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 新版功能.

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.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，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。

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。

sys.breakpointhook()

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

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

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

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

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

3.7 新版功能.

sys._debugmallocstats()

将有关 CPython 内存分配器状态的底层的信息打印至 stderr。

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

3.3 新版功能.

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

sys.dllhandle

指向 Python DLL 句柄的整数。

可用性: Windows。

sys.displayhook(value)

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

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

伪代码:

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

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

sys.dont_write_bytecode

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

sys.excepthook(type, value, traceback)

本函数会将所给的回溯和异常输出到 sys.stderr 中。

当抛出一个异常，且未被捕获时，解释器将调用 sys.excepthook 并带有三个参数：异常类、异常实例和一个回溯对象。在交互式会话中，这会在控制权返回到提示符之前发生。在 Python 程序中，这会在程序退出之前发生。如果要自定义此类顶级异常的处理过程，可以将另一个 3 个参数的函数赋给 sys.excepthook。

sys.__breakpointhook__

sys.__displayhook__

sys.__excepthook__

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

3.7 新版功能: __breakpointhook__

sys.exc_info()

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

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.

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.

attribute -- 属性	标志
debug	-d
inspect	-i
interactive	-i
isolated	-I
optimize	-O 或 -OO
dont_write_bytecode	-B
no_user_site	-s
no_site	-S
ignore_environment	-E
verbose	-v
bytes_warning	-b
quiet	-q
hash_randomization	-R
dev_mode	-X dev
utf8_mode	-X utf8

在 3.2 版更改: 为新的 -q 标志添加了 quiet 属性

3.2.3 新版功能: hash_randomization 属性

在 3.3 版更改: 删除了过时的 division_warning 属性

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

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

sys.float_info

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.

attribute -- 属性	float.h 宏	解释
epsilon	DBL_EPSILON	difference between 1.0 and the least value greater than 1.0 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 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
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.getcheckinterval()

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

3.2 版后已移除: 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).

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 。

• 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'。

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.

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.get_coroutine_wrapper()

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

3.5 新版功能: See PEP 492 for more details.

注解

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

3.7 版后已移除: The coroutine wrapper functionality has been deprecated, and will be removed in 3.8. See bpo-32591 for details.

sys.hash_info

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

attribute -- 属性	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 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	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 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.

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, 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'

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.

参见

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

platform 模块对系统的标识有更详细的检查。

sys.prefix

A string giving the site-specific directory prefix where the platform independent Python files are installed; by default, this is the string '/usr/local'. This can be set at build time with the --prefix argument to the configure script. The main collection of Python library modules is installed in the directory prefix/lib/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.setcheckinterval(interval)

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

3.2 版后已移除: This function doesn't have an effect anymore, as the internal logic for thread switching and asynchronous tasks has been rewritten. Use setswitchinterval() instead.

sys.setdlopenflags(n)

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

可用性: Unix。

sys.setprofile(profilefunc)

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. 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.

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.

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.

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.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 新版功能: See PEP 492 for more details.

注解

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

3.7 版后已移除: The coroutine wrapper functionality has been deprecated, and will be removed in 3.8. See bpo-32591 for details.

sys._enablelegacywindowsfsencoding()

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

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

可用性: Windows。

3.6 新版功能: 有关更多详细信息，请参阅 PEP 529。

sys.stdin

sys.stdout

sys.stderr

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

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

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

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

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

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

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

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

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

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

注解

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

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

sys.__stdin__

sys.__stdout__

sys.__stderr__

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

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

注解

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

sys.thread_info

A named tuple holding information about the thread implementation.

属性	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	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.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.