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.

sys.abiflags¶

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

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

New in version 3.2.

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.

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.

New 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.

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.

New 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).

New 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).

New 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 giving 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.)

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 some other code.

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.

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.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.

New in version 3.7.

sys._debugmallocstats()¶

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

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

New 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.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.

New in version 3.8.

sys.excepthook(type,value,traceback)¶

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

When an exception 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.

New in version 3.7:__breakpointhook__

New in version 3.8:__unraisablehook__

sys.exc_info()¶

This function returns a tuple of three values that give information about theexception that is currently being handled. The information returned is specificboth to the current thread and to the current stack frame. If the current stackframe is not handling an exception, the information is taken from the callingstack frame, or its caller, and so on until a stack frame is found that ishandling an exception. Here, “handling an exception” is defined as “executingan except clause.” For any stack frame, only information about the exceptionbeing currently handled is accessible.

If no exception is being handled anywhere on the stack, a tuple containingthreeNone values is returned. Otherwise, the values returned are(type,value,traceback). Their meaning is:type gets the type of theexception being handled (a subclass ofBaseException);value getsthe exception instance (an instance of the exception type);traceback getsatraceback object which encapsulates the callstack at the point where the exception originally occurred.

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])¶

Exit from Python. This is implemented by raising theSystemExitexception, so cleanup actions specified by finally clauses oftrystatements are honored, and it is possible to intercept the exit attempt atan outer level.

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.

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.

attribute	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	-Xdev
utf8_mode	-Xutf8
int_max_str_digits	-Xint_max_str_digits (integer string conversion length limitation)

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

New 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:Addeddev_mode attribute for the new-Xdev flagandutf8_mode attribute for the new-Xutf8 flag.

Changed in version 3.8.14: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.

attribute	float.h macro	explanation
epsilon	DBL_EPSILON	difference between 1.0 and the least valuegreater than 1.0 that is representable as a float
dig	DBL_DIG	maximum number of decimal digits that can befaithfully represented in a float; see below
mant_dig	DBL_MANT_DIG	float precision: the number of base-radixdigits in the significand of a float
max	DBL_MAX	maximum representable positive finite float
max_exp	DBL_MAX_EXP	maximum integere such thatradix**(e-1) isa representable finite float
max_10_exp	DBL_MAX_10_EXP	maximum integere such that10**e is in therange of representable finite floats
min	DBL_MIN	minimum representable positivenormalized float
min_exp	DBL_MIN_EXP	minimum integere such thatradix**(e-1) isa normalized float
min_10_exp	DBL_MIN_10_EXP	minimum integere such that10**e is anormalized float
radix	FLT_RADIX	radix of exponent representation
rounds	FLT_ROUNDS	integer constant representing the rounding modeused for arithmetic operations. This reflectsthe value of the system FLT_ROUNDS macro atinterpreter startup time. See section 5.2.4.2.2of the C99 standard for an explanation of thepossible values and their meanings.

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.

New 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_type_cache() andgc.collect() to get morepredictable results.

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

New in version 3.4.

sys.getandroidapilevel()¶

Return the build time API version of Android as an integer.

Availability: Android.

New in version 3.7.

sys.getcheckinterval()¶

Return the interpreter’s “check interval”; seesetcheckinterval().

Deprecated since version 3.2:Usegetswitchinterval() instead.

sys.getdefaultencoding()¶

Return the name of the current default string encoding used by the Unicodeimplementation.

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()¶

Return the name of the encoding used to convert between Unicodefilenames and bytes filenames. For best compatibility, str should beused for filenames in all cases, although representing filenames as bytesis also supported. Functions accepting or returning filenames should supporteither str or bytes and internally convert to the system’s preferredrepresentation.

This encoding is always ASCII-compatible.

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

• In the UTF-8 mode, the encoding isutf-8 on any platform.

• On macOS, the encoding is'utf-8'.

• On Unix, the encoding is the locale encoding.

• On Windows, the encoding may be'utf-8' or'mbcs', dependingon user configuration.

• On Android, the encoding is'utf-8'.

• On VxWorks, the encoding is'utf-8'.

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’ in the UTF-8 mode.

sys.getfilesystemencodeerrors()¶

Return the name of the error mode used to convert between Unicode filenamesand bytes filenames. The encoding name is returned fromgetfilesystemencoding().

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

New 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().

New in version 3.8.14.

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().

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”; seesetswitchinterval().

New 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 no arguments.

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 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.

New 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().

New 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.

attribute	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 acomplex 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

New 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.

New 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.

Attribute	Explanation
bits_per_digit	number of bits held in each digit. Pythonintegers are stored internally in base2**int_info.bits_per_digit
sizeof_digit	size in bytes of the C type used torepresent a digit
default_max_str_digits	default value forsys.get_int_max_str_digits() when itis not otherwise explicitly configured.
str_digits_check_threshold	minimum non-zero value forsys.set_int_max_str_digits(),PYTHONINTMAXSTRDIGITS, or-Xint_max_str_digits.

New in version 3.1.

Changed in version 3.8.14: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.

New 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 not immortal; you must keep a reference to the returnvalue ofintern() around to benefit from it.

sys.is_finalizing()¶

ReturnTrue if the Python interpreter isshutting down,False otherwise.

New in version 3.5.

sys.last_type¶

sys.last_value¶

sys.last_traceback¶

These three variables are not always defined; they are set when an exception isnot 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 moduleand engage in post-mortem debugging without having to re-execute the commandthat caused the error. (Typical use isimportpdb;pdb.pm() to enter thepost-mortem debugger; seepdb module formore information.)

The meaning of the variables is the same as that of the return values fromexc_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. 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. Earlier versions of Python looked for a method calledfind_module().This is still called as a fallback if ameta_path entry doesn’thave 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.

sys.path¶

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

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 Pythoninterpreter. If the script directory is not available (e.g. if the interpreteris invoked interactively or if the script is read from standard input),path[0] is the empty string, which directs Python to search modules in thecurrent directory first. Notice that the script directory is insertedbeforethe entries inserted as a result ofPYTHONPATH.

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

See also

Modulesite This describes how to use .pth files to extendsys.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.

Changed in version 3.3:None is stored instead ofimp.NullImporter when no finderis found.

sys.platform¶

This string contains a platform identifier that can be used to appendplatform-specific components tosys.path, for instance.

For Unix systems, except on Linux and AIX, this is the lowercased OS name asreturned 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...elifsys.platform.startswith('linux'):# Linux-specific code here...elifsys.platform.startswith('aix'):# AIX-specific code here...

For other systems, the values are:

System	platform value
AIX	'aix'
Linux	'linux'
Windows	'win32'
Windows/Cygwin	'cygwin'
macOS	'darwin'

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'. Sinceolder Python versions include the version number, it is recommended toalways use thestartswith idiom presented above.

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'. Sinceolder Python versions include the version number, it is recommended toalways use thestartswith idiom presented above.

See also

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

Theplatform module provides detailed checks for thesystem’s identity.

sys.prefix¶

A string giving the site-specific directory prefix where the platformindependent Python files are installed; by default, this is the string'/usr/local'. This can be set at build time with the--prefixargument to theconfigure script. The main collection of Pythonlibrary modules is installed in the directoryprefix/lib/pythonX.Ywhile the platform independent header files (all exceptpyconfig.h) arestored inprefix/include/pythonX.Y, whereX.Y is the versionnumber of Python, for example3.2.

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.setcheckinterval(interval)¶

Set the interpreter’s “check interval”. This integer value determines how oftenthe interpreter checks for periodic things such as thread switches and signalhandlers. The default is100, meaning the check is performed every 100Python virtual instructions. Setting it to a larger value may increaseperformance for programs using threads. Setting it to a value<= 0 checksevery virtual instruction, maximizing responsiveness as well as overhead.

Deprecated since version 3.2:This function doesn’t have an effect anymore, as the internal logic forthread switching and asynchronous tasks has been rewritten. Usesetswitchinterval() instead.

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(n)¶

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

New in version 3.8.14.

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.

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.

Raises anauditing eventsys.setprofile with no arguments.

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.

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.

New 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 for further tracing in that scope), orNone to turn off tracingin that scope.

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

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 that frame.

'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.

New 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 willbe None.

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.

New in version 3.7.

Note

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

sys._enablelegacywindowsfsencoding()¶

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

This is equivalent to defining thePYTHONLEGACYWINDOWSFSENCODINGenvironment variable before launching Python.

Availability: Windows.

New in version 3.6:SeePEP 529 for more details.

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 character encoding is platform-dependent. Non-Windowsplatforms use the locale encoding (seelocale.getpreferredencoding()).

  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 thesystem locale 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,stdout andstderr streams are line-buffered.Otherwise, they are block-buffered like regular text files. You canoverride this value with the-u command-line option.

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.thread_info¶

Anamed tuple holding information about the threadimplementation.

Attribute	Explanation
name	Name of the thread implementation: 'nt': Windows threads 'pthread': POSIX threads 'solaris': Solaris threads
lock	Name of the lock implementation: 'semaphore': a lock uses a semaphore 'mutex+cond': a lock uses a mutexand a condition variable None if this information is unknown
version	Name and version of the thread library. It is a string,orNone if this information is unknown.

New 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.

Storingexc_value using a custom hook can create a reference cycle. Itshould 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.

See alsoexcepthook() which handles uncaught exceptions.

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.

New 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 thefirst three characters ofversion. It is provided in thesysmodule for informational purposes; modifying this value has no effect on theregistry keys used by Python.

Availability: Windows.

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.

New in version 3.2.

Citations

C99

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