# `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()`。

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

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

  3.7 新版功能.

`sys.``_debugmallocstats`()

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

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

  3.3 新版功能.

  CPython implementation detail: This function is specific to CPython. The exact output format is not defined here, and may change.

`sys.``dllhandle`

  Integer specifying the handle of the Python DLL.

  可用性: Windows。

`sys.``displayhook`(value)

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

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

  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: Use `'backslashreplace'` error handler on `UnicodeEncodeError`.

`sys.``dont_write_bytecode`

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

`sys.``pycache_prefix`

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

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

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

  3.8 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`.

  Raise an auditing event `sys.excepthook` with arguments `hook`, `type`, `value`, `traceback` when an uncaught exception occurs. If no hook has been set, `hook` may be `None`. If any hook raises an exception derived from `RuntimeError` the call to the hook will be suppressed. Otherwise, the audit hook exception will be reported as unraisable and `sys.excepthook` will be called.

  参见

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

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

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

  3.7 新版功能: __breakpointhook__

  3.8 新版功能: __unraisablehook__

`sys.``exc_info`()

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

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

`sys.``exec_prefix`

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

`sys.``executable`

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

`sys.``exit`([arg])

  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。

  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.

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

`sys.``flags`

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

  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` (Python Development Mode)
  `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 the `dev_mode` attribute for the new Python Development Mode and the `utf8_mode` attribute for the new `-X` `utf8` flag.

`sys.``float_info`

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

  属性 | float.h 宏 | 说明
  --- | --- | ---
  `epsilon` | DBL_EPSILON | difference between 1.0 and the least value greater than 1.0 that is representable as a float See also `math.ulp()`.
  `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 positive 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 representable positive normalized float Use `math.ulp(0.0)` to get the smallest positive denormalized representable float.
  `min_exp` | DBL_MIN_EXP | minimum integer e such that `radix**(e-1)` is a normalized float
  `min_10_exp` | DBL_MIN_10_EXP | minimum integer e such that `10**e` is a normalized float
  `radix` | FLT_RADIX | radix of exponent representation
  `rounds` | FLT_ROUNDS | integer constant representing the rounding mode used for arithmetic operations. This reflects the value of the system FLT_ROUNDS macro at interpreter startup time. See section 5.2.4.2.2 of the C99 standard for an explanation of the possible values and their meanings.

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

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

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

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

`sys.``float_repr_style`

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

  3.1 新版功能.

`sys.``getallocatedblocks`()

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

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

  3.4 新版功能.

`sys.``getandroidapilevel`()

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

  Availability: Android.

  3.7 新版功能.

`sys.``getdefaultencoding`()

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

`sys.``getdlopenflags`()

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

  可用性: Unix。

`sys.``getfilesystemencoding`()

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

  This encoding is always ASCII-compatible.

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

  - 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()` result cannot be `None` anymore.

  在 3.6 版更改: Windows is no longer guaranteed to return `'mbcs'`. See PEP 529 and `_enablelegacywindowsfsencoding()` for more information.

  在 3.7 版更改: 在UTF-8模式下返回 'utf-8' 。

`sys.``getfilesystemencodeerrors`()

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

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

  3.6 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.

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

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

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

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

`sys.``getswitchinterval`()

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

  3.2 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.

  Raises an auditing event `sys._getframe` with no arguments.

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

`sys.``getprofile`()

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

`sys.``gettrace`()

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

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

`sys.``getwindowsversion`()

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

  platform will be `2 (VER_PLATFORM_WIN32_NT)`.

  product_type may be one of the following values:

  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.

  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` 模块提供了对系统标识更详细的检查。

`sys.``platlibdir`

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

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

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

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

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

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

  3.9 新版功能.

`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/pythonX.Y` while the platform independent header files (all except `pyconfig.h`) are stored in `prefix/include/pythonX.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.``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)

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

  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.

  Raises an auditing event `sys.setprofile` with no arguments.

  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)

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

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

  在 3.9 版更改: Non-interactive `stderr` is now line-buffered instead of fully buffered.

  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.