"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.abiflags

   在POSIX系统上，以标准的 "configure" 脚本构建的 Python 中，这个变量
   会包含 **PEP 3149** 中定义的ABI标签。

   在 3.8 版更改: 默认的 flags 变为了空字符串（用于 pymalloc 的 "m" 旗
   标已经移除）

   3.2 新版功能.

sys.addaudithook(hook)

   将可调用的 *hook* 附加到当前解释器的活动的审核钩子列表中。

   通过 "sys.audit()" 函数引发审计事件时，将按照加入钩子的先后顺序调用
   每个钩子，调用时将带有事件名称和参数元组。首先调用由
   "PySys_AddAuditHook()" 添加的静态钩子，然后调用添加到当前解释器中的
   钩子。

   引发一个 审计事件 "sys.addaudithook"，没有附带参数。

   3.8 新版功能.

   在 3.8.1 版更改: 派生自 "Exception" （而非 "RuntimeError" ）的异常
   不会被抑制。

   **CPython implementation detail:** 启用跟踪时（参阅 "settrace()" ）
   ，仅当可调用对象（钩子）的 "__cantrace__" 成员设置为 true 时，才会
   跟踪该钩子。否则，跟踪功能将跳过该钩子。

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

   注解:

     在 Unix 上，系统传递的命令行参数是字节类型的。Python 使用文件系统
     编码和 "surrogateescape" 错误处理程序对它们进行解码。当需要原始字
     节时，可以通过 "[os.fsencode(arg) for arg in sys.argv]" 来获取。

sys.audit(event, *args)

   利用某些活动的钩子引发审计事件。事件名称是一个字符串，用于标记事件
   及其关联的架构（即参数的数量和类型）。给定事件的架构应视作公开且稳
   定的 API，并且不应在版本之间进行修改。

   任何钩子抛出第一个异常时，此函数也将抛出该异常。通常不应处理这些错
   误，而应尽快终止该进程。

   钩子程序由 "sys.addaudithook()" 或 "PySys_AddAuditHook()" 函数添加
   。

   与本函数相等效的原生函数是 "PySys_Audit()"，应尽量使用原生函数。

   参阅 审计事件表 以获取 CPython 定义的所有审计事件。

   3.8 新版功能.

sys.base_exec_prefix

   在 "site.py" 运行之前， Python 启动的时候被设置为跟 "exec_prefix"
   同样的值。如果不是运行在 虚拟环境 中，两个值会保持相同；如果
   "site.py" 发现处于一个虚拟环境中， "prefix" 和 "exec_prefix" 将会指
   向虚拟环境。然而 "base_prefix" 和 "base_exec_prefix" 将仍然会指向基
   础的 Python 环境（用来创建虚拟环境的 Python 环境）

   3.3 新版功能.

sys.base_prefix

   在 "site.py" 运行之前， Python 启动的时候被设置为跟 "prefix" 同样的
   值。如果不是运行在 虚拟环境 中, 两个值会保持相同；如果 "site.py" 发
   现处于一个虚拟环境中， "prefix" 和 "exec_prefix" 将会指向虚拟环境。
   然而 "base_prefix" 和 "base_exec_prefix" 将仍然会指向基础的 Python
   环境（用来创建虚拟环境的 Python 环境）

   3.3 新版功能.

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'" 。

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。

sys._current_frames()

   返回一个字典，存放着每个线程的标识符与（调用本函数时）该线程栈顶的
   帧（当前活动的帧）之间的映射。注意 "traceback" 模块中的函数可以在给
   定某一帧的情况下构建调用堆栈。

   这对于调试死锁最有用：本函数不需要死锁线程的配合，并且只要这些线程
   的调用栈保持死锁，它们就是冻结的。在调用本代码来检查栈顶的帧的那一
   刻，非死锁线程返回的帧可能与该线程当前活动的帧没有任何关系。

   这个函数应该只在内部为了一些特定的目的使用。

   引发一个 审计事件 "sys._current_frames"，没有附带参数。

sys.breakpointhook()

   本钩子函数由内建函数 "breakpoint()" 调用。默认情况下，它将进入
   "pdb" 调试器，但可以将其改为任何其他函数，以选择使用哪个调试器。

   该函数的特征取决于其调用的函数。例如，默认绑定（即
   "pdb.set_trace()" ）不要求提供参数，但可以将绑定换成要求提供附加参
   数（位置参数/关键字参数）的函数。内建函数 "breakpoint()" 直接将其
   "*args" 和 "**kws" 传入。"breakpointhooks()" 返回的所有内容都会从
   "breakpoint()" 返回。

   默认的实现首先会查询环境变量 "PYTHONBREAKPOINT"。如果将该变量设置为
   ""0""，则本函数立即返回，表示在断点处无操作。如果未设置该环境变量或
   将其设置为空字符串，则调用 "pdb.set_trace()"。否则，此变量应指定要
   运行的函数，指定函数时应使用 Python 的点导入命名法，如
   "package.subpackage.module.function"。这种情况下将导入
   "package.subpackage.module"，且导入的模块必须有一个名为
   "function()" 的可调用对象。该可调用对象会运行，"*args" 和 "**kws"
   会传入，且无论 "function()" 返回什么，"sys.breakpointhook()" 都将返
   回到內建函数 "breakpoint()"。

   注意，如果在导入 "PYTHONBREAKPOINT" 指定的可调用对象时出错，则将报
   告一个 "RuntimeWarning" 并忽略断点。

   另请注意，如果以编程方式覆盖 "sys.breakpointhook()"，则 *不会* 查询
   "PYTHONBREAKPOINT"。

   3.7 新版功能.

sys._debugmallocstats()

   将有关 CPython 内存分配器状态的底层的信息打印至 stderr。

   如果 Python 被配置为 --with-pydebug，本方法还将执行一些开销较大的内
   部一致性检查。

   3.3 新版功能.

   **CPython implementation detail:** 本函数仅限 CPython。此处没有定义
   确切的输出格式，且可能会更改。

sys.dllhandle

   指向 Python DLL 句柄的整数。

   可用性: Windows。

sys.displayhook(value)

   如果 *value* 不是 "None"，则本函数会将 "repr(value)" 打印至
   "sys.stdout"，并将 *value* 保存在 "builtins._" 中。如果
   "repr(value)" 无法用 "sys.stdout.errors" 错误回调方法（可能较为“严
   格”）编码为 "sys.stdout.encoding"，请用 "'backslashreplace'" 错误回
   调方法将其编码为 "sys.stdout.encoding"。

   在交互式 Python 会话中运行 *expression* 产生结果后，将在结果上调用
   "sys.displayhook"。若要自定义这些 value 的显示，可以将
   "sys.displayhook" 指定为另一个单参数函数。

   伪代码:

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

   在 3.2 版更改: 在发生 "UnicodeEncodeError" 时使用
   "'backslashreplace'" 错误回调方法。

sys.dont_write_bytecode

   如果该值为 true，则 Python 在导入源码模块时将不会尝试写入 ".pyc" 文
   件。该值会被初始化为 "True" 或 "False"，依据是 "-B" 命令行选项和
   "PYTHONDONTWRITEBYTECODE" 环境变量，可以自行设置该值，来控制是否生
   成字节码文件。

sys.pycache_prefix

   如果将该选项设为某个目录（不是 "None" ），Python 会将字节码缓存文件
   ".pyc" 写入到以该目录为根的并行目录树中（并从中读取），而不是在源码
   树中的 "__pycache__" 目录下读写。源码树中所有的 "__pycache__" 目录
   都将被忽略，并将在 pycache prefix 内写入新的 *.pyc* 文件。因此，如
   果使用 "compileall" 作为预构建步骤，则必须确保预构建时使用的
   pycache prefix （如果有）与将来运行的时候相同。

   相对路径将解释为相对于当前工作目录。

   该值的初值设置，依据 "-X" "pycache_prefix=PATH" 命令行选项或
   "PYTHONPYCACHEPREFIX" 环境变量的值（命令行优先）。如果两者均未设置
   ，则为 "None"。

   3.8 新版功能.

sys.excepthook(type, value, traceback)

   本函数会将所给的回溯和异常输出到 "sys.stderr" 中。

   当抛出一个异常，且未被捕获时，解释器将调用 "sys.excepthook" 并带有
   三个参数：异常类、异常实例和一个回溯对象。在交互式会话中，这会在控
   制权返回到提示符之前发生。在 Python 程序中，这会在程序退出之前发生
   。如果要自定义此类顶级异常的处理过程，可以将另一个 3 个参数的函数赋
   给 "sys.excepthook"。

   引发一个 审计事件 "sys.excepthook"，附带参数 "hook", "type",
   "value", "traceback"。

   参见:

     "sys.unraisablehook()" 函数处理无法抛出的异常，
     "threading.excepthook()" 函数处理 "threading.Thread.run()" 抛出的
     异常。

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

   程序开始时，这些对象存有 "breakpointhook"、"displayhook"、
   "excepthook" 和 "unraisablehook" 的初始值。保存它们是为了可以在
   "breakpointhook"、"displayhook" 和 "excepthook"、"unraisablehook"
   被破坏或被替换时恢复它们。

   3.7 新版功能: __breakpointhook__

   3.8 新版功能: __unraisablehook__

sys.exc_info()

   本函数返回的元组包含三个值，它们给出当前正在处理的异常的信息。返回
   的信息仅限于当前线程和当前堆栈帧。如果当前堆栈帧没有正在处理的异常
   ，则信息将从调用的下级堆栈帧或上级调用者等位置获取，依此类推，直到
   找到正在处理异常的堆栈帧为止。此处的“处理异常”被定义为“执行 except
   子句”。任何堆栈帧都只能访问当前正在处理的异常的信息。

   如果整个堆栈都没有正在处理的异常，则返回包含三个 "None" 值的元组。
   否则返回值为 "(type, value, traceback)"。它们的含义是：*type* 是正
   在处理的异常类型（它是 "BaseException" 的子类）；*value* 是异常实例
   （异常类型的实例）；*traceback* 是一个 回溯对象，该对象封装了最初发
   生异常时的调用堆栈。

sys.exec_prefix

   一个字符串，给出特定域的目录前缀，该目录中安装了与平台相关的 Python
   文件，默认也是 "'/usr/local'"。该目录前缀可以在构建时使用
   **configure** 脚本的 "--exec-prefix" 参数进行设置。具体而言，所有配
   置文件（如 "pyconfig.h" 头文件）都安装在目录
   "*exec_prefix*/lib/python*X.Y*/config" 中，共享库模块安装在
   "*exec_prefix*/lib/python*X.Y*/lib-dynload" 中，其中 *X.Y* 是
   Python 的版本号，如 "3.2"。

   注解:

     如果在一个 虚拟环境 中，那么该值将在 "site.py" 中被修改，指向虚拟
     环境。Python 安装位置仍然可以用 "base_exec_prefix" 来获取。

sys.executable

   一个字符串，提供 Python 解释器的可执行二进制文件的绝对路径，仅在部
   分系统中此值有意义。如果 Python 无法获取其可执行文件的真实路径，则
   "sys.executable" 将为空字符串或 "None"。

sys.exit([arg])

   从Python中退出。实现方式是抛出一个 "SystemExit" 异常。异常抛出后
   "try" 声明的 finally 分支语句的清除动作将被出发。此动作有可能打断更
   外层的退出尝试。

   可选参数 *arg* 可以是表示退出状态的整数（默认为 0），也可以是其他类
   型的对象。如果它是整数，则 shell 等将 0 视为“成功终止”，非零值视为“
   异常终止”。大多数系统要求该值的范围是 0--127，否则会产生不确定的结
   果。某些系统为退出代码约定了特定的含义，但通常尚不完善；Unix 程序通
   常用 2 表示命令行语法错误，用 1 表示所有其他类型的错误。传入其他类
   型的对象，如果传入 "None" 等同于传入 0，如果传入其他对象则将其打印
   至 "stderr"，且退出代码为 1。特别地，"sys.exit("some error
   message")" 可以在发生错误时快速退出程序。

   由于 "exit()" 最终“只是”抛出一个异常，因此当从主线程调用时，只会从
   进程退出；而异常不会因此被打断。

   在 3.6 版更改: 在 Python 解释器捕获 "SystemExit" 后，如果在清理中发
   生错误（如清除标准流中的缓冲数据时出错），则退出状态码将变为 120。

sys.flags

   *具名元组* *flags* 含有命令行标志的状态。这些属性是只读的。

   +-------------------------------+-------------------------------+
   | attribute -- 属性             | 标志                          |
   |===============================|===============================|
   | "debug"                       | "-d"                          |
   +-------------------------------+-------------------------------+
   | "inspect"                     | "-i"                          |
   +-------------------------------+-------------------------------+
   | "interactive"                 | "-i"                          |
   +-------------------------------+-------------------------------+
   | "isolated"                    | "-I"                          |
   +-------------------------------+-------------------------------+
   | "optimize"                    | "-O" 或 "-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"                          |
   +-------------------------------+-------------------------------+
   | "dev_mode"                    | "-X" "dev"                    |
   +-------------------------------+-------------------------------+
   | "utf8_mode"                   | "-X" "utf8"                   |
   +-------------------------------+-------------------------------+

   在 3.2 版更改: 为新的 "-q" 标志添加了 "quiet" 属性

   3.2.3 新版功能: "hash_randomization" 属性

   在 3.3 版更改: 删除了过时的 "division_warning" 属性

   在 3.4 版更改: 为 "-I" "isolated" 标志添加了 "isolated" 属性。

   在 3.7 版更改: Added "dev_mode" attribute for the new "-X" "dev"
   flag and "utf8_mode" attribute for the new  "-X" "utf8" flag.

sys.float_info

   一个 *具名元组*，存有浮点型的相关信息。它包含的是关于精度和内部表示
   的底层信息。这些值与标准头文件 "float.h" 中为 C 语言定义的各种浮点
   常量对应，详情请参阅 1999 ISO/IEC C 标准 [C99] 的 5.2.4.2.2 节，
   'Characteristics of floating types（浮点型的特性）'。

   +-----------------------+------------------+----------------------------------------------------+
   | attribute -- 属性     | float.h 宏       | 说明                                               |
   |=======================|==================|====================================================|
   | "epsilon"             | DBL_EPSILON      | 大于 1.0 的最小值和 1.0 之间的差，表示为浮点数     |
   +-----------------------+------------------+----------------------------------------------------+
   | "dig"                 | DBL_DIG          | 浮点数可以真实表示的最大十进制数字；见下文         |
   +-----------------------+------------------+----------------------------------------------------+
   | "mant_dig"            | DBL_MANT_DIG     | 浮点数精度："radix" 基数下的浮点数有效位数         |
   +-----------------------+------------------+----------------------------------------------------+
   | "max"                 | DBL_MAX          | 最大可表示的正有限浮点数                           |
   +-----------------------+------------------+----------------------------------------------------+
   | "max_exp"             | DBL_MAX_EXP      | 使得 "radix**(e-1)" 是可表示的浮点数（非无穷）的最 |
   |                       |                  | 大整数 *e*                                         |
   +-----------------------+------------------+----------------------------------------------------+
   | "max_10_exp"          | DBL_MAX_10_EXP   | 使得 "10**e" 在可表示的浮点数（非无穷）范围内的最  |
   |                       |                  | 大整数 *e*                                         |
   +-----------------------+------------------+----------------------------------------------------+
   | "min"                 | DBL_MIN          | 可表示的最小正 *规格化* 浮点数                     |
   +-----------------------+------------------+----------------------------------------------------+
   | "min_exp"             | DBL_MIN_EXP      | 使得 "radix**(e-1)" 是规格化浮点数的最小整数 *e*   |
   +-----------------------+------------------+----------------------------------------------------+
   | "min_10_exp"          | DBL_MIN_10_EXP   | 使得 "10**e" 是规格化浮点数的最小整数 *e*          |
   +-----------------------+------------------+----------------------------------------------------+
   | "radix"               | FLT_RADIX        | 指数表示法中采用的基数                             |
   +-----------------------+------------------+----------------------------------------------------+
   | "rounds"              | FLT_ROUNDS       | 整数常数，表示算术运算中的舍入方式。它反映了解释器 |
   |                       |                  | 启动时系统的 FLT_ROUNDS 宏的值。关于可能的值及其含 |
   |                       |                  | 义的说明，请参阅 C99 标准 5.2.4.2.2 节。           |
   +-----------------------+------------------+----------------------------------------------------+

   关于 "sys.float_info.dig" 属性的进一步说明。如果 "s" 是表示十进制数
   的字符串，而该数最多有 "sys.float_info.dig" 位有效数字，则将 "s" 转
   换为 float 再转回去将恢复原先相同十进制值的字符串:

      >>> 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'

   但是对于超过 "sys.float_info.dig" 位有效数字的字符串，转换前后并非
   总是相同:

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

sys.float_repr_style

   一个字符串，反映 "repr()" 函数在浮点数上的行为。如果该字符串是
   "'short'"，那么对于（非无穷的）浮点数 "x"，"repr(x)" 将会生成一个短
   字符串，满足 "float(repr(x)) == x" 的特性。这是 Python 3.1 及更高版
   本中的常见行为。否则 "float_repr_style" 的值将是 "'legacy'"，此时
   "repr(x)" 的行为方式将与 Python 3.1 之前的版本相同。

   3.1 新版功能.

sys.getallocatedblocks()

   返回解释器当前已分配的内存块数，无论它们大小如何。本函数主要用于跟
   踪和调试内存泄漏。因为解释器有内部缓存，所以不同调用之间结果会变化
   。可能需要调用 "_clear_type_cache()" 和 "gc.collect()" 使结果更容易
   预测。

   如果当前 Python 构建或实现无法合理地计算此信息，允许
   "getallocatedblocks()" 返回 0。

   3.4 新版功能.

sys.getandroidapilevel()

   返回一个整数，表示 Android 构建时 API 版本。

   可用性：Android。

   3.7 新版功能.

sys.getcheckinterval()

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

   3.2 版后已移除: Use "getswitchinterval()" instead.

sys.getdefaultencoding()

   返回当前 Unicode 实现所使用的默认字符串编码名称。

sys.getdlopenflags()

   返回当前 "dlopen()" 调用所使用的标志位的值。标志值对应的符号名称可
   以在 "os" 模块中找到（形如 "RTLD_xxx" 的常量，如 "os.RTLD_LAZY" ）
   。

   可用性: Unix。

sys.getfilesystemencoding()

   返回编码名称，该编码用于在 Unicode 文件名和 bytes 文件名之间转换。
   为获得最佳兼容性，任何时候都应使用 str 表示文件名，尽管用字节来表示
   文件名也是支持的。函数如果需要接受或返回文件名，它应支持 str 或
   bytes，并在内部将其转换为系统首选的表示形式。

   该编码始终是 ASCII 兼容的。

   应使用 "os.fsencode()" 和 "os.fsdecode()" 来保证所采用的编码和错误
   模式都是正确的。

   * 在UTF-8模式下，任何平台上的编码均为 "utf-8" 。

   * 在macOS上，编码为 "'utf-8'"。

   * 在Unix上，编码是语言环境编码。

   * 在Windows上取决于用户配置，编码可能是 "'utf-8'" 或 "'mbcs'"。

   * 在Android上，编码为 "'utf-8'"。

   * 在VxWorks上，编码为 "'utf-8'"。

   在 3.2 版更改: "getfilesystemencoding()" 的结果将不再有可能是
   "None"。

   在 3.6 版更改: Windows 不再保证会返回 "'mbcs'"。详情请参阅 **PEP
   529** 和 "_enablelegacywindowsfsencoding()"。

   在 3.7 版更改: 在UTF-8模式下返回 'utf-8' 。

sys.getfilesystemencodeerrors()

   返回错误回调函数的名称，该错误回调函数将在 Unicode 文件名和 bytes
   文件名转换时生效。编码的名称是由 "getfilesystemencoding()" 返回的。

   应使用 "os.fsencode()" 和 "os.fsdecode()" 来保证所采用的编码和错误
   模式都是正确的。

   3.6 新版功能.

sys.getrefcount(object)

   返回 *object* 的引用计数。返回的计数通常比预期的多一，因为它包括了
   作为 "getrefcount()" 参数的这一次（临时）引用。

sys.getrecursionlimit()

   返回当前的递归限制值，即 Python 解释器堆栈的最大深度。此限制可防止
   无限递归导致的 C 堆栈溢出和 Python 崩溃。该值可以通过
   "setrecursionlimit()" 设置。

sys.getsizeof(object[, default])

   返回对象的大小（以字节为单位）。该对象可以是任何类型。所有内建对象
   返回的结果都是正确的，但对于第三方扩展不一定正确，因为这与具体实现
   有关。

   只计算直接分配给对象的内存消耗，不计算它所引用的对象的内存消耗。

   对象不提供计算大小的方法时，如果有给出 *default* 则返回它，否则抛出
   "TypeError" 异常。

   如果对象由垃圾回收器管理，则 "getsizeof()" 将调用对象的
   "__sizeof__" 方法，并在上层添加额外的垃圾回收器。

   可以参考 recursive sizeof recipe 中的示例，关于递归调用
   "getsizeof()" 来得到各个容器及其所有内容物的大小。

sys.getswitchinterval()

   返回解释器的“线程切换间隔时间”，请参阅 "setswitchinterval()"。

   3.2 新版功能.

sys._getframe([depth])

   返回来自调用栈的一个帧对象。如果传入可选整数 *depth*，则返回从栈顶
   往下相应调用层数的帧对象。如果该数比调用栈更深，则抛出 "ValueError"
   。*depth* 的默认值是 0，返回调用栈顶部的帧。

   引发一个 审计事件 "sys._getframe"，没有附带参数。

   **CPython implementation detail:** 这个函数应该只在内部为了一些特定
   的目的使用。不保证它在所有 Python 实现中都存在。

sys.getprofile()

   返回由 "setprofile()" 设置的性能分析函数

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:

   +-----------------------------------------+-----------------------------------+
   | 常数                                    | 含义                              |
   |=========================================|===================================|
   | "1 (VER_NT_WORKSTATION)"                | 系统是工作站。                    |
   +-----------------------------------------+-----------------------------------+
   | "2 (VER_NT_DOMAIN_CONTROLLER)"          | 系统是域控制器。                  |
   +-----------------------------------------+-----------------------------------+
   | "3 (VER_NT_SERVER)"                     | 系统是服务器，但不是域控制器。    |
   +-----------------------------------------+-----------------------------------+

   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.

   可用性: Windows。

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

   在 3.6 版更改: 添加了 *platform_version*

sys.get_asyncgen_hooks()

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

   3.6 新版功能: See **PEP 525** for more details.

   注解:

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

sys.get_coroutine_origin_tracking_depth()

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

   3.7 新版功能.

   注解:

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

sys.hash_info

   A *named tuple* giving parameters of the numeric hash
   implementation.  For more details about hashing of numeric types,
   see 数字类型的哈希运算.

   +-----------------------+----------------------------------------------------+
   | attribute -- 属性     | 说明                                               |
   |=======================|====================================================|
   | "width"               | 用于哈希值的位宽度                                 |
   +-----------------------+----------------------------------------------------+
   | "modulus"             | 用于数字散列方案的素数模数P。                      |
   +-----------------------+----------------------------------------------------+
   | "inf"                 | 为正无穷大返回的哈希值                             |
   +-----------------------+----------------------------------------------------+
   | "nan"                 | 为nan返回的哈希值                                  |
   +-----------------------+----------------------------------------------------+
   | "imag"                | 用于复数虚部的乘数                                 |
   +-----------------------+----------------------------------------------------+
   | "algorithm"           | 字符串、字节和内存视图的哈希算法的名称             |
   +-----------------------+----------------------------------------------------+
   | "hash_bits"           | 哈希算法的内部输出大小。                           |
   +-----------------------+----------------------------------------------------+
   | "seed_bits"           | 散列算法的种子密钥的大小                           |
   +-----------------------+----------------------------------------------------+

   3.2 新版功能.

   在 3.4 版更改: 添加了 *algorithm*, *hash_bits* 和 *seed_bits*

sys.hexversion

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

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

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

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

sys.implementation

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

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

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

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

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

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

   3.3 新版功能.

   注解:

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

sys.int_info

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

   +---------------------------+------------------------------------------------+
   | 属性                      | 说明                                           |
   |===========================|================================================|
   | "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"            | 用于表示数字的C类型的字节大小                  |
   +---------------------------+------------------------------------------------+

   3.1 新版功能.

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.

   Raises an auditing event "cpython.run_interactivehook" with the
   hook object as the argument when the hook is called on startup.

   3.4 新版功能.

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 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 "pdb" module for more information.)

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

sys.maxsize

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

sys.maxunicode

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

   在 3.3 版更改: 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.

   参见:

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

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

   在 3.4 版更改: *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.

   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 finder is found on "sys.path_hooks" then "None" is stored.

   Originally specified in **PEP 302**.

   在 3.3 版更改: "None" is stored instead of "imp.NullImporter" when
   no finder is found.

sys.platform

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

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

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

   对于其他系统，值是：

   +------------------+-----------------------------+
   | 系统             | "平台" 值                   |
   |==================|=============================|
   | AIX              | "'aix'"                     |
   +------------------+-----------------------------+
   | Linux            | "'linux'"                   |
   +------------------+-----------------------------+
   | Windows          | "'win32'"                   |
   +------------------+-----------------------------+
   | Windows/Cygwin   | "'cygwin'"                  |
   +------------------+-----------------------------+
   | macOS            | "'darwin'"                  |
   +------------------+-----------------------------+

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

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

   参见:

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

     "platform" 模块对系统的标识有更详细的检查。

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 "3.2".

   注解:

     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 版后已移除: 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").

   可用性: Unix。

sys.setprofile(profilefunc)

   设置系统的性能分析函数，该函数使得在 Py​​thon 中能够实现一个 Python
   源代码性能分析器。关于 Python Profiler 的更多信息请参阅 Python
   Profilers 分析器 章节。性能分析函数的调用方式类似于系统的跟踪函数（
   参阅 "settrace()" ），但它是通过不同的事件调用的，例如，不是每执行
   一行代码就调用它一次（仅在调用某函数和从某函数返回时才会调用性能分
   析函数，但即使某函数发生异常也会算作返回事件）。该函数是特定于线程
   的，但是性能分析器无法得知线程之间的上下文切换，因此在存在多个线程
   的情况下使用它是没有意义的。另外，因为它的返回值不会被用到，所以可
   以简单地返回 "None"。性能分析函数中的错误将导致其自身被解除设置。

   性能分析函数应接收三个参数：*frame*、*event* 和 *arg*。*frame* 是当
   前的堆栈帧。*event* 是一个字符串："'call'"、"'return'"、"'c_call'"
   、"'c_return'" 或 "'c_exception'"。*arg* 取决于事件类型。

   引发一个 审计事件 "sys.setprofile"，不附带任何参数。

   这些事件具有以下含义：

   "'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.

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

sys.setswitchinterval(interval)

   设置解释器的线程切换间隔时间（单位为秒）。该浮点数决定了“时间片”的
   理想持续时间，时间片将分配给同时运行的 Python 线程。请注意，实际值
   可能更高，尤其是使用了运行时间长的内部函数或方法时。同时，在时间间
   隔末尾调度哪个线程是操作系统的决定。解释器没有自己的调度程序。

   3.2 新版功能.

sys.settrace(tracefunc)

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

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

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

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

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

   这些事件具有以下含义：

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

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

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

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

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

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

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

   For more information on code and frame objects, refer to 标准类型层
   级结构.

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

   **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.

   在 3.7 版更改: "'opcode'" event type added; "f_trace_lines" and
   "f_trace_opcodes" attributes added to frames

sys.set_asyncgen_hooks(firstiter, finalizer)

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

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

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

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

   3.6 新版功能: 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

   注解:

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

sys.set_coroutine_origin_tracking_depth(depth)

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

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

   This setting is thread-specific.

   3.7 新版功能.

   注解:

     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.

   可用性: Windows。

   3.6 新版功能: 有关更多详细信息，请参阅 **PEP 529**。

sys.stdin
sys.stdout
sys.stderr

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

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

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

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

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

   * The character encoding is platform-dependent.  Non-Windows
     platforms use the locale encoding (see
     "locale.getpreferredencoding()").

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

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

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

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

   注解:

     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.

   注解:

     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 *named tuple* holding information about the thread
   implementation.

   +--------------------+-----------------------------------------------------------+
   | 属性               | 说明                                                      |
   |====================|===========================================================|
   | "name"             | 线程实现的名称：  * "'nt'": Windows 线程  * "'pthread'":  |
   |                    | POSIX 线程  * "'solaris'": Solaris 线程                   |
   +--------------------+-----------------------------------------------------------+
   | "lock"             | 锁实现的名称：  * "'semaphore'": 锁使用信号量  *          |
   |                    | "'mutex+cond'": 锁使用互斥和条件变量  * "None" 如果此信息 |
   |                    | 未知                                                      |
   +--------------------+-----------------------------------------------------------+
   | "version"          | 线程库的名称和版本。它是一个字符串，如果此信息未知，则为  |
   |                    | "None" 。                                                 |
   +--------------------+-----------------------------------------------------------+

   3.3 新版功能.

sys.tracebacklimit

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

sys.unraisablehook(unraisable, /)

   Handle an unraisable exception.

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

   The *unraisable* argument has the following attributes:

   * *exc_type*: 异常类型

   * *exc_value*: 异常值，可以是 "None".

   * *exc_traceback*: 异常回溯，可以是 "None".

   * *err_msg*: 错误信息，可以是 "None".

   * *object*: 导致异常的对象，可以为 "None".

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

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

   使用定制钩子存放 *exc_value* 可能会创建引用循环。 它应当在不再需要
   异常时被显式地清空以打破引用循环。

   Storing *object* using a custom hook can resurrect it if it is set
   to an object which is being finalized. Avoid storing *object* after
   the custom hook completes to avoid resurrecting objects.

   See also "excepthook()" which handles uncaught exceptions.

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

   3.8 新版功能.

sys.version

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

sys.api_version

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

sys.version_info

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

   在 3.1 版更改: 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.

   可用性: 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 新版功能.

-[ 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.
