# 29.1. `sys` — 系统相关的参数和函数

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

`sys.``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。

  3.2 版新加入.

`sys.``argv`

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

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

`sys.``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 版新加入.

`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 版新加入.

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

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

  这个函数应该只在内部为了一些特定的目的使用。

`sys.``_debugmallocstats`()

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

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

  3.3 版新加入.

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

`sys.``dllhandle`

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

`sys.``displayhook`(value)

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

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

  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

  3.2 版更變: Use `'backslashreplace'` error handler on `UnicodeEncodeError`.

`sys.``dont_write_bytecode`

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

`sys.``excepthook`(type, value, traceback)

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

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

`sys.``__displayhook__`
`sys.``__excepthook__`

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

`sys.``exc_info`()

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

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

`sys.``exec_prefix`

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

  備註

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

`sys.``executable`

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

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

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

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

  Since `exit()` ultimately 「only」 raises an exception, it will only exit the process when called from the main thread, and the exception is not intercepted.

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

`sys.``flags`

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

  attribute – 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`

  3.2 版更變: 为新的 `-q` 标志添加了 `quiet` 属性

  3.2.3 版新加入: `hash_randomization` 属性

  3.3 版更變: 删除了过时的 `division_warning` 属性

  3.4 版更變: 为 `-I` `isolated` 标志添加了 `isolated` 属性。

`sys.``float_info`

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

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

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

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

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

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

`sys.``float_repr_style`

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

  3.1 版新加入.

`sys.``getallocatedblocks`()

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

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

  3.4 版新加入.

`sys.``getcheckinterval`()

  Return the interpreter’s 「check interval」; see `setcheckinterval()`.

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

`sys.``getdefaultencoding`()

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

`sys.``getdlopenflags`()

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

`sys.``getfilesystemencoding`()

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

  This encoding is always ASCII-compatible.

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

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

  - 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'`。

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

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

`sys.``getfilesystemencodeerrors`()

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

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

  3.6 版新加入.

`sys.``getrefcount`(object)

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

`sys.``getrecursionlimit`()

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

`sys.``getsizeof`(object[, default])

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

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

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

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

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

`sys.``getswitchinterval`()

  Return the interpreter’s 「thread switch interval」; see `setswitchinterval()`.

  3.2 版新加入.

`sys.``_getframe`([depth])

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

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

`sys.``getprofile`()

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

`sys.``gettrace`()

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

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

`sys.``getwindowsversion`()

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

  platform will be `2 (VER_PLATFORM_WIN32_NT)`.

  product_type may be one of the following values:

  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.

  Availability: Windows.

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

  3.6 版更變: 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 asychronous generator by an event loop.

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

  備註

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

`sys.``get_coroutine_wrapper`()

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

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

  備註

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

`sys.``hash_info`

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

  attribute – 属性 | 解释
  --- | ---
  `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 版新加入.

  3.4 版更變: 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 struct sequence `sys.version_info` may be used for a more human-friendly encoding of the same information.

  More details of `hexversion` can be found at API 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 版新加入.

`sys.``int_info`

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

  属性 | 解释
  --- | ---
  `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 版新加入.

`sys.``__interactivehook__`

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

  3.4 版新加入.

`sys.``intern`(string)

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

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

`sys.``is_finalizing`()

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

  3.5 版新加入.

`sys.``last_type`
`sys.``last_value`
`sys.``last_traceback`

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

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

`sys.``maxsize`

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

`sys.``maxunicode`

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

  3.3 版更變: Before PEP 393, `sys.maxunicode` used to be either `0xFFFF` or `0x10FFFF`, depending on the configuration option that specified whether Unicode characters were stored as UCS-2 or UCS-4.

`sys.``meta_path`

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

  也參考

  `importlib.abc.MetaPathFinder`

    The abstract base class defining the interface of finder objects on `meta_path`.

  `importlib.machinery.ModuleSpec`

    The concrete class which `find_spec()` should return instances of.

  3.4 版更變: Module specs were introduced in Python 3.4, by PEP 451. Earlier versions of Python looked for a method called `find_module()`. This is still called as a fallback if a `meta_path` entry doesn’t have a `find_spec()` method.

`sys.``modules`

  This is a dictionary that maps module names to modules which have already been loaded. This can be manipulated to force reloading of modules and other tricks. However, replacing the dictionary will not necessarily work as expected and deleting essential items from the dictionary may cause Python to fail.

`sys.``path`

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

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

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

  也參考

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

`sys.``path_hooks`

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

  Originally specified in PEP 302.

`sys.``path_importer_cache`

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

  Originally specified in PEP 302.

  3.3 版更變: `None` is stored instead of `imp.NullImporter` when no finder is found.

`sys.``platform`

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

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

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

  对于其他系统，值是：

  系统 | `平台` 值
  --- | ---
  Linux | `'linux'`
  Windows | `'win32'`
  Windows/Cygwin | `'cygwin'`
  Mac OS X | `'darwin'`

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

  也參考

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

  `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/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`.

  備註

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

`sys.``ps1`
`sys.``ps2`

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

`sys.``setcheckinterval`(interval)

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

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

`sys.``setdlopenflags`(n)

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

  Availability: Unix.

`sys.``setprofile`(profilefunc)

  Set the system’s profile function, which allows you to implement a Python source code profiler in Python. See chapter Python 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`.

  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.

  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.

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

`sys.``setswitchinterval`(interval)

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

  3.2 版新加入.

`sys.``settrace`(tracefunc)

  Set the system’s trace function, which allows you to implement a Python source code debugger in Python. The function is thread-specific; for a debugger to support multiple threads, it must be registered using `settrace()` for each thread being debugged.

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

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

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

  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.

  `'return'`

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

  `'exception'`

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

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

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

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

`sys.``set_asyncgen_hooks`(firstiter, finalizer)

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

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

  備註

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

`sys.``set_coroutine_wrapper`(wrapper)

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

  The wrapper argument must be either:

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

  - `None`, to reset the wrapper.

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

  The wrapper callable cannot define new coroutines directly or indirectly:

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

      async def foo():
          pass

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

  See also `get_coroutine_wrapper()`.

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

  備註

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

`sys.``_enablelegacywindowsfsencoding`()

  Changes the default filesystem encoding and errors mode to 『mbcs』 and 『replace』 respectively, for consistency with versions of Python prior to 3.6.

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

  Availability: Windows

  3.6 版新加入: 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. Under Windows, if the stream is interactive (that is, if its `isatty()` method returns `True`), the console codepage is used, otherwise the ANSI code page. Under other platforms, the locale encoding is used (see `locale.getpreferredencoding()`).

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

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

  備註

  To write or read binary data from/to the standard streams, use the underlying binary `buffer` object. For example, to write bytes to `stdout`, use `sys.stdout.buffer.write(b'abc')`.

  However, if you are writing a library (and do not control in which context its code will be executed), be aware that the standard streams may be replaced with file-like objects like `io.StringIO` which do not support the `buffer` attribute.

`sys.``__stdin__`
`sys.``__stdout__`
`sys.``__stderr__`

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

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

  備註

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

`sys.``thread_info`

  A struct sequence holding information about the thread implementation.

  属性 | 解释
  --- | ---
  `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` | Name and version of the thread library. It is a string, or `None` if these informations are unknown.

  3.3 版新加入.

`sys.``tracebacklimit`

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

`sys.``version`

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

`sys.``api_version`

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

`sys.``version_info`

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

  3.1 版更變: Added named component attributes.

`sys.``warnoptions`

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

`sys.``winver`

  The version number used to form registry keys on Windows platforms. This is stored as string resource 1000 in the Python DLL. The value is normally the first three characters of `version`. It is provided in the `sys` module for informational purposes; modifying this value has no effect on the registry keys used by Python. Availability: Windows.

`sys.``_xoptions`

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

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

  CPython implementation detail: This is a CPython-specific way of accessing options passed through `-X`. Other implementations may export them through other means, or not at all.

  3.2 版新加入.

Citations

C99

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