
   | "1 (VER_PLATFORM_WIN32_WINDOWS)"          | Windows 95/98/ME          |
   +-------------------------------------------+---------------------------+
   | "2 (VER_PLATFORM_WIN32_NT)"               | Windows NT/2000/XP/x64    |
   +-------------------------------------------+---------------------------+
   | "3 (VER_PLATFORM_WIN32_CE)"               | Windows CE                |
   +-------------------------------------------+---------------------------+

   *product_type* may be one of the following values:

   +-----------------------------------------+-----------------------------------+
   | 常数                                    | 意义                              |
   |=========================================|===================================|
   | "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.

   Availability: Windows.

   2.3 新版功能.

   在 2.7 版更改: Changed to a named tuple and added
   *service_pack_minor*, *service_pack_major*, *suite_mask*, and
   *product_type*.

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 "version_info" value may be used for a more human-
   friendly encoding of the same information.

   The "hexversion" is a 32-bit number with the following layout:

   +---------------------------+--------------------------------------------------+
   | Bits (big endian order)   | 意义                                             |
   |===========================|==================================================|
   | "1-8"                     | "PY_MAJOR_VERSION"  (the "2" in "2.1.0a3")       |
   +---------------------------+--------------------------------------------------+
   | "9-16"                    | "PY_MINOR_VERSION"  (the "1" in "2.1.0a3")       |
   +---------------------------+--------------------------------------------------+
   | "17-24"                   | "PY_MICRO_VERSION"  (the "0" in "2.1.0a3")       |
   +---------------------------+--------------------------------------------------+
   | "25-28"                   | "PY_RELEASE_LEVEL"  ("0xA" for alpha, "0xB" for  |
   |                           | beta, "0xC" for release candidate and "0xF" for  |
   |                           | final)                                           |
   +---------------------------+--------------------------------------------------+
   | "29-32"                   | "PY_RELEASE_SERIAL"  (the "3" in "2.1.0a3", zero |
   |                           | for final releases)                              |
   +---------------------------+--------------------------------------------------+

   Thus "2.1.0a3" is hexversion "0x020100a3".

   1.5.2 新版功能.

sys.long_info

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

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

   2.7 新版功能.

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 chapter pdb — Python的调试器 for more
   information.)

   The meaning of the variables is the same as that of the return
   values from "exc_info()" above.  (Since there is only one
   interactive thread, thread-safety is not a concern for these
   variables, unlike for "exc_type" etc.)

sys.maxint

   The largest positive integer supported by Python’s regular integer
   type.  This is at least 2**31-1.  The largest negative integer is
   "-maxint-1" — the asymmetry results from the use of 2’s complement
   binary arithmetic.

sys.maxsize

   The largest positive integer supported by the platform’s Py_ssize_t
   type, and thus the maximum size lists, strings, dicts, and many
   other containers can have.

sys.maxunicode

   An integer giving the largest supported code point for a Unicode
   character.  The value of this depends on the configuration option
   that specifies whether Unicode characters are stored as UCS-2 or
   UCS-4.

sys.meta_path

   A list of *finder* objects that have their "find_module()" methods
   called to see if one of the objects can find the module to be
   imported. The "find_module()" method is called at least with the
   absolute name of the module being imported. If the module to be
   imported is contained in package then the parent package’s
   "__path__" attribute is passed in as a second argument. The method
   returns "None" if the module cannot be found, else returns a
   *loader*.

   "sys.meta_path" is searched before any implicit default finders or
   "sys.path".

   See **PEP 302** for the original specification.

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. Note that removing a module from this
   dictionary is *not* the same as calling "reload()" on the
   corresponding module object.

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.

   在 2.3 版更改: Unicode strings are no longer ignored.

   参见: 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 explicit finder is found on "sys.path_hooks" then "None" is
   stored to represent the implicit default finder should be used. If
   the path is not an existing path then "imp.NullImporter" is set.

   Originally specified in **PEP 302**.

sys.platform

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

   For most Unix systems, 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'", *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...

   在 2.7.3 版更改: Since lots of code check for "sys.platform ==
   'linux2'", and there is no essential change between Linux 2.x and
   3.x, "sys.platform" is always set to "'linux2'", even on Linux 3.x.
   In Python 3.3 and later, the value will always be set to "'linux'",
   so it is recommended to always use the "startswith" idiom presented
   above.

   For other systems, the values are:

   +-----------------------+-----------------------------+
   | System                | "platform" value            |
   |=======================|=============================|
   | Linux (2.x *and* 3.x) | "'linux2'"                  |
   +-----------------------+-----------------------------+
   | Windows               | "'win32'"                   |
   +-----------------------+-----------------------------+
   | Windows/Cygwin        | "'cygwin'"                  |
   +-----------------------+-----------------------------+
   | Mac OS X              | "'darwin'"                  |
   +-----------------------+-----------------------------+
   | OS/2                  | "'os2'"                     |
   +-----------------------+-----------------------------+
   | OS/2 EMX              | "'os2emx'"                  |
   +-----------------------+-----------------------------+
   | RiscOS                | "'riscos'"                  |
   +-----------------------+-----------------------------+
   | AtheOS                | "'atheos'"                  |
   +-----------------------+-----------------------------+

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

     "platform" 模块提供了对系统标识更详细的检查。

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 "2.7".

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.py3kwarning

   Bool containing the status of the Python 3 warning flag. It’s
   "True" when Python is started with the -3 option.  (This should be
   considered read-only; setting it to a different value doesn’t have
   an effect on Python 3 warnings.)

   2.6 新版功能.

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.

sys.setdefaultencoding(name)

   Set the current default string encoding used by the Unicode
   implementation.  If *name* does not match any available encoding,
   "LookupError" is raised. This function is only intended to be used
   by the "site" module implementation and, where needed, by
   "sitecustomize".  Once used by the "site" module, it is removed
   from the "sys" module’s namespace.

   2.0 新版功能.

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(dl.RTLD_NOW |
   dl.RTLD_GLOBAL)".  Symbolic names for the flag modules can be
   either found in the "dl" module, or in the "DLFCN" module. If
   "DLFCN" is not available, it can be generated from
   "/usr/include/dlfcn.h" using the **h2py** script. Availability:
   Unix.

   2.2 新版功能.

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 she has 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.

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 标准类型层
   级结构.

   **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.settscdump(on_flag)

   Activate dumping of VM measurements using the Pentium timestamp
   counter, if *on_flag* is true. Deactivate these dumps if *on_flag*
   is off. The function is available only if Python was compiled with
   "--with-tsc". To understand the output of this dump, read
   "Python/ceval.c" in the Python sources.

   2.4 新版功能.

   **CPython implementation detail:** This function is intimately
   bound to CPython implementation details and thus not likely to be
   implemented elsewhere.

sys.stdin
sys.stdout
sys.stderr

   File objects corresponding to the interpreter’s standard input,
   output and error streams.  "stdin" is used for all interpreter
   input except for scripts but including calls to "input()" and
   "raw_input()".  "stdout" is used for the output of "print" and
   *expression* statements and for the prompts of "input()" and
   "raw_input()". The interpreter’s own prompts and (almost all of)
   its error messages go to "stderr".  "stdout" and "stderr" needn’t
   be built-in file objects: any object is acceptable as long as it
   has a "write()" method that takes a string argument.  (Changing
   these objects doesn’t affect the standard I/O streams of processes
   executed by "os.popen()", "os.system()" or the "exec*()" family of
   functions in the "os" module.)

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.

sys.subversion

   A triple (repo, branch, version) representing the Subversion
   information of the Python interpreter. *repo* is the name of the
   repository, "'CPython'". *branch* is a string of one of the forms
   "'trunk'", "'branches/name'" or "'tags/name'". *version* is the
   output of "svnversion", if the interpreter was built from a
   Subversion checkout; it contains the revision number (range) and
   possibly a trailing ‘M’ if there were local modifications. If the
   tree was exported (or svnversion was not available), it is the
   revision of "Include/patchlevel.h" if the branch is a tag.
   Otherwise, it is "None".

   2.5 新版功能.

   注解: Python is now developed using Git.  In recent Python 2.7
     bugfix releases, "subversion" therefore contains placeholder
     information.  It is removed in Python 3.3.

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.

   2.3 新版功能.

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.

   2.0 新版功能.

   在 2.7 版更改: 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.

-[ 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.
