`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. Unless explicitly noted otherwise, all variables are read-only.

sys.abiflags¶: On POSIX systems where Python was built with the standard configure script, this contains the ABI flags as specified by PEP 3149.

Added in version 3.2.

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

Availability: Unix.

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 by PySys_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 as ctypes) should be completely removed or closely monitored.

Calling sys.addaudithook() will itself raise an auditing event named sys.addaudithook with no arguments. If any existing hooks raise an exception derived from RuntimeError, 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.

Added in version 3.8.

Changed in version 3.8.1: Exceptions derived from Exception but not RuntimeError 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() or PySys_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.

Added in version 3.8.

sys.base_exec_prefix¶

Equivalent to exec_prefix, but referring to the base Python installation.

When running under Virtual Environments, exec_prefix gets overwritten to the virtual environment prefix. base_exec_prefix, conversely, does not change, and always points to the base Python installation. Refer to Virtual Environments for more information.

Added in version 3.3.

sys.base_prefix¶

Equivalent to prefix, but referring to the base Python installation.

When running under virtual environment, prefix gets overwritten to the virtual environment prefix. base_prefix, conversely, does not change, and always points to the base Python installation. Refer to Virtual Environments for more information.

Added 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 or profile some other code.

Tracing is suspended while calling a tracing function set by settrace() or setprofile() 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.

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

sys._clear_internal_caches()¶: Clear all internal performance-related caches. Use this function only to release unnecessary references and memory blocks when hunting for leaks.

Added in version 3.13.

sys._current_frames()¶

Return a dictionary mapping each thread’s identifier to the topmost stack 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.

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

sys.breakpointhook()¶

This hook function is called by built-in breakpoint(). By default, it drops you into the pdb 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 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 named function(). This is run, passing in *args and **kws, and whatever function() returns, sys.breakpointhook() returns to the built-in breakpoint() function.

Note that if anything goes wrong while importing the callable named by PYTHONBREAKPOINT, a RuntimeWarning is reported and the breakpoint is ignored.

Also note that if sys.breakpointhook() is overridden programmatically, PYTHONBREAKPOINT is not consulted.

Added 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.

Added 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 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 expression 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:

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 on UnicodeEncodeError.

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 -B command line option and the PYTHONDONTWRITEBYTECODE 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.

_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.js v14.18.2', or 'UNKNOWN'.

_emscripten_info.pthreads¶: True if Python is compiled with Emscripten pthreads support.

_emscripten_info.shared_memory¶: True if Python is compiled with shared memory support.

Availability: Emscripten.

Added in version 3.11.

sys.pycache_prefix¶

If this is set (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 compileall 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 the PYTHONPYCACHEPREFIX environment variable (command-line takes precedence). If neither are set, it is None.

Added 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 SystemExit 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 RuntimeError the call to the hook will be suppressed. Otherwise, the audit hook exception will be reported as unraisable and sys.excepthook will be called.

See also

The sys.unraisablehook() function handles unraisable exceptions and the threading.excepthook() function handles exception raised by threading.Thread.run().

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.

Added in version 3.7: __breakpointhook__

Added in version 3.8: __unraisablehook__

sys.exception()¶

This function, when called while an exception handler is executing (such 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.

Added 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 exception() would return e), exc_info() returns the tuple (type(e), e, e.__traceback__). That is, a tuple containing the type of the exception (a subclass of BaseException), 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 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 exc_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. 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 virtual environment is in effect, this exec_prefix will point to the virtual environment. The value for the Python installation will still be available, via base_exec_prefix. Refer to Virtual Environments for more information.

Changed in version 3.14: When running under a virtual environment, prefix and exec_prefix are now set to the virtual environment prefix by the path initialization, instead of site. This means that prefix and exec_prefix always point to the virtual environment, even when site is disabled (-S).

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 or None.

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 to stderr and results in an exit code of 1. In particular, sys.exit("some error message") is a quick way to exit a program when an error occurs.

Since exit() ultimately “only” raises an exception, it will only exit the process when called from the main thread, and the exception is not intercepted. Cleanup actions specified by finally clauses of try 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. Flags should only be accessed only by name and not by index. The attributes are read only.

flags.debug¶	`-d`
flags.inspect¶	`-i`
flags.interactive¶	`-i`
flags.isolated¶	`-I`
flags.optimize¶	`-O` or `-OO`
flags.dont_write_bytecode¶	`-B`
flags.no_user_site¶	`-s`
flags.no_site¶	`-S`
flags.ignore_environment¶	`-E`
flags.verbose¶	`-v`
flags.bytes_warning¶	`-b`
flags.quiet¶	`-q`
flags.hash_randomization¶	`-R`
flags.dev_mode¶	`-X dev` (Python Development Mode)
flags.utf8_mode¶	`-X utf8`
flags.safe_path¶	`-P`
flags.int_max_str_digits¶	`-X int_max_str_digits` (integer string conversion length limitation)
flags.warn_default_encoding¶	`-X warn_default_encoding`
flags.gil¶	`-X gil` and `PYTHON_GIL`
flags.thread_inherit_context¶	`-X thread_inherit_context` and `PYTHON_THREAD_INHERIT_CONTEXT`
flags.context_aware_warnings¶	`-X context_aware_warnings` and `PYTHON_CONTEXT_AWARE_WARNINGS`

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

Added 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 the utf8_mode attribute for the new -X utf8 flag.

Changed in version 3.10: Added warn_default_encoding attribute for -X warn_default_encoding 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.

Changed in version 3.13: Added the gil attribute.

Changed in version 3.14: Added the thread_inherit_context attribute.

Changed in version 3.14: Added the context_aware_warnings 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.

Attributes of the `float_info` named tuple¶
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 `math.ulp()`.
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 `math.ulp(0.0)` 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 sys.float_info.dig needs further explanation. If s is any string representing a decimal number with at most sys.float_info.dig significant digits, then converting s to a float and back again will recover a string representing the same decimal value:

>>> import sys
>>> sys.float_info.dig
15
>>> s = '3.14159265358979'    # decimal string with 15 significant digits
>>> format(float(s), '.15g')  # convert to float and back -> same value
'3.14159265358979'

But for strings with more than sys.float_info.dig significant digits, this isn’t always true:

>>> s = '9876543211234567'    # 16 significant digits is too many!
>>> format(float(s), '.16g')  # conversion changes value
'9876543211234568'

sys.float_repr_style¶: A string indicating how the repr() function behaves for floats. If the string has value 'short' then for a finite float x, repr(x) aims to produce a short string with the property that float(repr(x)) == x. This is the usual behaviour in Python 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.

Added in version 3.1.

sys.getallocatedblocks()¶

Return the number of memory blocks currently allocated by the interpreter, regardless of their size. This function is mainly useful for 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_internal_caches() and gc.collect() to get more predictable results.

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

Added in version 3.4.

sys.getunicodeinternedsize()¶: Return the number of unicode objects that have been interned.

Added in version 3.12.

sys.getandroidapilevel()¶: Return the build-time API level of Android as an integer. This represents the minimum version of Android this build of Python can run on. For runtime version information, see platform.android_ver().

Availability: Android.

Added in version 3.7.

sys.getdefaultencoding()¶: Return 'utf-8'. This is the name of the default string encoding, used in methods like str.encode().

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 os 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 getfilesystemencodeerrors().

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() and os.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: see filesystem_encoding and filesystem_errors members of PyConfig.

Changed in version 3.2: getfilesystemencoding() result cannot be None 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

sys — System-specific parameters and functions¶

`sys` — System-specific parameters and functions¶