sys — System-specific parameters and functions¶

This module provides access to some variables used or maintained by theinterpreter and to functions that interact strongly with the interpreter. It isalways available. Unless explicitly noted otherwise, all variables are read-only.

sys.abiflags¶

On POSIX systems where Python was built with the standardconfigurescript, this contains the ABI flags as specified byPEP 3149.

Added in version 3.2.

Changed in version 3.8:Default flags became an empty string (m flag for pymalloc has beenremoved).

Availability: Unix.

sys.addaudithook(hook)¶

Append the callablehook to the list of active auditing hooks for thecurrent (sub)interpreter.

When an auditing event is raised through thesys.audit() function, eachhook will be called in the order it was added with the event name and thetuple of arguments. Native hooks added byPySys_AddAuditHook() arecalled first, followed by hooks added in the current (sub)interpreter. Hookscan then log the event, raise an exception to abort the operation,or terminate the process entirely.

Note that audit hooks are primarily for collecting information about internalor otherwise unobservable actions, whether by Python or libraries written inPython. They are not suitable for implementing a “sandbox”. In particular,malicious code can trivially disable or bypass hooks added using thisfunction. At a minimum, any security-sensitive hooks must be added using theC APIPySys_AddAuditHook() before initialising the runtime, and anymodules allowing arbitrary memory modification (such asctypes) shouldbe completely removed or closely monitored.

Callingsys.addaudithook() will itself raise an auditing eventnamedsys.addaudithook with no arguments. If anyexisting hooks raise an exception derived fromRuntimeError, thenew hook will not be added and the exception suppressed. As a result,callers cannot assume that their hook has been added unless they controlall existing hooks.

See theaudit events table for all events raised byCPython, andPEP 578 for the original design discussion.

Added in version 3.8.

Changed in version 3.8.1:Exceptions derived fromException but notRuntimeErrorare no longer suppressed.

CPython implementation detail: When tracing is enabled (seesettrace()), Python hooks are onlytraced if the callable has a__cantrace__ member that is set to atrue value. Otherwise, trace functions will skip the hook.

sys.argv¶

The list of command line arguments passed to a Python script.argv[0] is thescript name (it is operating system dependent whether this is a full pathname ornot). If the command was executed using the-c command line option tothe interpreter,argv[0] is set to the string'-c'. If no script namewas passed to the Python interpreter,argv[0] is the empty string.

To loop over the standard input, or the list of files given on thecommand line, see thefileinput module.

See alsosys.orig_argv.

Note

On Unix, command line arguments are passed by bytes from OS. Python decodesthem with filesystem encoding and “surrogateescape” error handler.When you need original bytes, you can get it by[os.fsencode(arg)forarginsys.argv].

sys.audit(event,*args)¶

Raise an auditing event and trigger any active auditing hooks.event is a string identifying the event, andargs may containoptional arguments with more information about the event. Thenumber and types of arguments for a given event are considered apublic and stable API and should not be modified between releases.

For example, one auditing event is namedos.chdir. This event hasone argument calledpath that will contain the requested newworking directory.

sys.audit() will call the existing auditing hooks, passingthe event name and arguments, and will re-raise the first exceptionfrom any hook. In general, if an exception is raised, it should notbe handled and the process should be terminated as quickly aspossible. This allows hook implementations to decide how to respondto particular events: they can merely log the event or abort theoperation by raising an exception.

Hooks are added using thesys.addaudithook() orPySys_AddAuditHook() functions.

The native equivalent of this function isPySys_Audit(). Using thenative function is preferred when possible.

See theaudit events table for all events raised byCPython.

Added in version 3.8.

sys.base_exec_prefix¶

Set during Python startup, beforesite.py is run, to the same value asexec_prefix. If not running in avirtual environment, the values will stay the same; ifsite.py finds that a virtual environment is in use, the values ofprefix andexec_prefix will be changed to point to thevirtual environment, whereasbase_prefix andbase_exec_prefix will remain pointing to the base Pythoninstallation (the one which the virtual environment was created from).

Added in version 3.3.

sys.base_prefix¶

Set during Python startup, beforesite.py is run, to the same value asprefix. If not running in avirtual environment, the valueswill stay the same; ifsite.py finds that a virtual environment is inuse, the values ofprefix andexec_prefix will be changed topoint to the virtual environment, whereasbase_prefix andbase_exec_prefix will remain pointing to the base Pythoninstallation (the one which the virtual environment was created from).

Added in version 3.3.

sys.byteorder¶

An indicator of the native byte order. This will have the value'big' onbig-endian (most-significant byte first) platforms, and'little' onlittle-endian (least-significant byte first) platforms.

sys.builtin_module_names¶

A tuple of strings containing the names of all modules that are compiled into thisPython interpreter. (This information is not available in any other way —modules.keys() only lists the imported modules.)

See also thesys.stdlib_module_names list.

sys.call_tracing(func,args)¶

Callfunc(*args), while tracing is enabled. The tracing state is saved,and restored afterwards. This is intended to be called from a debugger froma checkpoint, to recursively debug or profile some other code.

Tracing is suspended while calling a tracing function set bysettrace() orsetprofile() to avoid infinite recursion.call_tracing() enables explicit recursion of the tracing function.

sys.copyright¶

A string containing the copyright pertaining to the Python interpreter.

sys._clear_type_cache()¶

Clear the internal type cache. The type cache is used to speed up attributeand method lookups. Use the functiononly to drop unnecessary referencesduring reference leak debugging.

This function should be used for internal and specialized purposes only.

Deprecated since version 3.13:Use the more general_clear_internal_caches() function instead.

sys._clear_internal_caches()¶

Clear all internal performance-related caches. Use this functiononly torelease unnecessary references and memory blocks when hunting for leaks.

Added in version 3.13.

sys._current_frames()¶

Return a dictionary mapping each thread’s identifier to the topmost stack framecurrently active in that thread at the time the function is called. Note thatfunctions in thetraceback module can build the call stack given such aframe.

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

This function should be used for internal and specialized purposes only.

Raises anauditing eventsys._current_frames with no arguments.

sys._current_exceptions()¶

Return a dictionary mapping each thread’s identifier to the topmost exceptioncurrently active in that thread at the time the function is called.If a thread is not currently handling an exception, it is not included inthe result dictionary.

This is most useful for statistical profiling.

This function should be used for internal and specialized purposes only.

Raises anauditing eventsys._current_exceptions with no arguments.

Changed in version 3.12:Each value in the dictionary is now a single exception instance, ratherthan a 3-tuple as returned fromsys.exc_info().

sys.breakpointhook()¶

This hook function is called by built-inbreakpoint(). By default,it drops you into thepdb debugger, but it can be set to any otherfunction 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, butyou might bind it to a function that expects additional arguments(positional and/or keyword). The built-inbreakpoint() function passesits*args and**kws straight through. Whateverbreakpointhooks() returns is returned frombreakpoint().

The default implementation first consults the environment variablePYTHONBREAKPOINT. If that is set to"0" then this functionreturns immediately; i.e. it is a no-op. If the environment variable isnot set, or is set to the empty string,pdb.set_trace() is called.Otherwise this variable should name a function to run, using Python’sdotted-import nomenclature, e.g.package.subpackage.module.function.In this case,package.subpackage.module would be imported and theresulting module must have a callable namedfunction(). This is run,passing in*args and**kws, and whateverfunction() returns,sys.breakpointhook() returns to the built-inbreakpoint()function.

Note that if anything goes wrong while importing the callable named byPYTHONBREAKPOINT, aRuntimeWarning is reported and thebreakpoint is ignored.

Also note that ifsys.breakpointhook() is overridden programmatically,PYTHONBREAKPOINT isnot consulted.

Added in version 3.7.

sys._debugmallocstats()¶

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

If Python isbuilt in debug mode (configure--with-pydebugoption), it also performs some expensiveinternal consistency checks.

Added in version 3.3.

CPython implementation detail: This function is specific to CPython. The exact output format is notdefined here, and may change.

sys.dllhandle¶

Integer specifying the handle of the Python DLL.

Availability: Windows.

sys.displayhook(value)¶

Ifvalue is notNone, this function printsrepr(value) tosys.stdout, and savesvalue inbuiltins._. Ifrepr(value) isnot encodable tosys.stdout.encoding withsys.stdout.errors errorhandler (which is probably'strict'), encode it tosys.stdout.encoding with'backslashreplace' error handler.

sys.displayhook is called on the result of evaluating anexpressionentered in an interactive Python session. The display of these values can becustomized by assigning another one-argument function tosys.displayhook.

Pseudo-code:

defdisplayhook(value):ifvalueisNone:return# Set '_' to None to avoid recursionbuiltins._=Nonetext=repr(value)try:sys.stdout.write(text)exceptUnicodeEncodeError:bytes=text.encode(sys.stdout.encoding,'backslashreplace')ifhasattr(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

Changed in version 3.2:Use'backslashreplace' error handler onUnicodeEncodeError.

sys.dont_write_bytecode¶

If this is true, Python won’t try to write.pyc files on theimport of source modules. This value is initially set toTrue orFalse depending on the-B command line option and thePYTHONDONTWRITEBYTECODE environment variable, but you can set ityourself to control bytecode file generation.

sys._emscripten_info¶

Anamed tuple holding information about the environment on thewasm32-emscripten platform. The named tuple is provisional and may changein the future.

_emscripten_info.emscripten_version¶

Emscripten version as tuple of ints (major, minor, micro), e.g.(3,1,8).

_emscripten_info.runtime¶

Runtime string, e.g. browser user agent,'Node.jsv14.18.2', or'UNKNOWN'.

_emscripten_info.pthreads¶

True if Python is compiled with Emscripten pthreads support.

_emscripten_info.shared_memory¶

True if Python is compiled with shared memory support.

Availability: Emscripten.

Added in version 3.11.

sys.pycache_prefix¶

If this is set (notNone), Python will write bytecode-cache.pycfiles to (and read them from) a parallel directory tree rooted at thisdirectory, rather than from__pycache__ directories in the source codetree. Any__pycache__ directories in the source code tree will be ignoredand new.pyc files written within the pycache prefix. Thus if you usecompileall as a pre-build step, you must ensure you run it with thesame pycache prefix (if any) that you will use at runtime.

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

This value is initially set based on the value of the-Xpycache_prefix=PATH command-line option or thePYTHONPYCACHEPREFIX environment variable (command-line takesprecedence). If neither are set, it isNone.

Added in version 3.8.

sys.excepthook(type,value,traceback)¶

This function prints out a given traceback and exception tosys.stderr.

When an exception other thanSystemExit is raised and uncaught, the interpreter callssys.excepthook with three arguments, the exception class, exceptioninstance, and a traceback object. In an interactive session this happens justbefore control is returned to the prompt; in a Python program this happens justbefore the program exits. The handling of such top-level exceptions can becustomized by assigning another three-argument function tosys.excepthook.

Raise an auditing eventsys.excepthook with argumentshook,type,value,traceback when an uncaught exception occurs.If no hook has been set,hook may beNone. If any hook raisesan exception derived fromRuntimeError the call to the hook willbe suppressed. Otherwise, the audit hook exception will be reported asunraisable andsys.excepthook will be called.

See also

Thesys.unraisablehook() function handles unraisable exceptionsand thethreading.excepthook() function handles exception raisedbythreading.Thread.run().

sys.__breakpointhook__¶

sys.__displayhook__¶

sys.__excepthook__¶

sys.__unraisablehook__¶

These objects contain the original values ofbreakpointhook,displayhook,excepthook, andunraisablehook at the start of theprogram. They are saved so thatbreakpointhook,displayhook andexcepthook,unraisablehook can be restored in case they happen toget replaced with broken or alternative objects.

Added in version 3.7:__breakpointhook__

Added in version 3.8:__unraisablehook__

sys.exception()¶

This function, when called while an exception handler is executing (such asanexcept orexcept* clause), returns the exception instance thatwas caught by this handler. When exception handlers are nested within oneanother, only the exception handled by the innermost handler is accessible.

If no exception handler is executing, this function returnsNone.

Added in version 3.11.

sys.exc_info()¶

This function returns the old-style representation of the handledexception. If an exceptione is currently handled (soexception() would returne),exc_info() returns thetuple(type(e),e,e.__traceback__).That is, a tuple containing the type of the exception (a subclass ofBaseException), the exception itself, and atracebackobject which typically encapsulates the callstack at the point where the exception last occurred.

If no exception is being handled anywhere on the stack, this functionreturn a tuple containing threeNone values.

Changed in version 3.11:Thetype andtraceback fields are now derived from thevalue(the exception instance), so when an exception is modified while it isbeing handled, the changes are reflected in the results of subsequentcalls toexc_info().

sys.exec_prefix¶

A string giving the site-specific directory prefix where the platform-dependentPython files are installed; by default, this is also'/usr/local'. This canbe set at build time with the--exec-prefix argument to theconfigure script. Specifically, all configuration files (e.g. thepyconfig.h header file) are installed in the directoryexec_prefix/lib/pythonX.Y/config, and shared library modules areinstalled inexec_prefix/lib/pythonX.Y/lib-dynload, whereX.Yis the version number of Python, for example3.2.

Note

If avirtual environment is in effect, thisvalue will be changed insite.py to point to the virtual environment.The value for the Python installation will still be available, viabase_exec_prefix.

sys.executable¶

A string giving the absolute path of the executable binary for the Pythoninterpreter, on systems where this makes sense. If Python is unable to retrievethe real path to its executable,sys.executable will be an empty stringorNone.

sys.exit([arg])¶

Raise aSystemExit exception, signaling an intention to exit the interpreter.

The optional argumentarg can be an integer giving the exit status(defaulting to zero), or another type of object. If it is an integer, zerois considered “successful termination” and any nonzero value is considered“abnormal termination” by shells and the like. Most systems require it to bein the range 0–127, and produce undefined results otherwise. Some systemshave a convention for assigning specific meanings to specific exit codes, butthese are generally underdeveloped; Unix programs generally use 2 for commandline syntax errors and 1 for all other kind of errors. If another type ofobject is passed,None is equivalent to passing zero, and any otherobject is printed tostderr and results in an exit code of 1. Inparticular,sys.exit("someerrormessage") is a quick way to exit aprogram when an error occurs.

Sinceexit() ultimately “only” raises an exception, it will only exitthe process when called from the main thread, and the exception is notintercepted. Cleanup actions specified by finally clauses oftry statementsare honored, and it is possible to intercept the exit attempt at an outer level.

Changed in version 3.6:If an error occurs in the cleanup after the Python interpreterhas caughtSystemExit (such as an error flushing buffered datain the standard streams), the exit status is changed to 120.

sys.flags¶

Thenamed tupleflags exposes the status of command lineflags. The attributes are read only.

flags.debug¶	-d
flags.inspect¶	-i
flags.interactive¶	-i
flags.isolated¶	-I
flags.optimize¶	-O or-OO
flags.dont_write_bytecode¶	-B
flags.no_user_site¶	-s
flags.no_site¶	-S
flags.ignore_environment¶	-E
flags.verbose¶	-v
flags.bytes_warning¶	-b
flags.quiet¶	-q
flags.hash_randomization¶	-R
flags.dev_mode¶	-Xdev (Python Development Mode)
flags.utf8_mode¶	-Xutf8
flags.safe_path¶	-P
flags.int_max_str_digits¶	-Xint_max_str_digits(integer string conversion length limitation)
flags.warn_default_encoding¶	-Xwarn_default_encoding

Changed in version 3.2:Addedquiet attribute for the new-q flag.

Added in version 3.2.3:Thehash_randomization attribute.

Changed in version 3.3:Removed obsoletedivision_warning attribute.

Changed in version 3.4:Addedisolated attribute for-Iisolated flag.

Changed in version 3.7:Added thedev_mode attribute for the newPython DevelopmentMode and theutf8_mode attribute for the new-Xutf8 flag.

Changed in version 3.10:Addedwarn_default_encoding attribute for-Xwarn_default_encoding flag.

Changed in version 3.11:Added thesafe_path attribute for-P option.

Changed in version 3.11:Added theint_max_str_digits attribute.

sys.float_info¶

Anamed tuple holding information about the float type. Itcontains low level information about the precision and internalrepresentation. The values correspond to the various floating-pointconstants defined in the standard header filefloat.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.

Attributes of thefloat_infonamed tuple¶

attribute	float.h macro	explanation
float_info.epsilon¶	DBL_EPSILON	difference between 1.0 and the least value greater than 1.0 that isrepresentable as a float. See alsomath.ulp().
float_info.dig¶	DBL_DIG	The maximum number of decimal digits that can be faithfullyrepresented in a float; see below.
float_info.mant_dig¶	DBL_MANT_DIG	Float precision: the number of base-radix digits in thesignificand of a float.
float_info.max¶	DBL_MAX	The maximum representable positive finite float.
float_info.max_exp¶	DBL_MAX_EXP	The maximum integere such thatradix**(e-1) is a representablefinite float.
float_info.max_10_exp¶	DBL_MAX_10_EXP	The maximum integere such that10**e is in the range ofrepresentable finite floats.
float_info.min¶	DBL_MIN	The minimum representable positivenormalized float. Usemath.ulp(0.0) to get the smallest positivedenormalized representable float.
float_info.min_exp¶	DBL_MIN_EXP	The minimum integere such thatradix**(e-1) is a normalizedfloat.
float_info.min_10_exp¶	DBL_MIN_10_EXP	The minimum integere such that10**e is a normalized float.
float_info.radix¶	FLT_RADIX	The radix of exponent representation.
float_info.rounds¶	FLT_ROUNDS	An integer representing the rounding mode for floating-point arithmetic.This reflects the value of the systemFLT_ROUNDS macroat interpreter startup time: -1: indeterminable 0: toward zero 1: to nearest 2: toward positive infinity 3: toward negative infinity All other values forFLT_ROUNDS characterizeimplementation-defined rounding behavior.

The attributesys.float_info.dig needs further explanation. Ifs is any string representing a decimal number with at mostsys.float_info.dig significant digits, then convertings to afloat and back again will recover a string representing the same decimalvalue:

>>>importsys>>>sys.float_info.dig15>>>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 thansys.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 therepr() function behaves forfloats. If the string has value'short' then for a finitefloatx,repr(x) aims to produce a short string with theproperty thatfloat(repr(x))==x. This is the usual behaviourin Python 3.1 and later. Otherwise,float_repr_style has value'legacy' andrepr(x) behaves in the same way as it did inversions of Python prior to 3.1.

Added in version 3.1.

sys.getallocatedblocks()¶

Return the number of memory blocks currently allocated by the interpreter,regardless of their size. This function is mainly useful for trackingand debugging memory leaks. Because of the interpreter’s internalcaches, the result can vary from call to call; you may have to call_clear_internal_caches() andgc.collect() to get morepredictable results.

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

Added in version 3.4.

sys.getunicodeinternedsize()¶

Return the number of unicode objects that have been interned.

Added in version 3.12.

sys.getandroidapilevel()¶

Return the build-time API level of Android as an integer. This represents theminimum version of Android this build of Python can run on. For runtimeversion information, seeplatform.android_ver().

Availability: Android.

Added in version 3.7.

sys.getdefaultencoding()¶

Return'utf-8'. This is the name of the default string encoding, usedin methods likestr.encode().

sys.getdlopenflags()¶

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

Availability: Unix.

sys.getfilesystemencoding()¶

Get thefilesystem encoding:the encoding used with thefilesystem error handler to convert between Unicode filenames and bytesfilenames. The filesystem error handler is returned fromgetfilesystemencodeerrors().

For best compatibility, str should be used for filenames in all cases,although representing filenames as bytes is also supported. Functionsaccepting or returning filenames should support either str or bytes andinternally convert to the system’s preferred representation.

os.fsencode() andos.fsdecode() should be used to ensure thatthe correct encoding and errors mode are used.

Thefilesystem encoding and error handler are configured at Pythonstartup by thePyConfig_Read() function: seefilesystem_encoding andfilesystem_errors members ofPyConfig.

Changed in version 3.2:getfilesystemencoding() result cannot beNone anymore.

Changed in version 3.6:Windows is no longer guaranteed to return'mbcs'. SeePEP 529and_enablelegacywindowsfsencoding() for more information.

Changed in version 3.7:Return'utf-8' if thePython UTF-8 Mode isenabled.

sys.getfilesystemencodeerrors()¶

Get thefilesystem error handler: the error handler used with thefilesystem encoding to convert between Unicodefilenames and bytes filenames. The filesystem encoding is returned fromgetfilesystemencoding().

os.fsencode() andos.fsdecode() should be used to ensure thatthe correct encoding and errors mode are used.

Thefilesystem encoding and error handler are configured at Pythonstartup by thePyConfig_Read() function: seefilesystem_encoding andfilesystem_errors members ofPyConfig.

Added in version 3.6.

sys.get_int_max_str_digits()¶

Returns the current value for theinteger string conversion lengthlimitation. See alsoset_int_max_str_digits().

Added in version 3.11.

sys.getrefcount(object)¶

Return the reference count of theobject. The count returned is generally onehigher than you might expect, because it includes the (temporary) reference asan argument togetrefcount().

Note that the returned value may not actually reflect how manyreferences to the object are actually held. For example, someobjects areimmortal and have a very high refcount that does notreflect the actual number of references. Consequently, do not relyon the returned value to be accurate, other than a value of 0 or 1.

Changed in version 3.12:Immortal objects have very large refcounts that do not matchthe actual number of references to the object.

sys.getrecursionlimit()¶

Return the current value of the recursion limit, the maximum depth of the Pythoninterpreter stack. This limit prevents infinite recursion from causing anoverflow of the C stack and crashing Python. It can be set bysetrecursionlimit().

sys.getsizeof(object[,default])¶

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

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

If given,default will be returned if the object does not provide means toretrieve the size. Otherwise aTypeError will be raised.

getsizeof() calls the object’s__sizeof__ method and adds anadditional garbage collector overhead if the object is managed by the garbagecollector.

Seerecursive sizeof recipefor an example of usinggetsizeof() recursively to find the size ofcontainers and all their contents.

sys.getswitchinterval()¶

Return the interpreter’s “thread switch interval” in seconds; seesetswitchinterval().

Added in version 3.2.

sys._getframe([depth])¶

Return a frame object from the call stack. If optional integerdepth isgiven, return the frame object that many calls below the top of the stack. Ifthat is deeper than the call stack,ValueError is raised. The defaultfordepth is zero, returning the frame at the top of the call stack.

Raises anauditing eventsys._getframe with argumentframe.

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._getframemodulename([depth])¶

Return the name of a module from the call stack. If optional integerdepthis given, return the module that many calls below the top of the stack. Ifthat is deeper than the call stack, or if the module is unidentifiable,None is returned. The default fordepth is zero, returning themodule at the top of the call stack.

Raises anauditing eventsys._getframemodulename with argumentdepth.

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.getobjects(limit[,type])¶

This function only exists if CPython was built using thespecialized configure option--with-trace-refs.It is intended only for debugging garbage-collection issues.

Return a list of up tolimit dynamically allocated Python objects.Iftype is given, only objects of that exact type (not subtypes)are included.

Objects from the list are not safe to use.Specifically, the result will include objects from all interpreters thatshare their object allocator state (that is, ones created withPyInterpreterConfig.use_main_obmalloc set to 1or usingPy_NewInterpreter(), and themain interpreter).Mixing objects from different interpreters may lead to crashesor other unexpected behavior.

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

Changed in version 3.13.1:The result may include objects from other interpreters.

sys.getprofile()¶

Get the profiler function as set bysetprofile().

sys.gettrace()¶

Get the trace function as set bysettrace().

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

sys.getwindowsversion()¶

Return a named tuple describing the Windows versioncurrently running. The named elements aremajor,minor,build,platform,service_pack,service_pack_minor,service_pack_major,suite_mask,product_type andplatform_version.service_pack contains a string,platform_version a 3-tuple and all other values areintegers. The components can also be accessed by name, sosys.getwindowsversion()[0] is equivalent tosys.getwindowsversion().major. For compatibility with priorversions, only the first 5 elements are retrievable by indexing.

platform will be2 (VER_PLATFORM_WIN32_NT).

product_type may be one of the following values:

Constant	Meaning
1 (VER_NT_WORKSTATION)	The system is a workstation.
2 (VER_NT_DOMAIN_CONTROLLER)	The system is a domaincontroller.
3 (VER_NT_SERVER)	The system is a server, but nota domain controller.

This function wraps the Win32GetVersionEx() function; see theMicrosoft documentation onOSVERSIONINFOEX() for more informationabout these fields.

platform_version returns the major version, minor version andbuild number of the current operating system, rather than the version thatis being emulated for the process. It is intended for use in logging ratherthan for feature detection.

Note

platform_version derives the version from kernel32.dll which can be of a differentversion than the OS version. Please useplatform module for achieving accurateOS version.

Availability: Windows.

Changed in version 3.2:Changed to a named tuple and addedservice_pack_minor,service_pack_major,suite_mask, andproduct_type.

Changed in version 3.6:Addedplatform_version

sys.get_asyncgen_hooks()¶

Returns anasyncgen_hooks object, which is similar to anamedtuple of the form(firstiter,finalizer),wherefirstiter andfinalizer are expected to be eitherNone orfunctions which take anasynchronous generator iterator as anargument, and are used to schedule finalization of an asynchronousgenerator by an event loop.

Added in version 3.6:SeePEP 525 for more details.

Note

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

sys.get_coroutine_origin_tracking_depth()¶

Get the current coroutine origin tracking depth, as set byset_coroutine_origin_tracking_depth().

Added in version 3.7.

Note

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

sys.hash_info¶

Anamed tuple giving parameters of the numeric hashimplementation. For more details about hashing of numeric types, seeHashing of numeric types.

hash_info.width¶

The width in bits used for hash values

hash_info.modulus¶

The prime modulus P used for numeric hash scheme

hash_info.inf¶

The hash value returned for a positive infinity

hash_info.nan¶

(This attribute is no longer used)

hash_info.imag¶

The multiplier used for the imaginary part of a complex number

hash_info.algorithm¶

The name of the algorithm for hashing of str, bytes, and memoryview

hash_info.hash_bits¶

The internal output size of the hash algorithm

hash_info.seed_bits¶

The size of the seed key of the hash algorithm

Added in version 3.2.

Changed in version 3.4:Addedalgorithm,hash_bits andseed_bits

sys.hexversion¶

The version number encoded as a single integer. This is guaranteed to increasewith each version, including proper support for non-production releases. Forexample, to test that the Python interpreter is at least version 1.5.2, use:

ifsys.hexversion>=0x010502F0:# use some advanced feature...else:# use an alternative implementation or warn the user...

This is calledhexversion since it only really looks meaningful when viewedas the result of passing it to the built-inhex() function. Thenamed tuplesys.version_info may be used for a morehuman-friendly encoding of the same information.

More details ofhexversion can be found atAPI and ABI Versioning.

sys.implementation¶

An object containing information about the implementation of thecurrently running Python interpreter. The following attributes arerequired to exist in all Python implementations.

name is the implementation’s identifier, e.g.'cpython'. The actualstring is defined by the Python implementation, but it is guaranteed to belower case.

version is a named tuple, in the same format assys.version_info. It represents the version of the Pythonimplementation. This has a distinct meaning from the specificversion of the Pythonlanguage to which the currently runninginterpreter conforms, whichsys.version_info represents. Forexample, for PyPy 1.8sys.implementation.version might besys.version_info(1,8,0,'final',0), whereassys.version_infowould besys.version_info(2,7,2,'final',0). For CPython theyare the same value, since it is the reference implementation.

hexversion is the implementation version in hexadecimal format, likesys.hexversion.

cache_tag is the tag used by the import machinery in the filenames ofcached modules. By convention, it would be a composite of theimplementation’s name and version, like'cpython-33'. However, aPython implementation may use some other value if appropriate. Ifcache_tag is set toNone, it indicates that module caching shouldbe disabled.

sys.implementation may contain additional attributes specific tothe Python implementation. These non-standard attributes must start withan 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 Pythonlanguage versions, however.) SeePEP 421 for more information.

Added in version 3.3.

Note

The addition of new required attributes must go through the normal PEPprocess. SeePEP 421 for more information.

sys.int_info¶

Anamed tuple that holds information about Python’s internalrepresentation of integers. The attributes are read only.

int_info.bits_per_digit¶

The number of bits held in each digit.Python integers are stored internally in base2**int_info.bits_per_digit.

int_info.sizeof_digit¶

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

int_info.default_max_str_digits¶

The default value forsys.get_int_max_str_digits()when it is not otherwise explicitly configured.

int_info.str_digits_check_threshold¶

The minimum non-zero value forsys.set_int_max_str_digits(),PYTHONINTMAXSTRDIGITS, or-Xint_max_str_digits.

Added in version 3.1.

Changed in version 3.11:Addeddefault_max_str_digits andstr_digits_check_threshold.

sys.__interactivehook__¶

When this attribute exists, its value is automatically called (with noarguments) when the interpreter is launched ininteractive mode. This is done after thePYTHONSTARTUP file isread, so that you can set this hook there. Thesite modulesets this.

Raises anauditing eventcpython.run_interactivehook with the hook object as the argument whenthe hook is called on startup.

Added in version 3.4.

sys.intern(string)¶

Enterstring in the table of “interned” strings and return the interned string– which isstring itself or a copy. Interning strings is useful to gain alittle performance on dictionary lookup – if the keys in a dictionary areinterned, and the lookup key is interned, the key comparisons (after hashing)can be done by a pointer compare instead of a string compare. Normally, thenames used in Python programs are automatically interned, and the dictionariesused to hold module, class or instance attributes have interned keys.

Interned strings are notimmortal; you must keep a reference to thereturn value ofintern() around to benefit from it.

sys._is_gil_enabled()¶

ReturnTrue if theGIL is enabled andFalse ifit is disabled.

Added in version 3.13.

CPython implementation detail: It is not guaranteed to exist in all implementations of Python.

sys.is_finalizing()¶

ReturnTrue if the main Python interpreter isshutting down. ReturnFalse otherwise.

See also thePythonFinalizationError exception.

Added in version 3.5.

sys.last_exc¶

This variable is not always defined; it is set to the exception instancewhen an exception is not handled and the interpreter prints an error messageand a stack traceback. Its intended use is to allow an interactive user toimport a debugger module and engage in post-mortem debugging without havingto re-execute the command that caused the error. (Typical use isimportpdb;pdb.pm() to enter the post-mortem debugger; seepdbmodule for more information.)

Added in version 3.12.

sys._is_interned(string)¶

ReturnTrue if the given string is “interned”,Falseotherwise.

Added in version 3.13.

CPython implementation detail: It is not guaranteed to exist in all implementations of Python.

sys.last_type¶

sys.last_value¶

sys.last_traceback¶

These three variables are deprecated; usesys.last_exc instead.They hold the legacy representation ofsys.last_exc, as returnedfromexc_info() above.

sys.maxsize¶

An integer giving the maximum value a variable of typePy_ssize_t cantake. It’s usually2**31-1 on a 32-bit platform and2**63-1 on a64-bit platform.

sys.maxunicode¶

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

Changed in version 3.3:BeforePEP 393,sys.maxunicode used to be either0xFFFFor0x10FFFF, depending on the configuration option that specifiedwhether Unicode characters were stored as UCS-2 or UCS-4.

sys.meta_path¶

A list ofmeta path finder objects that have theirfind_spec() methods called to see if oneof the objects can find the module to be imported. By default, it holds entriesthat implement Python’s default import semantics. Thefind_spec() method is called with atleast the absolute name of the module being imported. If the module to beimported is contained in a package, then the parent package’s__path__attribute is passed in as a second argument. The method returns amodule spec, orNone if the module cannot be found.

See also

importlib.abc.MetaPathFinder

The abstract base class defining the interface of finder objects onmeta_path.

importlib.machinery.ModuleSpec

The concrete class whichfind_spec() should returninstances of.

Changed in version 3.4:Module specs were introduced in Python 3.4, byPEP 451.

Changed in version 3.12:Removed the fallback that looked for afind_module() methodif ameta_path entry didn’t have afind_spec() method.

sys.modules¶

This is a dictionary that maps module names to modules which have already beenloaded. This can be manipulated to force reloading of modules and other tricks.However, replacing the dictionary will not necessarily work as expected anddeleting essential items from the dictionary may cause Python to fail. Ifyou want to iterate over this global dictionary always usesys.modules.copy() ortuple(sys.modules) to avoid exceptions as itssize may change during iteration as a side effect of code or activity inother threads.

sys.orig_argv¶

The list of the original command line arguments passed to the Pythonexecutable.

The elements ofsys.orig_argv are the arguments to the Python interpreter,while the elements ofsys.argv are the arguments to the user’s program.Arguments consumed by the interpreter itself will be present insys.orig_argvand missing fromsys.argv.

Added in version 3.10.

sys.path¶

A list of strings that specifies the search path for modules. Initialized fromthe environment variablePYTHONPATH, plus an installation-dependentdefault.

By default, as initialized upon program startup, a potentially unsafe pathis prepended tosys.path (before the entries inserted as a resultofPYTHONPATH):

• python-mmodule command line: prepend the current workingdirectory.

• pythonscript.py command line: prepend the script’s directory.If it’s a symbolic link, resolve symbolic links.

• python-ccode andpython (REPL) command lines: prepend an emptystring, which means the current working directory.

To not prepend this potentially unsafe path, use the-P commandline option or thePYTHONSAFEPATH environment variable.

A program is free to modify this list for its own purposes. Only stringsshould be added tosys.path; all other data types areignored during import.

See also

• Modulesite This describes how to use .pth files toextendsys.path.

sys.path_hooks¶

A list of callables that take a path argument to try to create afinder for the path. If a finder can be created, it is to bereturned by the callable, else raiseImportError.

Originally specified inPEP 302.

sys.path_importer_cache¶

A dictionary acting as a cache forfinder objects. The keys arepaths that have been passed tosys.path_hooks and the values arethe finders that are found. If a path is a valid file system path but nofinder is found onsys.path_hooks thenNone isstored.

Originally specified inPEP 302.

sys.platform¶

A string containing a platform identifier. Known values are:

System	platform value
AIX	'aix'
Android	'android'
Emscripten	'emscripten'
iOS	'ios'
Linux	'linux'
macOS	'darwin'
Windows	'win32'
Windows/Cygwin	'cygwin'
WASI	'wasi'

On Unix systems not listed in the table, the value is the lowercased OS nameas returned byuname-s, with the first part of the version as returned byuname-r appended, e.g.'sunos5' or'freebsd8',at the timewhen Python was built. Unless you want to test for a specific systemversion, it is therefore recommended to use the following idiom:

ifsys.platform.startswith('freebsd'):# FreeBSD-specific code here...

Changed in version 3.3:On Linux,sys.platform doesn’t contain the major version anymore.It is always'linux', instead of'linux2' or'linux3'.

Changed in version 3.8:On AIX,sys.platform doesn’t contain the major version anymore.It is always'aix', instead of'aix5' or'aix7'.

Changed in version 3.13:On Android,sys.platform now returns'android' rather than'linux'.

See also

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

Theplatform module provides detailed checks for thesystem’s identity.

sys.platlibdir¶

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

It is equal to"lib" on most platforms. On Fedora and SuSE, it is equalto"lib64" on 64-bit platforms which gives the followingsys.pathpaths (whereX.Y is the Pythonmajor.minor version):

• /usr/lib64/pythonX.Y/:Standard library (likeos.py of theos module)

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

• /usr/lib/pythonX.Y/site-packages/ (always uselib, notsys.platlibdir): Third-party modules

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

Added in version 3.9.

sys.prefix¶

A string giving the site-specific directory prefix where the platformindependent Python files are installed; on Unix, the default is/usr/local. This can be set at build time with the--prefixargument to theconfigure script. SeeInstallation paths for derived paths.

Note

If avirtual environment is in effect, thisvalue will be changed insite.py to point to the virtualenvironment. The value for the Python installation will still beavailable, viabase_prefix.

sys.ps1¶

sys.ps2¶

Strings specifying the primary and secondary prompt of the interpreter. Theseare only defined if the interpreter is in interactive mode. Their initialvalues in this case are'>>>' and'...'. If a non-string object isassigned to either variable, itsstr() is re-evaluated each time theinterpreter prepares to read a new interactive command; this can be used toimplement a dynamic prompt.

sys.setdlopenflags(n)¶

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

Availability: Unix.

sys.set_int_max_str_digits(maxdigits)¶

Set theinteger string conversion length limitation used by this interpreter. See alsoget_int_max_str_digits().

Added in version 3.11.

sys.setprofile(profilefunc)¶

Set the system’s profile function, which allows you to implement a Python sourcecode profiler in Python. See chapterThe Python Profilers for more information on thePython profiler. The system’s profile function is called similarly to thesystem’s trace function (seesettrace()), 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 isthread-specific, but there is no way for the profiler to know about context switches betweenthreads, 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 returnNone. Error in the profilefunction will cause itself unset.

Note

The same tracing mechanism is used forsetprofile() assettrace().To trace calls withsetprofile() inside a tracing function(e.g. in a debugger breakpoint), seecall_tracing().

Profile functions should have three arguments:frame,event, andarg.frame is the current stack frame.event is a string:'call','return','c_call','c_return', or'c_exception'.arg dependson the event type.

The events have the following meaning:

'call'

A function is called (or some other code block entered). Theprofile function is called;arg isNone.

'return'

A function (or other code block) is about to return. The profilefunction is called;arg is the value that will be returned, orNoneif 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 ora 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.

Raises anauditing eventsys.setprofile with no arguments.

sys.setrecursionlimit(limit)¶

Set the maximum depth of the Python interpreter stack tolimit. This limitprevents infinite recursion from causing an overflow of the C stack and crashingPython.

The highest possible limit is platform-dependent. A user may need to set thelimit higher when they have a program that requires deep recursion and a platformthat supports a higher limit. This should be done with care, because a too-highlimit can lead to a crash.

If the new limit is too low at the current recursion depth, aRecursionError exception is raised.

Changed in version 3.5.1:ARecursionError exception is now raised if the new limit is toolow at the current recursion depth.

sys.setswitchinterval(interval)¶

Set the interpreter’s thread switch interval (in seconds). This floating-pointvalue determines the ideal duration of the “timeslices” allocated toconcurrently running Python threads. Please note that the actual valuecan be higher, especially if long-running internal functions or methodsare used. Also, which thread becomes scheduled at the end of the intervalis the operating system’s decision. The interpreter doesn’t have itsown scheduler.

Added in version 3.2.

sys.settrace(tracefunc)¶

Set the system’s trace function, which allows you to implement a Pythonsource code debugger in Python. The function is thread-specific; for adebugger to support multiple threads, it must register a trace function usingsettrace() for each thread being debugged or usethreading.settrace().

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

The trace function is invoked (withevent set to'call') whenever a newlocal scope is entered; it should return a reference to a local tracefunction to be used for the new scope, orNone if the scope shouldn’t betraced.

The local trace function should return a reference to itself, or to anotherfunction which would then be used as the local trace function for the scope.

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

Note

Tracing is disabled while calling the trace function (e.g. a function set bysettrace()). For recursive tracing seecall_tracing().

The events have the following meaning:

'call'

A function is called (or some other code block entered). Theglobal trace function is called;arg isNone; the return valuespecifies the local trace function.

'line'

The interpreter is about to execute a new line of code or re-execute thecondition of a loop. The local trace function is called;arg isNone; the return value specifies the new local trace function. SeeObjects/lnotab_notes.txt for a detailed explanation of how thisworks.Per-line events may be disabled for a frame by settingf_trace_lines toFalse on thatframe.

'return'

A function (or other code block) is about to return. The local tracefunction is called;arg is the value that will be returned, orNoneif the event is caused by an exception being raised. The trace function’sreturn value is ignored.

'exception'

An exception has occurred. The local trace function is called;arg is atuple(exception,value,traceback); the return value specifies thenew local trace function.

'opcode'

The interpreter is about to execute a new opcode (seedis foropcode details). The local trace function is called;arg isNone; the return value specifies the new local trace function.Per-opcode events are not emitted by default: they must be explicitlyrequested by settingf_trace_opcodes toTrue on theframe.

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 byassigningframe.f_trace=tracefunc explicitly, rather than relying onit being set indirectly via the return value from an already installedtrace function. This is also required for activating the trace function onthe current frame, whichsettrace() doesn’t do. Note that in orderfor this to work, a global tracing function must have been installedwithsettrace() in order to enable the runtime tracing machinery,but it doesn’t need to be the same tracing function (e.g. it could be alow overhead tracing function that simply returnsNone to disableitself immediately on each frame).

For more information on code and frame objects, refer toThe standard type hierarchy.

Raises anauditing eventsys.settrace with no arguments.

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

Changed in version 3.7:'opcode' event type added;f_trace_lines andf_trace_opcodes attributes added to frames

sys.set_asyncgen_hooks([firstiter][,finalizer])¶

Accepts two optional keyword arguments which are callables that accept anasynchronous generator iterator as an argument. Thefirstitercallable will be called when an asynchronous generator is iterated for thefirst time. Thefinalizer will be called when an asynchronous generatoris about to be garbage collected.

Raises anauditing eventsys.set_asyncgen_hooks_firstiter with no arguments.

Raises anauditing eventsys.set_asyncgen_hooks_finalizer with no arguments.

Two auditing events are raised because the underlying API consists of twocalls, each of which must raise its own event.

Added in version 3.6:SeePEP 525 for more details, and for a reference example of afinalizer method see the implementation ofasyncio.Loop.shutdown_asyncgens inLib/asyncio/base_events.py

Note

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

sys.set_coroutine_origin_tracking_depth(depth)¶

Allows enabling or disabling coroutine origin tracking. Whenenabled, thecr_origin attribute on coroutine objects willcontain a tuple of (filename, line number, function name) tuplesdescribing the traceback where the coroutine object was created,with the most recent call first. When disabled,cr_origin willbeNone.

To enable, pass adepth value greater than zero; this sets thenumber of frames whose information will be captured. To disable,pass setdepth to zero.

This setting is thread-specific.

Added in version 3.7.

Note

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

sys.activate_stack_trampoline(backend,/)¶

Activate the stack profiler trampolinebackend.The only supported backend is"perf".

Availability: Linux.

Added in version 3.12.

See also

• Python support for the Linux perf profiler

• https://perf.wiki.kernel.org

sys.deactivate_stack_trampoline()¶

Deactivate the current stack profiler trampoline backend.

If no stack profiler is activated, this function has no effect.

Availability: Linux.

Added in version 3.12.

sys.is_stack_trampoline_active()¶

ReturnTrue if a stack profiler trampoline is active.

Availability: Linux.

Added in version 3.12.

sys._enablelegacywindowsfsencoding()¶

Changes thefilesystem encoding and error handler to ‘mbcs’ and‘replace’ respectively, for consistency with versions of Python prior to3.6.

This is equivalent to defining thePYTHONLEGACYWINDOWSFSENCODINGenvironment variable before launching Python.

See alsosys.getfilesystemencoding() andsys.getfilesystemencodeerrors().

Availability: Windows.

Note

Changing the filesystem encoding after Python startup is risky becausethe old fsencoding or paths encoded by the old fsencoding may be cachedsomewhere. UsePYTHONLEGACYWINDOWSFSENCODING instead.

Added in version 3.6:SeePEP 529 for more details.

Deprecated since version 3.13, will be removed in version 3.16:UsePYTHONLEGACYWINDOWSFSENCODING instead.

sys.stdin¶

sys.stdout¶

sys.stderr¶

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

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

• stdout is used for the output ofprint() andexpressionstatements and for the prompts ofinput();

• The interpreter’s own prompts and its error messages go tostderr.

These streams are regulartext files like thosereturned by theopen() function. Their parameters are chosen asfollows:

• The encoding and error handling are is initialized fromPyConfig.stdio_encoding andPyConfig.stdio_errors.

  On Windows, UTF-8 is used for the console device. Non-characterdevices such as disk files and pipes use the system localeencoding (i.e. the ANSI codepage). Non-console characterdevices such as NUL (i.e. whereisatty() returnsTrue) use thevalue of the console input and output codepages at startup,respectively for stdin and stdout/stderr. This defaults to thesystemlocale encoding if the process is not initially attachedto a console.

  The special behaviour of the console can be overriddenby setting the environment variable PYTHONLEGACYWINDOWSSTDIObefore starting Python. In that case, the console codepages areused as for any other character device.

  Under all platforms, you can override the character encoding bysetting thePYTHONIOENCODING environment variable beforestarting Python or by using the new-Xutf8 commandline option andPYTHONUTF8 environment variable. However,for the Windows console, this only applies whenPYTHONLEGACYWINDOWSSTDIO is also set.

• When interactive, thestdout stream is line-buffered. Otherwise,it is block-buffered like regular text files. Thestderr streamis line-buffered in both cases. You can make both streams unbufferedby passing the-u command-line option or setting thePYTHONUNBUFFERED environment variable.

Changed in version 3.9:Non-interactivestderr is now line-buffered instead of fullybuffered.

Note

To write or read binary data from/to the standard streams, use theunderlying binarybuffer object. For example, towrite bytes tostdout, usesys.stdout.buffer.write(b'abc').

However, if you are writing a library (and do not control in whichcontext its code will be executed), be aware that the standard streamsmay be replaced with file-like objects likeio.StringIO whichdo not support thebuffer attribute.

sys.__stdin__¶

sys.__stdout__¶

sys.__stderr__¶

These objects contain the original values ofstdin,stderr andstdout 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 thesys.std* object has been redirected.

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

Note

Under some conditionsstdin,stdout andstderr as well as theoriginal values__stdin__,__stdout__ and__stderr__ can beNone. It is usually the case for Windows GUI apps that aren’t connectedto a console and Python apps started withpythonw.

sys.stdlib_module_names¶

A frozenset of strings containing the names of standard library modules.

It is the same on all platforms. Modules which are not available onsome platforms and modules disabled at Python build are also listed.All module kinds are listed: pure Python, built-in, frozen and extensionmodules. Test modules are excluded.

For packages, only the main package is listed: sub-packages and sub-modulesare not listed. For example, theemail package is listed, but theemail.mime sub-package and theemail.message sub-module are notlisted.

See also thesys.builtin_module_names list.

Added in version 3.10.

sys.thread_info¶

Anamed tuple holding information about the threadimplementation.

thread_info.name¶

The name of the thread implementation:

• "nt": Windows threads

• "pthread": POSIX threads

• "pthread-stubs": stub POSIX threads(on WebAssembly platforms without threading support)

• "solaris": Solaris threads

thread_info.lock¶

The name of the lock implementation:

• "semaphore": a lock uses a semaphore

• "mutex+cond": a lock uses a mutex and a condition variable

• None if this information is unknown

thread_info.version¶

The name and version of the thread library.It is a string, orNone if this information is unknown.

Added in version 3.3.

sys.tracebacklimit¶

When this variable is set to an integer value, it determines the maximum numberof levels of traceback information printed when an unhandled exception occurs.The default is1000. When set to0 or less, all traceback informationis suppressed and only the exception type and value are printed.

sys.unraisablehook(unraisable,/)¶

Handle an unraisable exception.

Called when an exception has occurred but there is no way for Python tohandle it. For example, when a destructor raises an exception or duringgarbage collection (gc.collect()).

Theunraisable argument has the following attributes:

• exc_type: Exception type.

• exc_value: Exception value, can beNone.

• exc_traceback: Exception traceback, can beNone.

• err_msg: Error message, can beNone.

• object: Object causing the exception, can beNone.

The default hook formatserr_msg andobject as:f'{err_msg}:{object!r}'; use “Exception ignored in” error messageiferr_msg isNone.

sys.unraisablehook() can be overridden to control how unraisableexceptions are handled.

See also

excepthook() which handles uncaught exceptions.

Warning

Storingexc_value using a custom hook can create a reference cycle.It should be cleared explicitly to break the reference cycle when theexception is no longer needed.

Storingobject using a custom hook can resurrect it if it is set to anobject which is being finalized. Avoid storingobject after the customhook completes to avoid resurrecting objects.

Raise an auditing eventsys.unraisablehook with argumentshook,unraisable when an exception that cannot be handled occurs.Theunraisable object is the same as what will be passed to the hook.If no hook has been set,hook may beNone.

Added in version 3.8.

sys.version¶

A string containing the version number of the Python interpreter plus additionalinformation on the build number and compiler used. This string is displayedwhen the interactive interpreter is started. Do not extract version informationout of it, rather, useversion_info and the functions provided by theplatform module.

sys.api_version¶

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

sys.version_info¶

A tuple containing the five components of the version number:major,minor,micro,releaselevel, andserial. All values exceptreleaselevel areintegers; the release level is'alpha','beta','candidate', or'final'. Theversion_info value corresponding to the Python version 2.0is(2,0,0,'final',0). The components can also be accessed by name,sosys.version_info[0] is equivalent tosys.version_info.majorand so on.

Changed in version 3.1:Added named component attributes.

sys.warnoptions¶

This is an implementation detail of the warnings framework; do not modify thisvalue. Refer to thewarnings module for more information on the warningsframework.

sys.winver¶

The version number used to form registry keys on Windows platforms. This isstored as string resource 1000 in the Python DLL. The value is normally themajor and minor versions of the running Python interpreter. It is provided in thesysmodule for informational purposes; modifying this value has no effect on theregistry keys used by Python.

Availability: Windows.

sys.monitoring

Namespace containing functions and constants for register callbacksand controlling monitoring events.Seesys.monitoring for details.

sys._xoptions¶

A dictionary of the various implementation-specific flags passed throughthe-X command-line option. Option names are either mapped totheir values, if given explicitly, or toTrue. Example:

$./python-Xa=b-XcPython 3.2a3+ (py3k, Oct 16 2010, 20:14:50)[GCC 4.4.3] on linux2Type "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 othermeans, or not at all.

Added in version 3.2.

Citations