Python Thread State

Get the current Python Thread State (tstate)

  • _PyRuntimeState_GetThreadState(runtime): read runtime->gilstate.tstate_current
  • _PyThreadState_GET(): internal C API, call _PyRuntimeState_GetThreadState(&_PyRuntime), new in Python 3.8.
  • PyThreadState_Get(): opaque function call
  • _PyThreadState_UncheckedGet(): added to Python 3.5.2 (bpo-26154)
  • PyGILState_GetThisThreadState()
  • PyThreadState_GET(): macro, alias to PyThreadState_Get(). When pycore_pystate.h is included: macro redefined as an alias to _PyThreadState_GET().

There was also _PyThreadState_Current: removed from Python 3.5.1.

History:

  • Python 3.7: PyThreadState_GET() reads _PyThreadState_Current (atomic variable).
  • Python 3.8: _PyThreadState_Current becomes _PyRuntime.gilstate.tstate_current

Get the current interpreter (interp)

  • PyInterpreterState_Get(): limited C API. new in Python 3.9. Known as _PyInterpreterState_Get() in Python 3.8. Implemented as _PyThreadState_GET()->interp.
  • _PyInterpreterState_GET(): internal C API, added to Python 3.8. Previously called _PyInterpreterState_GET_UNSAFE() in Python 3.8.
  • _PyGILState_GetInterpreterStateUnsafe(): read _PyRuntime.gilstate.autoInterpreterState, new in Python 3.6.

Python 3.9 also defines _PyInterpreterState_Get() as an alias to PyInterpreterState_Get() for backward compatibility.

Main thread and main interpreter

  • PyInterpreterState_Main(): get _PyRuntime.interpreters.main
  • _PyOS_IsMainThread(): call _Py_ThreadCanHandleSignals(_PyInterpreterState_GET())
  • _Py_IsMainThread(): Check if the current thread is the main thread.
  • _Py_IsMainInterpreter(tstate)
  • _Py_ThreadCanHandleSignals(interp): added by https://bugs.python.org/issue40010
  • _Py_ThreadCanHandlePendingCalls(): see https://bugs.python.org/issue40231

Identifier of the main thread (of the main interpreter): _PyRuntime.main_thread. Identifier read by PyThread_get_thread_ident(), it’s not a Python thread state. Used by _Py_IsMainThread().

Main interpreter: _PyRuntime.interpreters.main.

GIL

take_gil() and drop_gil() stores the Python thread state into _PyRuntime.ceval.gil.last_holder.

Signal handler

trip_signal() of signalmodule.c

Python 3.9 now always uses the main interpreter (_PyRuntime.interpreters.main), it no longer tries to get the current Python thread state: bpo-40082.

_PyEval_SignalReceived(interp) sets signals_pending and eval_breaker to 1.

Call _PyEval_AddPendingCall(interp, ...) if writing into the wakeup fd fails.

Python 3.9 made eval_breaker and pending calls per interpreter.

On Windows, SIGINT (CTRL+C) signal handler is called from a different thread at each call. MSDN signal documentation:

When a CTRL+C interrupt occurs, Win32 operating systems generate a new thread to specifically handle that interrupt.

This can cause a single-thread application, such as one in UNIX, to become multithreaded and cause unexpected behavior.

faulthandler

faulthandler_fatal_error() and faulthandler_user() signal handlers use PyGILState_GetThisThreadState() to get the current Python thread state:

/* SIGSEGV, SIGFPE, SIGABRT, SIGBUS and SIGILL are synchronous signals and
   are thus delivered to the thread that caused the fault. Get the Python
   thread state of the current thread.

   PyThreadState_Get() doesn't give the state of the thread that caused the
   fault if the thread released the GIL, and so this function cannot be
   used. Read the thread specific storage (TSS) instead: call
   PyGILState_GetThisThreadState(). */
tstate = PyGILState_GetThisThreadState();

enable() and register() use _PyThreadState_UncheckedGet() and then store PyThreadState_GetInterpreter(tstate) to use it from signal handlers.

tracemalloc

Hooks on memory allocators use PyGILState_GetThisThreadState() to get the current Python thread state. PyMem_RawMalloc() can be called without holding the GIL.

PyMem_RawMalloc() hook uses PyGILState_Ensure() and PyGILState_Release() to hold the GIL.

subinterpreters

It is currently possible for a single native thread to be associated to multiple Python thread states: one per interpreter.

Extract of _testcapi.run_in_subinterp() implementation:

PyThreadState *mainstate = PyThreadState_Get();

PyThreadState_Swap(NULL);

PyThreadState *substate = Py_NewInterpreter();
...
Py_EndInterpreter(substate);

PyThreadState_Swap(mainstate);

Py_NewInterpreter() creates a new Python thread state. Extract of its implementation:

PyInterpreterState *interp = PyInterpreterState_New();
...
PyThreadState *tstate = PyThreadState_New(interp);
..
PyThreadState_Swap(tstate);

Pass tstate explicitly

Pass the Python thread state explicitly (January 2020) by Victor Stinner.

Move global variables into PyInterpreterState

PyLong_FromLong() now requires to get the current interpreter to access PyInterpreterState.small_ints singletons.