sys
— System-specific parameters and functions¶
This module provides access to some variables used or maintained by the interpreter and to functions that interact strongly with the interpreter. It is always available.
- sys.abiflags¶
On POSIX systems where Python was built with the standard
configure
script, this contains the ABI flags as specified by PEP 3149.Changed in version 3.8: Default flags became an empty string (
m
flag for pymalloc has been removed).New in version 3.2.
- sys.addaudithook(hook)¶
Append the callable hook to the list of active auditing hooks for the current (sub)interpreter.
When an auditing event is raised through the
sys.audit()
function, each hook will be called in the order it was added with the event name and the tuple of arguments. Native hooks added byPySys_AddAuditHook()
are called first, followed by hooks added in the current (sub)interpreter. Hooks can 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 internal or otherwise unobservable actions, whether by Python or libraries written in Python. They are not suitable for implementing a “sandbox”. In particular, malicious code can trivially disable or bypass hooks added using this function. At a minimum, any security-sensitive hooks must be added using the C API
PySys_AddAuditHook()
before initialising the runtime, and any modules allowing arbitrary memory modification (such asctypes
) should be completely removed or closely monitored.Calling
sys.addaudithook()
will itself raise an auditing event namedsys.addaudithook
with no arguments. If any existing hooks raise an exception derived fromRuntimeError
, the new hook will not be added and the exception suppressed. As a result, callers cannot assume that their hook has been added unless they control all existing hooks.See the audit events table for all events raised by CPython, and PEP 578 for the original design discussion.
New in version 3.8.
Changed in version 3.8.1: Exceptions derived from
Exception
but notRuntimeError
are no longer suppressed.CPython implementation detail: When tracing is enabled (see
settrace()
), Python hooks are only traced if the callable has a__cantrace__
member that is set to a true value. Otherwise, trace functions will skip the hook.
- sys.argv¶
The list of command line arguments passed to a Python script.
argv[0]
is the script name (it is operating system dependent whether this is a full pathname or not). If the command was executed using the-c
command line option to the interpreter,argv[0]
is set to the string'-c'
. If no script name was passed to the Python interpreter,argv[0]
is the empty string.To loop over the standard input, or the list of files given on the command line, see the
fileinput
module.See also
sys.orig_argv
.Note
On Unix, command line arguments are passed by bytes from OS. Python decodes them with filesystem encoding and “surrogateescape” error handler. When you need original bytes, you can get it by
[os.fsencode(arg) for arg in sys.argv]
.
- sys.audit(event, *args)¶
Raise an auditing event and trigger any active auditing hooks. event is a string identifying the event, and args may contain optional arguments with more information about the event. The number and types of arguments for a given event are considered a public and stable API and should not be modified between releases.
For example, one auditing event is named
os.chdir
. This event has one argument called path that will contain the requested new working directory.sys.audit()
will call the existing auditing hooks, passing the event name and arguments, and will re-raise the first exception from any hook. In general, if an exception is raised, it should not be handled and the process should be terminated as quickly as possible. This allows hook implementations to decide how to respond to particular events: they can merely log the event or abort the operation by raising an exception.Hooks are added using the
sys.addaudithook()
orPySys_AddAuditHook()
functions.The native equivalent of this function is
PySys_Audit()
. Using the native function is preferred when possible.See the audit events table for all events raised by CPython.
New in version 3.8.
- sys.base_exec_prefix¶
Set during Python startup, before
site.py
is run, to the same value asexec_prefix
. If not running in a virtual 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 the virtual environment, whereasbase_prefix
andbase_exec_prefix
will remain pointing to the base Python installation (the one which the virtual environment was created from).New in version 3.3.
- sys.base_prefix¶
Set during Python startup, before
site.py
is run, to the same value asprefix
. If not running in a virtual 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 the virtual environment, whereasbase_prefix
andbase_exec_prefix
will remain pointing to the base Python installation (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'
on big-endian (most-significant byte first) platforms, and'little'
on little-endian (least-significant byte first) platforms.
- sys.builtin_module_names¶
A tuple of strings containing the names of all modules that are compiled into this Python interpreter. (This information is not available in any other way —
modules.keys()
only lists the imported modules.)See also the
sys.stdlib_module_names
list.
- sys.call_tracing(func, args)¶
Call
func(*args)
, while tracing is enabled. The tracing state is saved, and restored afterwards. This is intended to be called from a debugger from a checkpoint, to recursively debug some other code.
- sys.copyright¶
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 attribute and method lookups. Use the function only to drop unnecessary references during reference leak debugging.
This function should be used for internal and specialized purposes only.
- sys._current_frames()¶
Return a dictionary mapping each thread’s identifier to the topmost stack frame currently active in that thread at the time the function is called. Note that functions in the
traceback
module can build the call stack given such a frame.This is most useful for debugging deadlock: this function does not require the deadlocked threads’ cooperation, and such threads’ call stacks are frozen for as long as they remain deadlocked. The frame returned for a non-deadlocked thread may bear no relationship to that thread’s current activity by the time calling code examines the frame.
This function should be used for internal and specialized purposes only.
Raises an auditing event
sys._current_frames
with no arguments.
- sys._current_exceptions()¶
Return a dictionary mapping each thread’s identifier to the topmost exception currently active in that thread at the time the function is called. If a thread is not currently handling an exception, it is not included in the result dictionary.
This is most useful for statistical profiling.
This function should be used for internal and specialized purposes only.
Raises an auditing event
sys._current_exceptions
with no arguments.
- sys.breakpointhook()¶
This hook function is called by built-in
breakpoint()
. By default, it drops you into thepdb
debugger, but it can be set to any other function so that you can choose which debugger gets used.The signature of this function is dependent on what it calls. For example, the default binding (e.g.
pdb.set_trace()
) expects no arguments, but you might bind it to a function that expects additional arguments (positional and/or keyword). The built-inbreakpoint()
function passes its*args
and**kws
straight through. Whateverbreakpointhooks()
returns is returned frombreakpoint()
.The default implementation first consults the environment variable
PYTHONBREAKPOINT
. If that is set to"0"
then this function returns immediately; i.e. it is a no-op. If the environment variable is not set, or is set to the empty string,pdb.set_trace()
is called. Otherwise this variable should name a function to run, using Python’s dotted-import nomenclature, e.g.package.subpackage.module.function
. In this case,package.subpackage.module
would be imported and the resulting module must have a callable 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 by
PYTHONBREAKPOINT
, aRuntimeWarning
is reported and the breakpoint is ignored.Also note that if
sys.breakpointhook()
is overridden programmatically,PYTHONBREAKPOINT
is not consulted.New in version 3.7.
- sys._debugmallocstats()¶
Print low-level information to stderr about the state of CPython’s memory allocator.
If Python is built in debug mode (
configure --with-pydebug option
), it also performs some expensive internal consistency checks.New in version 3.3.
CPython implementation detail: This function is specific to CPython. The exact output format is not defined here, and may change.
- sys.dllhandle¶
Integer specifying the handle of the Python DLL.
Availability: Windows.
- sys.displayhook(value)¶
If value is not
None
, this function printsrepr(value)
tosys.stdout
, and saves value inbuiltins._
. Ifrepr(value)
is not encodable tosys.stdout.encoding
withsys.stdout.errors
error handler (which is probably'strict'
), encode it tosys.stdout.encoding
with'backslashreplace'
error handler.sys.displayhook
is called on the result of evaluating an expression entered in an interactive Python session. The display of these values can be customized by assigning another one-argument function tosys.displayhook
.Pseudo-code:
def displayhook(value): if value is None: return # Set '_' to None to avoid recursion builtins._ = None text = repr(value) try: sys.stdout.write(text) except UnicodeEncodeError: bytes = text.encode(sys.stdout.encoding, 'backslashreplace') if hasattr(sys.stdout, 'buffer'): sys.stdout.buffer.write(bytes) else: text = bytes.decode(sys.stdout.encoding, 'strict') sys.stdout.write(text) sys.stdout.write("\n") builtins._ = value
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 the import 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 it yourself to control bytecode file generation.
- sys._emscripten_info¶
A named tuple holding information about the environment on the wasm32-emscripten platform. The named tuple is provisional and may change in the future.
Attribute
Explanation
emscripten_version
Emscripten version as tuple of ints (major, minor, micro), e.g.
(3, 1, 8)
.runtime
Runtime string, e.g. browser user agent,
'Node.js v14.18.2'
, or'UNKNOWN'
.pthreads
True
if Python is compiled with Emscripten pthreads support.shared_memory
True
if Python is compiled with shared memory support.Availability: Emscripten.
New in version 3.11.
- sys.pycache_prefix¶
If this is set (not
None
), Python will write bytecode-cache.pyc
files to (and read them from) a parallel directory tree rooted at this directory, rather than from__pycache__
directories in the source code tree. Any__pycache__
directories in the source code tree will be ignored and new.pyc
files written within the pycache prefix. Thus if you usecompileall
as a pre-build step, you must ensure you run it with the same 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
-X
pycache_prefix=PATH
command-line option or thePYTHONPYCACHEPREFIX
environment variable (command-line takes precedence). 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 to
sys.stderr
.When an exception is raised and uncaught, the interpreter calls
sys.excepthook
with three arguments, the exception class, exception instance, and a traceback object. In an interactive session this happens just before control is returned to the prompt; in a Python program this happens just before the program exits. The handling of such top-level exceptions can be customized by assigning another three-argument function tosys.excepthook
.Raise an auditing event
sys.excepthook
with argumentshook
,type
,value
,traceback
when an uncaught exception occurs. If no hook has been set,hook
may beNone
. If any hook raises an exception derived fromRuntimeError
the call to the hook will be suppressed. Otherwise, the audit hook exception will be reported as unraisable andsys.excepthook
will be called.See also
The
sys.unraisablehook()
function handles unraisable exceptions and thethreading.excepthook()
function handles exception raised bythreading.Thread.run()
.
- sys.__breakpointhook__¶
- sys.__displayhook__¶
- sys.__excepthook__¶
- sys.__unraisablehook__¶
These objects contain the original values of
breakpointhook
,displayhook
,excepthook
, andunraisablehook
at the start of the program. They are saved so thatbreakpointhook
,displayhook
andexcepthook
,unraisablehook
can be restored in case they happen to get replaced with broken or alternative objects.New in version 3.7: __breakpointhook__
New in version 3.8: __unraisablehook__
- sys.exception()¶
This function, when called while an exception handler is executing (such as an
except
orexcept*
clause), returns the exception instance that was caught by this handler. When exception handlers are nested within one another, only the exception handled by the innermost handler is accessible.If no exception handler is executing, this function returns
None
.New in version 3.11.
- sys.exc_info()¶
This function returns the old-style representation of the handled exception. If an exception
e
is currently handled (soexception()
would returne
),exc_info()
returns the tuple(type(e), e, e.__traceback__)
. That is, a tuple containing the type of the exception (a subclass ofBaseException
), the exception itself, and a traceback object which typically encapsulates the call stack at the point where the exception last occurred.If no exception is being handled anywhere on the stack, this function return a tuple containing three
None
values.Changed in version 3.11: The
type
andtraceback
fields are now derived from thevalue
(the exception instance), so when an exception is modified while it is being handled, the changes are reflected in the results of subsequent calls toexc_info()
.
- sys.exec_prefix¶
A string giving the site-specific directory prefix where the platform-dependent Python files are installed; by default, this is also
'/usr/local'
. This can be set at build time with the--exec-prefix
argument to the configure script. Specifically, all configuration files (e.g. thepyconfig.h
header file) are installed in the directoryexec_prefix/lib/pythonX.Y/config
, and shared library modules are installed inexec_prefix/lib/pythonX.Y/lib-dynload
, where X.Y is the version number of Python, for example3.2
.Note
If a virtual environment is in effect, this value will be changed in
site.py
to point to the virtual environment. The value for the Python installation will still be available, viabase_exec_prefix
.
- sys.executable¶
A string giving the absolute path of the executable binary for the Python interpreter, on systems where this makes sense. If Python is unable to retrieve the real path to its executable,
sys.executable
will be an empty string orNone
.
- sys.exit([arg])¶
Raise a
SystemExit
exception, signaling an intention to exit the interpreter.The optional argument arg can be an integer giving the exit status (defaulting to zero), or another type of object. If it is an integer, zero is considered “successful termination” and any nonzero value is considered “abnormal termination” by shells and the like. Most systems require it to be in the range 0–127, and produce undefined results otherwise. Some systems have a convention for assigning specific meanings to specific exit codes, but these are generally underdeveloped; Unix programs generally use 2 for command line syntax errors and 1 for all other kind of errors. If another type of object is passed,
None
is equivalent to passing zero, and any other object is printed tostderr
and results in an exit code of 1. In particular,sys.exit("some error message")
is a quick way to exit a program when an error occurs.Since
exit()
ultimately “only” raises an exception, it will only exit the process when called from the main thread, and the exception is not intercepted. Cleanup actions specified by finally clauses oftry
statements are 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 interpreter has caught
SystemExit
(such as an error flushing buffered data in the standard streams), the exit status is changed to 120.
- sys.flags¶
The named tuple flags exposes the status of command line flags. The attributes are read only.
attribute
flag
debug
interactive
isolated
optimize
no_user_site
no_site
ignore_environment
verbose
bytes_warning
quiet
hash_randomization
dev_mode
utf8_mode
safe_path
int_max_str_digits
-X int_max_str_digits
(integer string conversion length limitation)Changed in version 3.2: Added
quiet
attribute for the new-q
flag.New in version 3.2.3: The
hash_randomization
attribute.Changed in version 3.3: Removed obsolete
division_warning
attribute.Changed in version 3.4: Added
isolated
attribute for-I
isolated
flag.Changed in version 3.7: Added the
dev_mode
attribute for the new Python Development Mode and theutf8_mode
attribute for the new-X
utf8
flag.Changed in version 3.11: Added the
safe_path
attribute for-P
option.Changed in version 3.11: Added the
int_max_str_digits
attribute.
- sys.float_info¶
A named tuple holding information about the float type. It contains low level information about the precision and internal representation. The values correspond to the various floating-point constants defined in the standard header file
float.h
for the ‘C’ programming language; see section 5.2.4.2.2 of the 1999 ISO/IEC C standard [C99], ‘Characteristics of floating types’, for details.attribute
float.h macro
explanation
epsilon
DBL_EPSILON
difference between 1.0 and the least value greater than 1.0 that is representable as a float
See also
math.ulp()
.dig
DBL_DIG
maximum number of decimal digits that can be faithfully represented in a float; see below
mant_dig
DBL_MANT_DIG
float precision: the number of base-
radix
digits in the significand of a floatDBL_MAX
maximum representable positive finite float
max_exp
DBL_MAX_EXP
maximum integer e such that
radix**(e-1)
is a representable finite floatmax_10_exp
DBL_MAX_10_EXP
maximum integer e such that
10**e
is in the range of representable finite floatsDBL_MIN
minimum representable positive normalized float
Use
math.ulp(0.0)
to get the smallest positive denormalized representable float.min_exp
DBL_MIN_EXP
minimum integer e such that
radix**(e-1)
is a normalized floatmin_10_exp
DBL_MIN_10_EXP
minimum integer e such that
10**e
is a normalized floatradix
FLT_RADIX
radix of exponent representation
rounds
FLT_ROUNDS
integer constant representing the rounding mode used for arithmetic operations. This reflects the value of the system FLT_ROUNDS macro at interpreter startup time. See section 5.2.4.2.2 of the C99 standard for an explanation of the possible values and their meanings.
The attribute
sys.float_info.dig
needs further explanation. Ifs
is any string representing a decimal number with at mostsys.float_info.dig
significant digits, then convertings
to a float and back again will recover a string representing the same decimal value:>>> import sys >>> sys.float_info.dig 15 >>> s = '3.14159265358979' # decimal string with 15 significant digits >>> format(float(s), '.15g') # convert to float and back -> same value '3.14159265358979'
But for strings with more than
sys.float_info.dig
significant digits, this isn’t always true:>>> s = '9876543211234567' # 16 significant digits is too many! >>> format(float(s), '.16g') # conversion changes value '9876543211234568'
- sys.float_repr_style¶
A string indicating how the
repr()
function behaves for floats. If the string has value'short'
then for a finite floatx
,repr(x)
aims to produce a short string with the property thatfloat(repr(x)) == x
. This is the usual behaviour in Python 3.1 and later. Otherwise,float_repr_style
has value'legacy'
andrepr(x)
behaves in the same way as it did in versions 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 tracking and debugging memory leaks. Because of the interpreter’s internal caches, the result can vary from call to call; you may have to call
_clear_type_cache()
andgc.collect()
to get more predictable results.If a Python build or implementation cannot reasonably compute this information,
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.getdefaultencoding()¶
Return the name of the current default string encoding used by the Unicode implementation.
- sys.getdlopenflags()¶
Return the current value of the flags that are used for
dlopen()
calls. Symbolic names for the flag values can be found in theos
module (RTLD_xxx
constants, e.g.os.RTLD_LAZY
).Availability: Unix.
- sys.getfilesystemencoding()¶
Get the filesystem encoding: the encoding used with the filesystem error handler to convert between Unicode filenames and bytes filenames. The filesystem error handler is returned from
getfilesystemencoding()
.For best compatibility, str should be used for filenames in all cases, although representing filenames as bytes is also supported. Functions accepting or returning filenames should support either str or bytes and internally convert to the system’s preferred representation.
os.fsencode()
andos.fsdecode()
should be used to ensure that the correct encoding and errors mode are used.The filesystem encoding and error handler are configured at Python startup by the
PyConfig_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'
. See PEP 529 and_enablelegacywindowsfsencoding()
for more information.Changed in version 3.7: Return
'utf-8'
if the Python UTF-8 Mode is enabled.
- sys.getfilesystemencodeerrors()¶
Get the filesystem error handler: the error handler used with the filesystem encoding to convert between Unicode filenames and bytes filenames. The filesystem encoding is returned from
getfilesystemencoding()
.os.fsencode()
andos.fsdecode()
should be used to ensure that the correct encoding and errors mode are used.The filesystem encoding and error handler are configured at Python startup by the
PyConfig_Read()
function: seefilesystem_encoding
andfilesystem_errors
members ofPyConfig
.New in version 3.6.
- sys.get_int_max_str_digits()¶
Returns the current value for the integer string conversion length limitation. See also
set_int_max_str_digits()
.New in version 3.11.
- sys.getrefcount(object)¶
Return the reference count of the object. The count returned is generally one higher than you might expect, because it includes the (temporary) reference as an argument to
getrefcount()
.
- sys.getrecursionlimit()¶
Return the current value of the recursion limit, the maximum depth of the Python interpreter stack. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python. It can be set by
setrecursionlimit()
.
- sys.getsizeof(object[, default])¶
Return the size of an object in bytes. The object can be any type of object. All built-in objects will return correct results, but this does not have to hold true for third-party extensions as it is implementation specific.
Only the memory consumption directly attributed to the object is accounted for, not the memory consumption of objects it refers to.
If given, default will be returned if the object does not provide means to retrieve the size. Otherwise a
TypeError
will be raised.getsizeof()
calls the object’s__sizeof__
method and adds an additional garbage collector overhead if the object is managed by the garbage collector.See recursive sizeof recipe for an example of using
getsizeof()
recursively to find the size of containers and all their contents.
- sys.getswitchinterval()¶
Return the interpreter’s “thread switch interval”; see
setswitchinterval()
.New in version 3.2.
- sys._getframe([depth])¶
Return a frame object from the call stack. If optional integer depth is given, return the frame object that many calls below the top of the stack. If that is deeper than the call stack,
ValueError
is raised. The default for depth is zero, returning the frame at the top of the call stack.Raises an auditing event
sys._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.getprofile()¶
Get the profiler function as set by
setprofile()
.
- sys.gettrace()¶
Get the trace function as set by
settrace()
.CPython implementation detail: The
gettrace()
function is intended only for implementing debuggers, profilers, coverage tools and the like. Its behavior is part of the implementation platform, rather than part of the language definition, and thus may not be available in all Python implementations.
- sys.getwindowsversion()¶
Return a named tuple describing the Windows version currently running. The named elements are major, minor, build, platform, service_pack, service_pack_minor, service_pack_major, suite_mask, product_type and platform_version. service_pack contains a string, platform_version a 3-tuple and all other values are integers. The components can also be accessed by name, so
sys.getwindowsversion()[0]
is equivalent tosys.getwindowsversion().major
. For compatibility with prior versions, only the first 5 elements are retrievable by indexing.platform will be
2 (VER_PLATFORM_WIN32_NT)
.product_type may be one of the following values:
Constant
Meaning
1 (VER_NT_WORKSTATION)
The system is a workstation.
2 (VER_NT_DOMAIN_CONTROLLER)
The system is a domain controller.
3 (VER_NT_SERVER)
The system is a server, but not a domain controller.
This function wraps the Win32
GetVersionEx()
function; see the Microsoft documentation onOSVERSIONINFOEX()
for more information about these fields.platform_version returns the major version, minor version and build number of the current operating system, rather than the version that is being emulated for the process. It is intended for use in logging rather than for feature detection.
Note
platform_version derives the version from kernel32.dll which can be of a different version than the OS version. Please use
platform
module for achieving accurate OS version.Availability: Windows.
Changed in version 3.2: Changed to a named tuple and added service_pack_minor, service_pack_major, suite_mask, and product_type.
Changed in version 3.6: Added platform_version
- sys.get_asyncgen_hooks()¶
Returns an asyncgen_hooks object, which is similar to a
namedtuple
of the form(firstiter, finalizer)
, where firstiter and finalizer are expected to be eitherNone
or functions which take an asynchronous generator iterator as an argument, and are used to schedule finalization of an asynchronous generator by an event loop.New in version 3.6: See PEP 525 for more details.
Note
This function has been added on a provisional basis (see PEP 411 for details.)
- sys.get_coroutine_origin_tracking_depth()¶
Get the current coroutine origin tracking depth, as set by
set_coroutine_origin_tracking_depth()
.New in version 3.7.
Note
This function has been added on a provisional basis (see PEP 411 for details.) Use it only for debugging purposes.
- sys.hash_info¶
A named tuple giving parameters of the numeric hash implementation. For more details about hashing of numeric types, see Hashing 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
(this attribute is no longer used)
imag
multiplier used for the imaginary part of a complex number
algorithm
name of the algorithm for hashing of str, bytes, and memoryview
hash_bits
internal output size of the hash algorithm
seed_bits
size of the seed key of the hash algorithm
New in version 3.2.
Changed in version 3.4: Added algorithm, hash_bits and seed_bits
- sys.hexversion¶
The version number encoded as a single integer. This is guaranteed to increase with each version, including proper support for non-production releases. For example, to test that the Python interpreter is at least version 1.5.2, use:
if sys.hexversion >= 0x010502F0: # use some advanced feature ... else: # use an alternative implementation or warn the user ...
This is called
hexversion
since it only really looks meaningful when viewed as the result of passing it to the built-inhex()
function. The named tuplesys.version_info
may be used for a more human-friendly encoding of the same information.More details of
hexversion
can be found at API and ABI Versioning.
- sys.implementation¶
An object containing information about the implementation of the currently running Python interpreter. The following attributes are required to exist in all Python implementations.
name is the implementation’s identifier, e.g.
'cpython'
. The actual string is defined by the Python implementation, but it is guaranteed to be lower case.version is a named tuple, in the same format as
sys.version_info
. It represents the version of the Python implementation. This has a distinct meaning from the specific version of the Python language to which the currently running interpreter conforms, whichsys.version_info
represents. For example, for PyPy 1.8sys.implementation.version
might besys.version_info(1, 8, 0, 'final', 0)
, whereassys.version_info
would besys.version_info(2, 7, 2, 'final', 0)
. For CPython they are the same value, since it is the reference implementation.hexversion is the implementation version in hexadecimal format, like
sys.hexversion
.cache_tag is the tag used by the import machinery in the filenames of cached modules. By convention, it would be a composite of the implementation’s name and version, like
'cpython-33'
. However, a Python implementation may use some other value if appropriate. Ifcache_tag
is set toNone
, it indicates that module caching should be disabled.sys.implementation
may contain additional attributes specific to the Python implementation. These non-standard attributes must start with an underscore, and are not described here. Regardless of its contents,sys.implementation
will not change during a run of the interpreter, nor between implementation versions. (It may change between Python language versions, however.) See PEP 421 for more information.New in version 3.3.
Note
The addition of new required attributes must go through the normal PEP process. See PEP 421 for more information.
- sys.int_info¶
A named tuple that holds information about Python’s internal representation of integers. The attributes are read only.
Attribute
Explanation
bits_per_digit
number of bits held in each digit. Python integers are stored internally in base
2**int_info.bits_per_digit
sizeof_digit
size in bytes of the C type used to represent a digit
default_max_str_digits
default value for
sys.get_int_max_str_digits()
when it is not otherwise explicitly configured.str_digits_check_threshold
minimum non-zero value for
sys.set_int_max_str_digits()
,PYTHONINTMAXSTRDIGITS
, or-X int_max_str_digits
.New in version 3.1.
Changed in version 3.11: Added
default_max_str_digits
andstr_digits_check_threshold
.
- sys.__interactivehook__¶
When this attribute exists, its value is automatically called (with no arguments) when the interpreter is launched in interactive mode. This is done after the
PYTHONSTARTUP
file is read, so that you can set this hook there. Thesite
module sets this.Raises an auditing event
cpython.run_interactivehook
with the hook object as the argument when the hook is called on startup.New in version 3.4.
- sys.intern(string)¶
Enter string in the table of “interned” strings and return the interned string – which is string itself or a copy. Interning strings is useful to gain a little performance on dictionary lookup – if the keys in a dictionary are interned, and the lookup key is interned, the key comparisons (after hashing) can be done by a pointer compare instead of a string compare. Normally, the names used in Python programs are automatically interned, and the dictionaries used to hold module, class or instance attributes have interned keys.
Interned strings are not immortal; you must keep a reference to the return value of
intern()
around to benefit from it.
- sys.is_finalizing()¶
Return
True
if the Python interpreter is shutting down,False
otherwise.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 is not handled and the interpreter prints an error message and a stack traceback. Their intended use is to allow an interactive user to import a debugger module and engage in post-mortem debugging without having to re-execute the command that caused the error. (Typical use is
import pdb; pdb.pm()
to enter the post-mortem debugger; seepdb
module for more information.)The meaning of the variables is the same as that of the return values from
exc_info()
above.
- sys.maxsize¶
An integer giving the maximum value a variable of type
Py_ssize_t
can take. It’s usually2**31 - 1
on a 32-bit platform and2**63 - 1
on a 64-bit platform.
- sys.maxunicode¶
An integer giving the value of the largest Unicode code point, i.e.
1114111
(0x10FFFF
in hexadecimal).Changed in version 3.3: Before PEP 393,
sys.maxunicode
used to be either0xFFFF
or0x10FFFF
, depending on the configuration option that specified whether Unicode characters were stored as UCS-2 or UCS-4.
- sys.meta_path¶
A list of meta path finder objects that have their
find_spec()
methods called to see if one of the objects can find the module to be imported. By default, it holds entries that implement Python’s default import semantics. Thefind_spec()
method is called with at least the absolute name of the module being imported. If the module to be imported is contained in a package, then the parent package’s__path__
attribute is passed in as a second argument. The method returns a module spec, orNone
if the module cannot be found.See also
importlib.abc.MetaPathFinder
The abstract base class defining the interface of finder objects on
meta_path
.importlib.machinery.ModuleSpec
The concrete class which
find_spec()
should return instances of.
Changed in version 3.4: Module specs were introduced in Python 3.4, by PEP 451. Earlier versions of Python looked for a method called
find_module()
. This is still called as a fallback if ameta_path
entry doesn’t have afind_spec()
method.
- sys.modules¶
This is a dictionary that maps module names to modules which have already been loaded. This can be manipulated to force reloading of modules and other tricks. However, replacing the dictionary will not necessarily work as expected and deleting essential items from the dictionary may cause Python to fail. If you want to iterate over this global dictionary always use
sys.modules.copy()
ortuple(sys.modules)
to avoid exceptions as its size may change during iteration as a side effect of code or activity in other threads.
- sys.orig_argv¶
The list of the original command line arguments passed to the Python executable.
See also
sys.argv
.New in version 3.10.
- sys.path¶
A list of strings that specifies the search path for modules. Initialized from the environment variable
PYTHONPATH
, plus an installation-dependent default.By default, as initialized upon program startup, a potentially unsafe path is prepended to
sys.path
(before the entries inserted as a result ofPYTHONPATH
):python -m module
command line: prepend the current working directory.python script.py
command line: prepend the script’s directory. If it’s a symbolic link, resolve symbolic links.python -c code
andpython
(REPL) command lines: prepend an empty string, which means the current working directory.
To not prepend this potentially unsafe path, use the
-P
command line option or thePYTHONSAFEPATH
environment variable.A program is free to modify this list for its own purposes. Only strings should be added to
sys.path
; all other data types are ignored during import.
- sys.path_hooks¶
A list of callables that take a path argument to try to create a finder for the path. If a finder can be created, it is to be returned by the callable, else raise
ImportError
.Originally specified in PEP 302.
- sys.path_importer_cache¶
A dictionary acting as a cache for finder objects. The keys are paths that have been passed to
sys.path_hooks
and the values are the finders that are found. If a path is a valid file system path but no finder is found onsys.path_hooks
thenNone
is stored.Originally specified in PEP 302.
Changed in version 3.3:
None
is stored instead ofimp.NullImporter
when no finder is found.
- sys.platform¶
This string contains a platform identifier that can be used to append platform-specific components to
sys.path
, for instance.For Unix systems, except on Linux and AIX, this is the lowercased OS name as returned by
uname -s
with the first part of the version as returned byuname -r
appended, e.g.'sunos5'
or'freebsd8'
, at the time when Python was built. Unless you want to test for a specific system version, it is therefore recommended to use the following idiom:if sys.platform.startswith('freebsd'): # FreeBSD-specific code here... elif sys.platform.startswith('linux'): # Linux-specific code here... elif sys.platform.startswith('aix'): # AIX-specific code here...
For other systems, the values are:
System
platform
valueAIX
'aix'
Emscripten
'emscripten'
Linux
'linux'
WASI
'wasi'
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'
. Since older Python versions include the version number, it is recommended to always 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'
. Since older Python versions include the version number, it is recommended to always use thestartswith
idiom presented above.See also
os.name
has a coarser granularity.os.uname()
gives system-dependent version information.The
platform
module provides detailed checks for the system’s identity.
- sys.platlibdir¶
Name of the platform-specific library directory. It is used to build the path of standard library and the paths of installed extension modules.
It is equal to
"lib"
on most platforms. On Fedora and SuSE, it is equal to"lib64"
on 64-bit platforms which gives the followingsys.path
paths (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
New in version 3.9.
- sys.prefix¶
A string giving the site-specific directory prefix where the platform independent Python files are installed; on Unix, the default is
'/usr/local'
. This can be set at build time with the--prefix
argument to the configure script. See Installation paths for derived paths.Note
If a virtual environment is in effect, this value will be changed in
site.py
to point to the virtual environment. The value for the Python installation will still be available, viabase_prefix
.
- sys.ps1¶
- sys.ps2¶
Strings specifying the primary and secondary prompt of the interpreter. These are only defined if the interpreter is in interactive mode. Their initial values in this case are
'>>> '
and'... '
. If a non-string object is assigned to either variable, itsstr()
is re-evaluated each time the interpreter prepares to read a new interactive command; this can be used to implement a dynamic prompt.
- sys.setdlopenflags(n)¶
Set the flags used by the interpreter for
dlopen()
calls, such as when the interpreter loads extension modules. Among other things, this will enable a lazy resolving of symbols when importing a module, if called assys.setdlopenflags(0)
. To share symbols across extension modules, call assys.setdlopenflags(os.RTLD_GLOBAL)
. Symbolic names for the flag values can be found in theos
module (RTLD_xxx
constants, e.g.os.RTLD_LAZY
).Availability: Unix.
- sys.set_int_max_str_digits(maxdigits)¶
Set the integer string conversion length limitation used by this interpreter. See also
get_int_max_str_digits()
.New in version 3.11.
- sys.setprofile(profilefunc)¶
Set the system’s profile function, which allows you to implement a Python source code profiler in Python. See chapter The Python Profilers for more information on the Python profiler. The system’s profile function is called similarly to the system’s trace function (see
settrace()
), but it is called with different events, for example it isn’t called for each executed line of code (only on call and return, but the return event is reported even when an exception has been set). The function is thread-specific, but there is no way for the profiler to know about context switches between threads, so it does not make sense to use this in the presence of multiple threads. Also, its return value is not used, so it can simply returnNone
. Error in the profile function will cause itself unset.Profile functions should have three arguments: frame, event, and arg. frame is the current stack frame. event is a string:
'call'
,'return'
,'c_call'
,'c_return'
, or'c_exception'
. arg depends on the event type.Raises an auditing event
sys.setprofile
with no arguments.The events have the following meaning:
'call'
A function is called (or some other code block entered). The profile function is called; arg is
None
.'return'
A function (or other code block) is about to return. The profile function is called; arg is the value that will be returned, or
None
if the event is caused by an exception being raised.'c_call'
A C function is about to be called. This may be an extension function or a built-in. arg is the C function object.
'c_return'
A C function has returned. arg is the C function object.
'c_exception'
A C function has raised an exception. arg is the C function object.
- sys.setrecursionlimit(limit)¶
Set the maximum depth of the Python interpreter stack to limit. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python.
The highest possible limit is platform-dependent. A user may need to set the limit higher when they have a program that requires deep recursion and a platform that supports a higher limit. This should be done with care, because a too-high limit can lead to a crash.
If the new limit is too low at the current recursion depth, a
RecursionError
exception is raised.Changed in version 3.5.1: A
RecursionError
exception is now raised if the new limit is too low at the current recursion depth.
- sys.setswitchinterval(interval)¶
Set the interpreter’s thread switch interval (in seconds). This floating-point value determines the ideal duration of the “timeslices” allocated to concurrently running Python threads. Please note that the actual value can be higher, especially if long-running internal functions or methods are used. Also, which thread becomes scheduled at the end of the interval is the operating system’s decision. The interpreter doesn’t have its own scheduler.
New in version 3.2.
- sys.settrace(tracefunc)¶
Set the system’s trace function, which allows you to implement a Python source code debugger in Python. The function is thread-specific; for a debugger to support multiple threads, it must register a trace function using
settrace()
for each thread being debugged or usethreading.settrace()
.Trace functions should have three arguments: frame, event, and arg. frame is the current stack frame. event is a string:
'call'
,'line'
,'return'
,'exception'
or'opcode'
. arg depends on the event type.The trace function is invoked (with event set to
'call'
) whenever a new local scope is entered; it should return a reference to a local trace function to be used for the new scope, orNone
if the scope shouldn’t be traced.The local trace function should return a reference to itself (or to another function for further tracing in that scope), or
None
to turn off tracing in that scope.If there is any error occurred in the trace function, it will be unset, just like
settrace(None)
is called.The events have the following meaning:
'call'
A function is called (or some other code block entered). The global trace function is called; arg is
None
; the return value specifies the local trace function.'line'
The interpreter is about to execute a new line of code or re-execute the condition of a loop. The local trace function is called; arg is
None
; the return value specifies the new local trace function. SeeObjects/lnotab_notes.txt
for a detailed explanation of how this works. 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 trace function is called; arg is the value that will be returned, or
None
if the event is caused by an exception being raised. The trace function’s return value is ignored.'exception'
An exception has occurred. The local trace function is called; arg is a tuple
(exception, value, traceback)
; the return value specifies the new local trace function.'opcode'
The interpreter is about to execute a new opcode (see
dis
for opcode details). The local trace function is called; arg isNone
; the return value specifies the new local trace function. Per-opcode events are not emitted by default: they must be explicitly requested by settingf_trace_opcodes
toTrue
on the frame.
Note that as an exception is propagated down the chain of callers, an
'exception'
event is generated at each level.For more fine-grained usage, it’s possible to set a trace function by assigning
frame.f_trace = tracefunc
explicitly, rather than relying on it being set indirectly via the return value from an already installed trace function. This is also required for activating the trace function on the current frame, whichsettrace()
doesn’t do. Note that in order for this to work, a global tracing function must have been installed withsettrace()
in order to enable the runtime tracing machinery, but it doesn’t need to be the same tracing function (e.g. it could be a low overhead tracing function that simply returnsNone
to disable itself immediately on each frame).For more information on code and frame objects, refer to The standard type hierarchy.
Raises an auditing event
sys.settrace
with no arguments.CPython implementation detail: The
settrace()
function is intended only for implementing debuggers, profilers, coverage tools and the like. Its behavior is part of the implementation platform, rather than part of the language definition, and thus may not be available in all Python implementations.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 an asynchronous generator iterator as an argument. The firstiter callable will be called when an asynchronous generator is iterated for the first time. The finalizer will be called when an asynchronous generator is about to be garbage collected.
Raises an auditing event
sys.set_asyncgen_hooks_firstiter
with no arguments.Raises an auditing event
sys.set_asyncgen_hooks_finalizer
with no arguments.Two auditing events are raised because the underlying API consists of two calls, each of which must raise its own event.
New in version 3.6: See PEP 525 for more details, and for a reference example of a finalizer method see the implementation of
asyncio.Loop.shutdown_asyncgens
in Lib/asyncio/base_events.pyNote
This function has been added on a provisional basis (see PEP 411 for details.)
- sys.set_coroutine_origin_tracking_depth(depth)¶
Allows enabling or disabling coroutine origin tracking. When enabled, the
cr_origin
attribute on coroutine objects will contain a tuple of (filename, line number, function name) tuples describing the traceback where the coroutine object was created, with the most recent call first. When disabled,cr_origin
will be None.To enable, pass a depth value greater than zero; this sets the number of frames whose information will be captured. To disable, pass set depth to zero.
This setting is thread-specific.
New in version 3.7.
Note
This function has been added on a provisional basis (see PEP 411 for details.) Use it only for debugging purposes.
- sys._enablelegacywindowsfsencoding()¶
Changes the filesystem encoding and error handler to ‘mbcs’ and ‘replace’ respectively, for consistency with versions of Python prior to 3.6.
This is equivalent to defining the
PYTHONLEGACYWINDOWSFSENCODING
environment variable before launching Python.See also
sys.getfilesystemencoding()
andsys.getfilesystemencodeerrors()
.Availability: Windows.
New in version 3.6: See PEP 529 for more details.
- sys.stdin¶
- sys.stdout¶
- sys.stderr¶
File objects used by the interpreter for standard input, output and errors:
stdin
is used for all interactive input (including calls toinput()
);stdout
is used for the output ofprint()
and expression statements and for the prompts ofinput()
;The interpreter’s own prompts and its error messages go to
stderr
.
These streams are regular text files like those returned by the
open()
function. Their parameters are chosen as follows:The encoding and error handling are is initialized from
PyConfig.stdio_encoding
andPyConfig.stdio_errors
.On Windows, UTF-8 is used for the console device. Non-character devices such as disk files and pipes use the system locale encoding (i.e. the ANSI codepage). Non-console character devices such as NUL (i.e. where
isatty()
returnsTrue
) use the value of the console input and output codepages at startup, respectively for stdin and stdout/stderr. This defaults to the system locale encoding if the process is not initially attached to a console.The special behaviour of the console can be overridden by setting the environment variable PYTHONLEGACYWINDOWSSTDIO before starting Python. In that case, the console codepages are used as for any other character device.
Under all platforms, you can override the character encoding by setting the
PYTHONIOENCODING
environment variable before starting Python or by using the new-X
utf8
command line option andPYTHONUTF8
environment variable. However, for the Windows console, this only applies whenPYTHONLEGACYWINDOWSSTDIO
is also set.When interactive, the
stdout
stream is line-buffered. Otherwise, it is block-buffered like regular text files. Thestderr
stream is line-buffered in both cases. You can make both streams unbuffered by passing the-u
command-line option or setting thePYTHONUNBUFFERED
environment variable.
Changed in version 3.9: Non-interactive
stderr
is now line-buffered instead of fully buffered.Note
To write or read binary data from/to the standard streams, use the underlying binary
buffer
object. For example, to write bytes tostdout
, usesys.stdout.buffer.write(b'abc')
.However, if you are writing a library (and do not control in which context its code will be executed), be aware that the standard streams may be replaced with file-like objects like
io.StringIO
which do not support thebuffer
attribute.
- sys.__stdin__¶
- sys.__stdout__¶
- sys.__stderr__¶
These objects contain the original values of
stdin
,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 objects in case they have been overwritten with a broken object. However, the preferred way to do this is to explicitly save the previous stream before replacing it, and restore the saved object.
Note
Under some conditions
stdin
,stdout
andstderr
as well as the original values__stdin__
,__stdout__
and__stderr__
can beNone
. It is usually the case for Windows GUI apps that aren’t connected to a console and Python apps started with pythonw.
- 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 on some platforms and modules disabled at Python build are also listed. All module kinds are listed: pure Python, built-in, frozen and extension modules. Test modules are excluded.
For packages, only the main package is listed: sub-packages and sub-modules are not listed. For example, the
email
package is listed, but theemail.mime
sub-package and theemail.message
sub-module are not listed.See also the
sys.builtin_module_names
list.New in version 3.10.
- sys.thread_info¶
A named tuple holding information about the thread implementation.
Attribute
Explanation
name
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
lock
Name of the lock implementation:
'semaphore'
: a lock uses a semaphore'mutex+cond'
: a lock uses a mutex and a condition variableNone
if this information is unknown
Name and version of the thread library. It is a string, or
None
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 number of levels of traceback information printed when an unhandled exception occurs. The default is
1000
. When set to0
or less, all traceback information is 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 to handle it. For example, when a destructor raises an exception or during garbage collection (
gc.collect()
).The unraisable argument has the following attributes:
exc_type: Exception type.
exc_value: Exception value, can be
None
.exc_traceback: Exception traceback, can be
None
.err_msg: Error message, can be
None
.object: Object causing the exception, can be
None
.
The default hook formats err_msg and object as:
f'{err_msg}: {object!r}'
; use “Exception ignored in” error message if err_msg isNone
.sys.unraisablehook()
can be overridden to control how unraisable exceptions are handled.Storing exc_value using a custom hook can create a reference cycle. It should be cleared explicitly to break the reference cycle when the exception is no longer needed.
Storing object using a custom hook can resurrect it if it is set to an object which is being finalized. Avoid storing object after the custom hook completes to avoid resurrecting objects.
See also
excepthook()
which handles uncaught exceptions.Raise an auditing event
sys.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 additional information on the build number and compiler used. This string is displayed when the interactive interpreter is started. Do not extract version information out of it, rather, use
version_info
and the functions provided by theplatform
module.
- sys.api_version¶
The C API version for this interpreter. Programmers may find this useful when debugging version conflicts between Python and extension modules.
- sys.version_info¶
A tuple containing the five components of the version number: major, minor, micro, releaselevel, and serial. All values except releaselevel are integers; the release level is
'alpha'
,'beta'
,'candidate'
, or'final'
. Theversion_info
value corresponding to the Python version 2.0 is(2, 0, 0, 'final', 0)
. The components can also be accessed by name, sosys.version_info[0]
is equivalent tosys.version_info.major
and 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 this value. Refer to the
warnings
module for more information on the warnings framework.
- sys.winver¶
The version number used to form registry keys on Windows platforms. This is stored as string resource 1000 in the Python DLL. The value is normally the major and minor versions of the running Python interpreter. It is provided in the
sys
module for informational purposes; modifying this value has no effect on the registry keys used by Python.Availability: Windows.
- sys._xoptions¶
A dictionary of the various implementation-specific flags passed through the
-X
command-line option. Option names are either mapped to their values, if given explicitly, or toTrue
. Example:$ ./python -Xa=b -Xc Python 3.2a3+ (py3k, Oct 16 2010, 20:14:50) [GCC 4.4.3] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> import sys >>> sys._xoptions {'a': 'b', 'c': True}
CPython implementation detail: This is a CPython-specific way of accessing options passed through
-X
. Other implementations may export them through other means, or not at all.New in version 3.2.
Citations
ISO/IEC 9899:1999. “Programming languages – C.” A public draft of this standard is available at https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf.