传递给 Python 脚本的命令行自变量列表。 argv 是脚本名称 (它是否为完整路径名从属操作系统)。若命令的执行是使用 -c 命令行选项到解释器， argv 被设为字符串 '-c' 。若脚本名称未被传递给 Python 解释器， argv 是空字符串。
要循环标准输入或命令行中给出的文件列表，见 fileinput 模块。
An indicator of the native byte order. This will have the value 'big' on big-endian (most-significant byte first) platforms, and 'little' on little-endian (least-significant byte first) platforms.
A tuple of strings giving the names of all modules that are compiled into this Python interpreter. (This information is not available in any other way — modules.keys() only lists the imported modules.)
调用 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.
A string containing the copyright pertaining to the Python interpreter.
Clear the internal type cache. The type cache is used to speed up attribute and method lookups. Use the function only to drop unnecessary references during reference leak debugging.
This function should be used for internal and specialized purposes only.
Return a dictionary mapping each thread’s identifier to the topmost stack frame currently active in that thread at the time the function is called. Note that functions in the traceback module can build the call stack given such a frame.
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.
This function should be used for internal and specialized purposes only.
Integer specifying the handle of the Python DLL. Availability: Windows.
若 value 不是 None , this function prints it to sys.stdout , and saves it in __builtin__._ .
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 .
If this is true, Python won’t try to write .pyc or .pyo 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.
此函数将给定回溯和异常输出到 sys.stderr .
当异常被引发但未被捕获时，解释器调用 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 .
这些对象包含原始值的 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.
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 or having executed an except clause.” For any stack frame, only information about the most recently handled exception is accessible.
若在堆栈的任何地方都没有要处理的异常，则元组包含 3 None 值被返回。否则，返回值是 (type, value, traceback) 。它们的含义： type gets the exception type of the exception being handled (a class object); value gets the exception parameter (its associated value or the second argument to raise , which is always a class instance if the exception type is a class object); traceback 获取回溯对象 (见参考手册) 封装最初发生异常点的调用堆栈。
若 exc_clear() is called, this function will return three None values until either another exception is raised in the current thread or the execution stack returns to a frame where another exception is being handled.
Assigning the traceback return value to a local variable in a function that is handling an exception will cause a circular reference. This will prevent anything referenced by a local variable in the same function or by the traceback from being garbage collected. Since most functions don’t need access to the traceback, the best solution is to use something like exctype, value = sys.exc_info()[:2] to extract only the exception type and value. If you do need the traceback, make sure to delete it after use (best done with a try ... finally statement) or to call exc_info() in a function that does not itself handle an exception.
Beginning with Python 2.2, such cycles are automatically reclaimed when garbage collection is enabled and they become unreachable, but it remains more efficient to avoid creating cycles.
This function clears all information relating to the current or last exception that occurred in the current thread. After calling this function, exc_info() will return three None values until another exception is raised in the current thread or the execution stack returns to a frame where another exception is being handled.
This function is only needed in only a few obscure situations. These include logging and error handling systems that report information on the last or current exception. This function can also be used to try to free resources and trigger object finalization, though no guarantee is made as to what objects will be freed, if any.
Deprecated since version 1.5: 使用 exc_info() 代替。
Since they are global variables, they are not specific to the current thread, so their use is not safe in a multi-threaded program. When no exception is being handled, exc_type 被设为 None and the other two are undefined.
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/python X.Y /config , and shared library modules are installed in exec_prefix /lib/python X.Y /lib-dynload ，其中 X.Y is the version number of Python, for example 2.7 .
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 将是空字符串或 None .
可选自变量 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.
This value is not actually defined by the module, but can be set by the user (or by a program) to specify a clean-up action at program exit. When set, it should be a parameterless function. This function will be called when the interpreter exits. Only one function may be installed in this way; to allow multiple functions which will be called at termination, use the atexit 模块。
The exit function is not called when the program is killed by a signal, when a Python fatal internal error is detected, or when os._exit() 被调用。
Deprecated since version 2.4: 使用 atexit 代替。
The struct sequence flags exposes the status of command line flags. The attributes are read only.
|optimize||-O or -OO|
|tabcheck||-t or -tt|
New in version 2.7.3: hash_randomization 属性。
A structseq 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 126.96.36.199.2 of the 1999 ISO/IEC C standard [C99] , ‘Characteristics of floating types’, for details.
|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_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|
|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 188.8.131.52.2 of the C99 standard for an explanation of the possible values and their meanings.|
属性 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'
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 2.7 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 2.7.
Return the name of the current default string encoding used by the Unicode implementation.
Return the current value of the flags that are used for dlopen() calls. The flag constants are defined in the dl and DLFCN modules. Availability: Unix.
Return the name of the encoding used to convert Unicode filenames into system file names, or None if the system default encoding is used. The result value depends on the operating system:
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() .
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() .
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.
若给定， default will be returned if the object does not provide means to retrieve the size. Otherwise a TypeError 会被引发。
getsizeof() calls the object’s __sizeof__ method and adds an additional garbage collector overhead if the object is managed by the garbage collector.
从调用堆栈返回帧对象。若可选整数 depth 有给定，返回堆栈顶部下方许多调用帧对象。若比调用堆栈更深， ValueError 被引发。默认 depth 为 0，返回调用堆栈顶部帧。
CPython 实现细节： 此函数只应用于内部和专用目的。它不保证在所有 Python 实现中均存在。
Get the trace function as set by settrace() .
CPython 实现细节： 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.
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 ，和 product_type . service_pack contains a string while all other values are integers. The components can also be accessed by name, so sys.getwindowsversion() 相当于 sys.getwindowsversion().major . For compatibility with prior versions, only the first 5 elements are retrievable by indexing.
platform may be one of the following values:
|0 (VER_PLATFORM_WIN32s)||Win32s on Windows 3.1|
|1 (VER_PLATFORM_WIN32_WINDOWS)||Windows 95/98/ME|
|2 (VER_PLATFORM_WIN32_NT)||Windows NT/2000/XP/x64|
|3 (VER_PLATFORM_WIN32_CE)||Windows CE|
product_type may be one of the following values:
此函数包裹 Win32 GetVersionEx() function; see the Microsoft documentation on OSVERSIONINFOEX() for more information about these fields.
2.7 版改变： Changed to a named tuple and added service_pack_minor , service_pack_major , suite_mask ，和 product_type .
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 version_info value may be used for a more human-friendly encoding of the same information.
hexversion is a 32-bit number with the following layout:
|1-8||PY_MAJOR_VERSION ( 2 in 2.1.0a3 )|
|9-16||PY_MINOR_VERSION ( 1 in 2.1.0a3 )|
|17-24||PY_MICRO_VERSION ( 0 in 2.1.0a3 )|
|25-28||PY_RELEASE_LEVEL ( 0xA 对于 Alpha， 0xB 对于 Beta， 0xC 对于发行候选和 0xF for final)|
|29-32||PY_RELEASE_SERIAL ( 3 in 2.1.0a3 ，0 对于最终发行)|
因此 2.1.0a3 是十六进制 0x020100a3 .
A struct sequence that holds information about Python’s internal representation of integers. The attributes are read only.
|bits_per_digit||number of bits held in each digit. Python integers are stored internally in base 2**long_info.bits_per_digit|
|sizeof_digit||size in bytes of the C type used to represent a digit|
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 chapter pdb — The Python Debugger 了解更多信息。)
The meaning of the variables is the same as that of the return values from exc_info() above. (Since there is only one interactive thread, thread-safety is not a concern for these variables, unlike for exc_type etc.)
The largest positive integer supported by Python’s regular integer type. This is at least 2**31-1. The largest negative integer is -maxint-1 — the asymmetry results from the use of 2’s complement binary arithmetic.
The largest positive integer supported by the platform’s Py_ssize_t type, and thus the maximum size lists, strings, dicts, and many other containers can have.
An integer giving the largest supported code point for a Unicode character. The value of this depends on the configuration option that specifies whether Unicode characters are stored as UCS-2 or UCS-4.
A list of finder objects that have their find_module() methods called to see if one of the objects can find the module to be imported. The find_module() method is called at least with the absolute name of the module being imported. If the module to be imported is contained in package then the parent package’s __path__ attribute is passed in as a second argument. The method returns None if the module cannot be found, else returns a loader .
见 PEP 302 for the original specification.
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. Note that removing a module from this dictionary is not the same as calling reload() on the corresponding module object.
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 , 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 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.
Changed in version 2.3: Unicode strings are no longer ignored.
最初的指定在 PEP 302 .
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 explicit finder is found on sys.path_hooks then None is stored to represent the implicit default finder should be used. If the path is not an existing path then imp.NullImporter 有设置。
最初的指定在 PEP 302 .
This string contains a platform identifier that can be used to append platform-specific components to sys.path ，例如。
For most Unix systems, 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' , 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...
Changed in version 2.7.3: Since lots of code check for sys.platform == 'linux2' , and there is no essential change between Linux 2.x and 3.x, sys.platform is always set to 'linux2' , even on Linux 3.x. In Python 3.3 and later, the value will always be set to 'linux' , so it is recommended to always use the startswith idiom presented above.
|Linux (2.x and 3.x)||'linux2'|
|Mac OS X||'darwin'|
A string giving the site-specific directory prefix where the platform independent Python files are installed; by default, this is the string '/usr/local' . This can be set at build time with the --prefix argument to the configure script. The main collection of Python library modules is installed in the directory prefix /lib/python X.Y while the platform independent header files (all except pyconfig.h ) are stored in prefix /include/python X.Y ，其中 X.Y is the version number of Python, for example 2.7 .
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.
Bool containing the status of the Python 3 warning flag. It’s True when Python is started with the -3 option. (This should be considered read-only; setting it to a different value doesn’t have an effect on Python 3 warnings.)
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.
Set the current default string encoding used by the Unicode implementation. If name does not match any available encoding, LookupError is raised. This function is only intended to be used by the site module implementation and, where needed, by sitecustomize . Once used by the site module, it is removed from the sys module’s namespace.
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(dl.RTLD_NOW | dl.RTLD_GLOBAL) . Symbolic names for the flag modules can be either found in the dl module, or in the DLFCN module. If DLFCN is not available, it can be generated from /usr/include/dlfcn.h 使用 h2py script. Availability: Unix.
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 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 .
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 she has 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.
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 ，和 arg . frame is the current stack frame. event is a string: 'call' , 'line' , 'return' , 'exception' , 'c_call' , 'c_return' ，或 'c_exception' . arg depends on the event type.
The trace function is invoked (with event 设为 '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.
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 标准类型层次结构 .
CPython 实现细节： 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.
Activate dumping of VM measurements using the Pentium timestamp counter, if on_flag is true. Deactivate these dumps if on_flag is off. The function is available only if Python was compiled with --with-tsc . To understand the output of this dump, read Python/ceval.c in the Python sources.
CPython 实现细节： This function is intimately bound to CPython implementation details and thus not likely to be implemented elsewhere.
File objects corresponding to the interpreter’s standard input, output and error streams. stdin is used for all interpreter input except for scripts but including calls to input() and raw_input() . stdout is used for the output of print and expression statements and for the prompts of input() and raw_input() . The interpreter’s own prompts and (almost all of) its error messages go to stderr . stdout and stderr needn’t be built-in file objects: any object is acceptable as long as it has a write() method that takes a string argument. (Changing these objects doesn’t affect the standard I/O streams of processes executed by os.popen() , os.system() 或 exec*() family of functions in the os 模块。)
这些对象包含原始值的 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.
A triple (repo, branch, version) representing the Subversion information of the Python interpreter. repo is the name of the repository, 'CPython' . branch is a string of one of the forms 'trunk' , 'branches/name' or 'tags/name' . version is the output of svnversion , if the interpreter was built from a Subversion checkout; it contains the revision number (range) and possibly a trailing ‘M’ if there were local modifications. If the tree was exported (or svnversion was not available), it is the revision of Include/patchlevel.h if the branch is a tag. Otherwise, it is None .
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.
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 模块。
The C API version for this interpreter. Programmers may find this useful when debugging version conflicts between Python and extension modules.
A tuple containing the five components of the version number: major , minor , micro , releaselevel ，和 serial . All values except releaselevel are integers; the release level is 'alpha' , 'beta' , 'candidate' ，或 'final' 。 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 相当于 sys.version_info.major and so on.
2.7 版改变： Added named component attributes
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.
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.
|[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 .|