———
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
.
New in version 3.2.
Changed in version 3.8: Default flags became an empty string (m flag for pymalloc has been removed).
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
function, each hook will be called in the order it was added with the event name and the tuple of arguments. Native hooks added by
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
before initialising the runtime, and any modules allowing arbitrary memory modification (such as
) should be completely removed or closely monitored.
Calling
will itself raise an auditing event named sys.addaudithook with no arguments. If any existing hooks raise an exception derived from
, 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
for all events raised by CPython, and
for the original design discussion.
New in version 3.8.
Changed in version 3.8.1: Exceptions derived from
but not
are no longer suppressed.
CPython implementation detail: When tracing is enabled (see
), 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
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
module.
See also
.
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)forarginsys.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.
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
or
functions.
The native equivalent of this function is
. Using the native function is preferred when possible.
See the
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 as
. If not running in a
, the values will stay the same; if site.py finds that a virtual environment is in use, the values of
and
will be changed to point to the virtual environment, whereas
and
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 as
. If not running in a
, the values will stay the same; if site.py finds that a virtual environment is in use, the values of
and
will be changed to point to the virtual environment, whereas
and
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
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 or profile some other code.
Tracing is suspended while calling a tracing function set by
or
to avoid infinite recursion. call_tracing() enables explicit recursion of the tracing function.
sys.copyright
A string containing the copyright pertaining to the Python interpreter.
sys._clear_type_cache()
Clear the internal type cache. The type cache is used to speed up 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
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
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
sys._current_exceptions with no arguments.
sys.breakpointhook()
This hook function is called by built-in
. By default, it drops you into the
debugger, but it can be set to any other function so that you can choose which debugger gets used.
The signature of this function is dependent on what it calls. For example, the default binding (e.g. pdb.set_trace()) expects no arguments, but you might bind it to a function that expects additional arguments (positional and/or keyword). The built-in breakpoint() function passes its *args and **kws straight through. Whatever breakpointhooks() returns is returned from breakpoint().
The default implementation first consults the environment variable
. If that is set to "0" then this function returns immediately; i.e. it is a no-op. If the environment variable is not set, or is set to the empty string, pdb.set_trace() is called. Otherwise this variable should name a function to run, using Python’s dotted-import nomenclature, e.g. package.subpackage.module.function. In this case, package.subpackage.module would be imported and the resulting module must have a callable named function(). This is run, passing in *args and **kws, and whatever function() returns, sys.breakpointhook() returns to the built-in
function.
Note that if anything goes wrong while importing the callable named by
, a
is reported and the breakpoint is ignored.
Also note that if sys.breakpointhook() is overridden programmatically,
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
(
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.
sys.displayhook(value)
If value is not None, this function prints repr(value) to sys.stdout, and saves value in builtins._. If repr(value) is not encodable to sys.stdout.encoding with sys.stdout.errors error handler (which is probably 'strict'), encode it to sys.stdout.encoding with 'backslashreplace' error handler.
sys.displayhook is called on the result of evaluating an
entered in an interactive Python session. The display of these values can be customized by assigning another one-argument function to sys.displayhook.
Pseudo-code:
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._=valueChanged in version 3.2: Use 'backslashreplace' error handler on
.
sys.dont_write_bytecode
If this is true, Python won’t try to write .pyc files on the import of source modules. This value is initially set to True or False depending on the
command line option and the
environment variable, but you can set it yourself to control bytecode file generation.
sys._emscripten_info
A
holding information about the environment on the wasm32-emscripten platform. The named tuple is provisional and may change in the future.
_emscripten_info.emscripten_version
Emscripten version as tuple of ints (major, minor, micro), e.g. (3,1,8).
_emscripten_info.runtime
Runtime string, e.g. browser user agent, 'Node.jsv14.18.2', or 'UNKNOWN'.
_emscripten_info.pthreads
True if Python is compiled with Emscripten pthreads support.
_emscripten_info.shared_memory
True if Python is compiled with shared memory support.
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 use
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
pycache_prefix=PATH command-line option or the
environment variable (command-line takes precedence). If neither are set, it is None.
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 other than
is raised and uncaught, the interpreter calls sys.excepthook with three arguments, the exception class, exception instance, and a traceback object. In an interactive session this happens just before control is returned to the prompt; in a Python program this happens just before the program exits. The handling of such top-level exceptions can be customized by assigning another three-argument function to sys.excepthook.
Raise an auditing event sys.excepthook with arguments hook, type, value, traceback when an uncaught exception occurs. If no hook has been set, hook may be None. If any hook raises an exception derived from
the call to the hook will be suppressed. Otherwise, the audit hook exception will be reported as unraisable and sys.excepthook will be called.
sys.__breakpointhook__
sys.__displayhook__
sys.__excepthook__
sys.__unraisablehook__
These objects contain the original values of breakpointhook, displayhook, excepthook, and unraisablehook at the start of the program. They are saved so that breakpointhook, displayhook and excepthook, 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 or except* 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 (so
would return e),
returns the tuple (type(e),e,e.__traceback__). That is, a tuple containing the type of the exception (a subclass of
), the exception itself, and a
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 and traceback fields are now derived from the value (the exception instance), so when an exception is modified while it is being handled, the changes are reflected in the results of subsequent calls to
.
sys.exec_prefix
A string giving the site-specific directory prefix where the platform-dependent Python files are installed; by default, this is also '/usr/local'. This can be set at build time with the --exec-prefix argument to the configure script. Specifically, all configuration files (e.g. the pyconfig.h header file) are installed in the directory exec_prefix/lib/pythonX.Y/config, and shared library modules are installed in exec_prefix/lib/pythonX.Y/lib-dynload, where X.Y is the version number of Python, for example 3.2.
Note
If a
is in effect, this value will be changed in site.py to point to the virtual environment. The value for the Python installation will still be available, via
.
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,
will be an empty string or None.
sys.exit([arg])
Raise a
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 to
and results in an exit code of 1. In particular, sys.exit("someerrormessage") is a quick way to exit a program when an error occurs.
Since
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 of
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
(such as an error flushing buffered data in the standard streams), the exit status is changed to 120.
sys.flags
The
flags exposes the status of command line flags. The attributes are read only.
flags.debug
flags.inspect
flags.interactive
flags.isolated
flags.optimize
or
flags.dont_write_bytecode
flags.no_user_site
flags.no_site
flags.ignore_environment
flags.verbose
flags.bytes_warning
flags.quiet
flags.hash_randomization
flags.dev_mode
(
)
flags.utf8_mode
flags.safe_path
flags.int_max_str_digits
(
integer string conversion length limitation
)
flags.warn_default_encoding
Changed in version 3.2: Added quiet attribute for the new
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
isolated flag.
Changed in version 3.7: Added the dev_mode attribute for the new
and the utf8_mode attribute for the new
utf8 flag.
Changed in version 3.10: Added warn_default_encoding attribute for
warn_default_encoding flag.
Changed in version 3.11: Added the safe_path attribute for
option.
Changed in version 3.11: Added the int_max_str_digits attribute.
sys.float_info
A
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
, ‘Characteristics of floating types’, for details.
Attributes of the float_info
attribute
float.h macro
explanation
float_info.epsilon
DBL_EPSILON
difference between 1.0 and the least value greater than 1.0 that is representable as a float.
See also
.
float_info.dig
DBL_DIG
The maximum number of decimal digits that can be faithfully represented in a float; see below.
float_info.mant_dig
DBL_MANT_DIG
Float precision: the number of base-radix digits in the significand of a float.
float_info.max
DBL_MAX
The maximum representable positive finite float.
float_info.max_exp
DBL_MAX_EXP
The maximum integer e such that radix**(e-1) is a representable finite float.
float_info.max_10_exp
DBL_MAX_10_EXP
The maximum integer e such that 10**e is in the range of representable finite floats.
float_info.min
DBL_MIN
The minimum representable positive normalized float.
Use
to get the smallest positive denormalized representable float.
float_info.min_exp
DBL_MIN_EXP
The minimum integer e such that radix**(e-1) is a normalized float.
float_info.min_10_exp
DBL_MIN_10_EXP
The minimum integer e such that 10**e is a normalized float.
float_info.radix
FLT_RADIX
The radix of exponent representation.
float_info.rounds
FLT_ROUNDS
An integer representing the rounding mode for floating-point arithmetic. This reflects the value of the system FLT_ROUNDS macro at interpreter startup time:
-1: indeterminable
0: toward zero
1: to nearest
2: toward positive infinity
3: toward negative infinity
All other values for FLT_ROUNDS characterize implementation-defined rounding behavior.
The attribute
needs further explanation. If s is any string representing a decimal number with at most sys.float_info.dig significant digits, then converting s to a float and back again will recover a string representing the same decimal value:
>>> 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 than
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
function behaves for floats. If the string has value 'short' then for a finite float x, repr(x) aims to produce a short string with the property that float(repr(x))==x. This is the usual behaviour in Python 3.1 and later. Otherwise, float_repr_style has value 'legacy' and repr(x) behaves in the same way as it did in versions of Python prior to 3.1.
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
and
to get more predictable results.
If a Python build or implementation cannot reasonably compute this information,
is allowed to return 0 instead.
New in version 3.4.
sys.getandroidapilevel()
Return the build time API version of Android as an integer.
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 the
module (RTLD_xxx constants, e.g.
).
sys.getfilesystemencoding()
Get the
: the encoding used with the
to convert between Unicode filenames and bytes filenames. The filesystem error handler is returned from
.
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.
and
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
function: see
and
members of
.
Changed in version 3.2:
result cannot be None anymore.
Changed in version 3.6: Windows is no longer guaranteed to return 'mbcs'. See
and
_enablelegacywindowsfsencoding()
for more information.
Changed in version 3.7: Return 'utf-8' if the
is enabled.
sys.getfilesystemencodeerrors()
Get the
: the error handler used with the
to convert between Unicode filenames and bytes filenames. The filesystem encoding is returned from
.
and
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
function: see
and
members of
.
New in version 3.6.
sys.get_int_max_str_digits()
Returns the current value for the
integer string conversion length limitation
. See also
.
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
.
Note that the returned value may not actually reflect how many references to the object are actually held. Consequently, do not rely on the returned value to be accurate, other than a value of 0 or 1.
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
.
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
will be raised.
calls the object’s __sizeof__ method and adds an additional garbage collector overhead if the object is managed by the garbage collector.
See
for an example of using
recursively to find the size of containers and all their contents.
sys.getswitchinterval()
Return the interpreter’s “thread switch interval”; see
.
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,
is raised. The default for depth is zero, returning the frame at the top of the call stack.
Raises an
sys._getframe with argument frame.
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
.
sys.gettrace()
Get the trace function as set by
.
CPython implementation detail: The
function is intended only for implementing debuggers, profilers, coverage tools and the like. Its behavior is part of the implementation platform, rather than part of the language definition, and thus may not be available in all Python implementations.
sys.getwindowsversion()
Return a named tuple describing the Windows version currently running. The named elements are major, minor, build, platform, service_pack, service_pack_minor, service_pack_major, suite_mask, product_type and platform_version. service_pack contains a string, platform_version a 3-tuple and all other values are integers. The components can also be accessed by name, so sys.getwindowsversion()[0] is equivalent to sys.getwindowsversion().major. For compatibility with prior versions, only the first 5 elements are retrievable by indexing.
platform will be 2 (VER_PLATFORM_WIN32_NT).
product_type may be one of the following values:
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 on OSVERSIONINFOEX() 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
module for achieving accurate OS version.
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
of the form (firstiter,finalizer), where firstiter and finalizer are expected to be either None or functions which take an
asynchronous generator iterator
as an argument, and are used to schedule finalization of an asynchronous generator by an event loop.
New in version 3.6: See
for more details.
Note
This function has been added on a provisional basis (see
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
for details.) Use it only for debugging purposes.
sys.hash_info
A
giving parameters of the numeric hash implementation. For more details about hashing of numeric types, see
.
hash_info.width
The width in bits used for hash values
hash_info.modulus
The prime modulus P used for numeric hash scheme
hash_info.inf
The hash value returned for a positive infinity
hash_info.nan
(This attribute is no longer used)
hash_info.imag
The multiplier used for the imaginary part of a complex number
hash_info.algorithm
The name of the algorithm for hashing of str, bytes, and memoryview
hash_info.hash_bits
The internal output size of the hash algorithm
hash_info.seed_bits
The size of the seed key of the hash algorithm
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:
ifsys.hexversion>=0x010502F0:# use some advanced feature...else:# use an alternative implementation or warn the user...This is called hexversion since it only really looks meaningful when viewed as the result of passing it to the built-in
function. The
may be used for a more human-friendly encoding of the same information.
More details of hexversion can be found at
.
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
. It represents the version of the Python implementation. This has a distinct meaning from the specific version of the Python language to which the currently running interpreter conforms, which sys.version_info represents. For example, for PyPy 1.8 sys.implementation.version might be sys.version_info(1,8,0,'final',0), whereas sys.version_info would be sys.version_info(2,7,2,'final',0). For CPython they are the same value, since it is the reference implementation.
hexversion is the implementation version in hexadecimal format, like
.
cache_tag is the tag used by the import machinery in the filenames of cached modules. By convention, it would be a composite of the implementation’s name and version, like 'cpython-33'. However, a Python implementation may use some other value if appropriate. If cache_tag is set to None, it indicates that module caching should be disabled.
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,
will not change during a run of the interpreter, nor between implementation versions. (It may change between Python language versions, however.) See
for more information.
New in version 3.3.
Note
The addition of new required attributes must go through the normal PEP process. See
for more information.
sys.int_info
A
that holds information about Python’s internal representation of integers. The attributes are read only.
int_info.bits_per_digit
The number of bits held in each digit. Python integers are stored internally in base 2**int_info.bits_per_digit.
int_info.sizeof_digit
The size in bytes of the C type used to represent a digit.
int_info.default_max_str_digits
The default value for
when it is not otherwise explicitly configured.
int_info.str_digits_check_threshold
The minimum non-zero value for
,
, or
.
New in version 3.1.
Changed in version 3.11: Added
and
.
sys.__interactivehook__
When this attribute exists, its value is automatically called (with no arguments) when the interpreter is launched in
. This is done after the
file is read, so that you can set this hook there. The
module
.
Raises an
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
around to benefit from it.
sys.is_finalizing()
Return
if the Python interpreter is
,
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 importpdb;pdb.pm() to enter the post-mortem debugger; see
module for more information.)
The meaning of the variables is the same as that of the return values from
above.
sys.maxsize
An integer giving the maximum value a variable of type
can take. It’s usually 2**31-1 on a 32-bit platform and 2**63-1 on a 64-bit platform.
sys.maxunicode
An integer giving the value of the largest Unicode code point, i.e. 1114111 (0x10FFFF in hexadecimal).
Changed in version 3.3: Before
, sys.maxunicode used to be either 0xFFFF or 0x10FFFF, depending on the configuration option that specified whether Unicode characters were stored as UCS-2 or UCS-4.
sys.meta_path
A list of
objects that have their
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. The
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
attribute is passed in as a second argument. The method returns a
, or None if the module cannot be found.
Changed in version 3.4:
were introduced in Python 3.4, by
. Earlier versions of Python looked for a method called
. This is still called as a fallback if a
entry doesn’t have a
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() or tuple(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.
The elements of
are the arguments to the Python interpreter, while the elements of
are the arguments to the user’s program. Arguments consumed by the interpreter itself will be present in
and missing from
.
New in version 3.10.
sys.path
A list of strings that specifies the search path for modules. Initialized from the environment variable
, plus an installation-dependent default.
By default, as initialized upon program startup, a potentially unsafe path is prepended to
(before the entries inserted as a result of
):
python-mmodule command line: prepend the current working directory.
pythonscript.py command line: prepend the script’s directory. If it’s a symbolic link, resolve symbolic links.
python-ccode and python (REPL) command lines: prepend an empty string, which means the current working directory.
To not prepend this potentially unsafe path, use the
command line option or the
environment variable.
A program is free to modify this list for its own purposes. Only strings should be added to
; all other data types are ignored during import.
See also
Module
This describes how to use .pth files to extend
.
sys.path_hooks
A list of callables that take a path argument to try to create a
for the path. If a finder can be created, it is to be returned by the callable, else raise
.
Originally specified in
.
sys.path_importer_cache
A dictionary acting as a cache for
objects. The keys are paths that have been passed to
and the values are the finders that are found. If a path is a valid file system path but no finder is found on
then None is stored.
Originally specified in
.
Changed in version 3.3: None is stored instead of
when no finder is found.
sys.platform
This string contains a platform identifier that can be used to append platform-specific components to
, 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 by uname-r appended, e.g. 'sunos5' or 'freebsd8', at the time when Python was built. Unless you want to test for a specific system version, it is therefore recommended to use the following idiom:
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'
Emscripten
'emscripten'
Linux
'linux'
WASI
'wasi'
Windows
'win32'
Windows/Cygwin
'cygwin'
macOS
'darwin'
Changed in version 3.3: On Linux,
doesn’t contain the major version anymore. It is always 'linux', instead of 'linux2' or 'linux3'. Since older Python versions include the version number, it is recommended to always use the startswith idiom presented above.
Changed in version 3.8: On AIX,
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 the startswith idiom presented above.
See also
has a coarser granularity.
gives system-dependent version information.
The
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 following sys.path paths (where X.Y is the Python major.minor version):
/usr/lib64/pythonX.Y/: Standard library (like os.py of the
module)
/usr/lib64/pythonX.Y/lib-dynload/: C extension modules of the standard library (like the
module, the exact filename is platform specific)
/usr/lib/pythonX.Y/site-packages/ (always use lib, not
): 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
argument to the configure script. See
for derived paths.
Note
If a
is in effect, this value will be changed in site.py to point to the virtual environment. The value for the Python installation will still be available, via
.
sys.ps1
sys.ps2
Strings specifying the primary and secondary prompt of the interpreter. These are only defined if the interpreter is in interactive mode. Their initial values in this case are '>>>' and '...'. If a non-string object is assigned to either variable, its
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 as sys.setdlopenflags(0). To share symbols across extension modules, call as sys.setdlopenflags(os.RTLD_GLOBAL). Symbolic names for the flag values can be found in the
module (RTLD_xxx constants, e.g.
).
sys.set_int_max_str_digits(maxdigits)
Set the
integer string conversion length limitation
used by this interpreter. See also
.
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
for more information on the Python profiler. The system’s profile function is called similarly to the system’s trace function (see
), but it is called with different events, for example it isn’t called for each executed line of code (only on call and return, but the return event is reported even when an exception has been set). The function is thread-specific, but there is no way for the profiler to know about context switches between threads, so it does not make sense to use this in the presence of multiple threads. Also, its return value is not used, so it can simply return None. Error in the profile function will cause itself unset.
Note
The same tracing mechanism is used for setprofile() as
. To trace calls with setprofile() inside a tracing function (e.g. in a debugger breakpoint), see
.
Profile functions should have three arguments: frame, event, and arg. frame is the current stack frame. event is a string: 'call', 'return', 'c_call', 'c_return', or 'c_exception'. arg depends on the event type.
The events have the following meaning:
'call'A function is called (or some other code block entered). The profile function is called; arg is None.
'return'A function (or other code block) is about to return. The profile function is called; arg is the value that will be returned, or None if the event is caused by an exception being raised.
'c_call'A C function is about to be called. This may be an extension function or a built-in. arg is the C function object.
'c_return'A C function has returned. arg is the C function object.
'c_exception'A C function has raised an exception. arg is the C function object.
Raises an
sys.setprofile with no arguments.
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
exception is raised.
Changed in version 3.5.1: A
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
for each thread being debugged or use
.
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, or None if the scope shouldn’t be traced.
The local trace function should return a reference to itself, or to another function which would then be used as the local trace function for the scope.
If there is any error occurred in the trace function, it will be unset, just like settrace(None) is called.
Note
Tracing is disabled while calling the trace function (e.g. a function set by settrace()). For recursive tracing see
.
The events have the following meaning:
'call'A function is called (or some other code block entered). The global trace function is called; arg is None; the return value specifies the local trace function.
'line'The interpreter is about to execute a new line of code or re-execute the condition of a loop. The local trace function is called; arg is None; the return value specifies the new local trace function. See Objects/lnotab_notes.txt for a detailed explanation of how this works. Per-line events may be disabled for a frame by setting
to
on that
.
'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
for opcode details). The local trace function is called; arg is None; the return value specifies the new local trace function. Per-opcode events are not emitted by default: they must be explicitly requested by setting
to
on the
.
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, which
doesn’t do. Note that in order for this to work, a global tracing function must have been installed with
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 returns None to disable itself immediately on each frame).
For more information on code and frame objects, refer to
.
Raises an
sys.settrace with no arguments.
CPython implementation detail: The
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.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
sys.set_asyncgen_hooks_firstiter with no arguments.
Raises an
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
for more details, and for a reference example of a finalizer method see the implementation of asyncio.Loop.shutdown_asyncgens in
Note
This function has been added on a provisional basis (see
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
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
environment variable before launching Python.
See also
and
sys.getfilesystemencodeerrors()
.
New in version 3.6: See
for more details.
sys.stdin
sys.stdout
sys.stderr
used by the interpreter for standard input, output and errors:
stdin is used for all interactive input (including calls to
);
stdout is used for the output of
and
statements and for the prompts of
;
The interpreter’s own prompts and its error messages go to stderr.
These streams are regular
like those returned by the
function. Their parameters are chosen as follows:
The encoding and error handling are is initialized from
and
.
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() returns True) use the value of the console input and output codepages at startup, respectively for stdin and stdout/stderr. This defaults to the system
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
environment variable before starting Python or by using the new
utf8 command line option and
environment variable. However, for the Windows console, this only applies when
is also set.
When interactive, the stdout stream is line-buffered. Otherwise, it is block-buffered like regular text files. The stderr stream is line-buffered in both cases. You can make both streams unbuffered by passing the
command-line option or setting the
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
object. For example, to write bytes to
, use sys.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
which do not support the buffer attribute.
sys.__stdin__
sys.__stdout__
sys.__stderr__
These objects contain the original values of stdin, stderr and stdout at the start of the program. They are used during finalization, and could be useful to print to the actual standard stream no matter if the sys.std* object has been redirected.
It can also be used to restore the actual files to known working file objects in case they have been overwritten with a broken object. However, the preferred way to do this is to explicitly save the previous stream before replacing it, and restore the saved object.
Note
Under some conditions stdin, stdout and stderr as well as the original values __stdin__, __stdout__ and __stderr__ can be None. 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 the email.mime sub-package and the email.message sub-module are not listed.
See also the
list.
New in version 3.10.
sys.thread_info
A
holding information about the thread implementation.
thread_info.name
The name of the thread implementation:
"nt": Windows threads
"pthread": POSIX threads
"pthread-stubs": stub POSIX threads (on WebAssembly platforms without threading support)
"solaris": Solaris threads
thread_info.lock
The name of the lock implementation:
"semaphore": a lock uses a semaphore
"mutex+cond": a lock uses a mutex and a condition variable
None if this information is unknown
thread_info.version
The name and version of the thread library. It is a string, 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 to 0 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 (
).
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 is None.
can be overridden to control how unraisable exceptions are handled.
See also
which handles uncaught exceptions.
Warning
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.
Raise an auditing event sys.unraisablehook with arguments hook, unraisable when an exception that cannot be handled occurs. The unraisable object is the same as what will be passed to the hook. If no hook has been set, hook may be None.
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
and the functions provided by the
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'. The version_info value corresponding to the Python version 2.0 is (2,0,0,'final',0). The components can also be accessed by name, so sys.version_info[0] is equivalent to sys.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
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
module for informational purposes; modifying this value has no effect on the registry keys used by Python.
sys._xoptions
A dictionary of the various implementation-specific flags passed through the
command-line option. Option names are either mapped to their values, if given explicitly, or to
. Example:
$ ./python-Xa=b-Xc Python 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
. Other implementations may export them through other means, or not at all.
New in version 3.2.
Citations