sys

The sys module exposes the running interpreter: command line, search path, streams, hooks, recursion limits, exception handling, GC and performance counters. Almost every CPython internal that needs a Python-visible knob surfaces it here.

Source-of-record: Python/sysmodule.c, sys docs.

Streams and command line

Name	Type	Notes
sys.argv	list[str]	Script name + args.
sys.stdin / stdout / stderr	TextIOWrapper	Line-buffered by default.
sys.__stdin__ / __stdout__ / __stderr__	originals	Snapshot at interpreter start.
sys.path	list[str]	Module search path.
sys.path_hooks	list[callable]	Path-entry finders.
sys.path_importer_cache	dict	Memoised path importers.
sys.modules	dict[str, Module]	Import cache.
sys.meta_path	list[MetaPathFinder]	Top-of-import dispatcher.
sys.builtin_module_names	tuple[str, ...]	Names compiled into the interpreter.
sys.stdlib_module_names	frozenset[str]	Stdlib module names (PEP 643).

Version and platform

Name	Type	Value
sys.version	str	Human-readable version string.
sys.version_info	sys.version_info	(major, minor, micro, releaselevel, serial).
sys.hexversion	int	Packed version number.
sys.implementation	SimpleNamespace	(name, version, hexversion, cache_tag).
sys.platform	str	'linux', 'darwin', 'win32', ...
sys.maxsize	int	Largest Py_ssize_t.
sys.maxunicode	int	0x10FFFF.
sys.byteorder	str	'little' or 'big'.
sys.float_info	named tuple	DBL_MAX, epsilon, etc.
sys.int_info	named tuple	Internal int representation info.
sys.hash_info	named tuple	Hash algorithm, width, modulus.
sys.thread_info	named tuple	Thread implementation info.
sys.flags	named tuple	Command-line flag values.

Hooks

Function	Role
sys.excepthook(type, value, traceback)	Called on unhandled top-level exception.
sys.unraisablehook(args)	Called when an exception is logged unraisable.
sys.displayhook(value)	REPL output.
sys.audit(event, *args)	Emit audit event (PEP 578).
sys.addaudithook(hook)	Register audit hook.
sys.breakpointhook(*args, **kw)	Backend for the breakpoint() builtin.
sys.exit(code=None)	Raise SystemExit(code).
sys.exception()	Currently handled exception, or None.
sys.exc_info()	(type, value, traceback) of current exception.
sys.last_type / last_value / last_traceback / last_exc	Most recent unhandled exception in REPL.

Runtime control

Function	Effect
sys.setrecursionlimit(n) / getrecursionlimit()	Frame depth cap.
sys.setswitchinterval(n) / getswitchinterval()	GIL switch interval in seconds.
sys.settrace(fn) / gettrace()	Per-frame trace function.
sys.setprofile(fn) / getprofile()	Per-call profile function.
sys.monitoring	PEP 669 monitoring API namespace.
sys.set_int_max_str_digits(n) / get_int_max_str_digits()	int/str length cap.
sys.set_coroutine_origin_tracking_depth(n)	Coroutine origin tracking depth.
sys.set_asyncgen_hooks(firstiter=, finalizer=)	Async generator hooks.
sys.intern(s)	Intern a string.
sys.getsizeof(obj, default=...)	Memory footprint of obj.
sys.getrefcount(obj)	Reference count incl. temp.
sys._getframe(depth=0)	Internal: caller frame.
sys._current_frames()	Frame per thread.
sys._current_exceptions()	Active exception per thread.
sys.is_finalizing()	True during interpreter shutdown.

Filesystem encoding

Name	Value
sys.getfilesystemencoding()	Encoding used for paths ('utf-8' on POSIX).
sys.getfilesystemencodeerrors()	Error handler ('surrogateescape').
sys.getdefaultencoding()	'utf-8'.

Interpreter shutdown

sys.exit(code) raises SystemExit. Cleanup runs atexit callbacks in LIFO order, then finalises GC. sys.is_finalizing() reports the finalisation phase.

Gopy status

Area	State
Streams, command line, paths	Complete.
Version / platform / impl info	Complete; cache_tag is gopy-NNN.
Hooks (excepthook, audit, display)	Complete.
settrace/setprofile	Complete.
sys.monitoring (PEP 669)	Complete.
setrecursionlimit	Complete; enforced by frame depth check.
set_int_max_str_digits	Complete.

Reference

• CPython 3.14: sys module.
• Python/sysmodule.c. C side.
• module/sys/. gopy port.