`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

On Unix, command line arguments are passed by bytes from OS. Python decodes them with filesystem encoding and "surrogateescape" error handler. When you need original bytes, you can get it by [os.fsencode(arg) for arg in sys.argv].

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)¶: Call func(*args), while tracing is enabled. The tracing state is saved, and restored afterwards. This is intended to be called from a debugger from a checkpoint, to recursively debug some other code.

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.breakpointhook()¶

This hook function is called by built-in breakpoint(). By default, it drops you into the pdb debugger, but it can be set to any other function so that you can choose which debugger gets used.

The signature of this function is dependent on what it calls. For example, the default binding (e.g. pdb.set_trace()) expects no arguments, but you might bind it to a function that expects additional arguments (positional and/or keyword). The built-in breakpoint() function passes its *args and **kws straight through. Whatever breakpointhooks() returns is returned from breakpoint().

The default implementation first consults the environment variable PYTHONBREAKPOINT. If that is set to "0" then this function returns immediately; i.e. it is a no-op. If the environment variable is not set, or is set to the empty string, pdb.set_trace() is called. Otherwise this variable should name a function to run, using Python's dotted-import nomenclature, e.g. package.subpackage.module.function. In this case, package.subpackage.module would be imported and the resulting module must have a callable named function(). This is run, passing in *args and **kws, and whatever function() returns, sys.breakpointhook() returns to the built-in breakpoint() function.

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.

Pseudo-code:

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.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.__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()¶

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.

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])¶

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.

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

属性	flag
`debug`	`-d`
`inspect`	`-i`
`interactive`	`-i`
`isolated`	`-I`
`optimize`	`-O` or `-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: Added quiet attribute for the new -q flag.

3.2.3 新版功能: The hash_randomization attribute.

在 3.3 版更改: Removed obsolete division_warning attribute.

在 3.4 版更改: Added isolated attribute for -I isolated flag.

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 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.

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

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

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

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

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

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.

In the UTF-8 mode, the encoding is utf-8 on any platform.
On Mac OS X, the encoding is 'utf-8'.
On Unix, the encoding is the locale encoding.
On Windows, the encoding may be 'utf-8' or 'mbcs', depending on user configuration.

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: Return 'utf-8' in the UTF-8 mode.

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	意义
`1 (VER_NT_WORKSTATION)`	The system is a workstation.
`2 (VER_NT_DOMAIN_CONTROLLER)`	The system is a domain controller.
`3 (VER_NT_SERVER)`	The system is a server, but not a domain controller.

This function wraps the Win32 GetVersionEx() function; see the Microsoft documentation on OSVERSIONINFOEX() for more information about these fields.

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: Added 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 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.

属性	explanation
`width`	width in bits used for hash values
`modulus`	prime modulus P used for numeric hash scheme
`inf`	hash value returned for a positive infinity
`nan`	hash value returned for a nan
`imag`	multiplier used for the imaginary part of a complex number
`algorithm`	name of the algorithm for hashing of str, bytes, and memoryview
`hash_bits`	internal output size of the hash algorithm
`seed_bits`	size of the seed key of the hash algorithm

3.2 新版功能.

在 3.4 版更改: Added algorithm, hash_bits and 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 新版功能.

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`	size in bytes of the C type used to represent a digit

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...

For other systems, the values are:

System	`platform` value
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 模块提供了对系统标识更详细的检查。

sys.prefix¶: A string giving the site-specific directory prefix where the platform independent Python files are installed; by default, this is the string '/usr/local'. This can be set at build time with the --prefix argument to the configure script. The main collection of Python library modules is installed in the directory prefix/lib/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 The Python Profilers for more information on the Python profiler. The system's profile function is called similarly to the system's trace function (see settrace()), but it is called with different events, for example it isn't called for each executed line of code (only on call and return, but the return event is reported even when an exception has been set). The function is thread-specific, but there is no way for the profiler to know about context switches between threads, so it does not make sense to use this in the presence of multiple threads. Also, its return value is not used, so it can simply return None. 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.

The events have the following meaning:

'call': A function is called (or some other code block entered). The profile function is called; arg is None.
'return': A function (or other code block) is about to return. The profile function is called; arg is the value that will be returned, or None if the event is caused by an exception being raised.
'c_call': A C function is about to be called. This may be an extension function or a built-in. arg is the C function object.
'c_return': A C function has returned. arg is the C function object.
'c_exception': A C function has raised an exception. arg is the C function object.

sys.setrecursionlimit(limit)¶

Set the maximum depth of the Python interpreter stack to limit. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python.

The highest possible limit is platform-dependent. A user may need to set the limit higher when 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 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', '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 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.

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

The events have the following meaning:

'call': A function is called (or some other code block entered). The global trace function is called; arg is None; the return value specifies the local trace function.
'line': The interpreter is about to execute a new line of code or re-execute the condition of a loop. The local trace function is called; arg is None; the return value specifies the new local trace function. See Objects/lnotab_notes.txt for a detailed explanation of how this works. 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 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()

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