Use thread inheritable contextvars.

Change `warnings` and `decimal` to use thread inheritable context vars.
This means new threads will start with bindings for these variables from
the starter thread, rather than having empty bindings (no matter the
value of the thread_inherit_context flag).

Author: nascheme

Lib/_py_warnings.py

@@ -83,7 +83,8 @@ def _filters(self):
 _global_context = _GlobalContext()
 
 
-_warnings_context = _contextvars.ContextVar('warnings_context')
+_warnings_context = _contextvars.ContextVar.thread_inheritable(
+    'warnings_context')
 
 
 def _get_context():

Lib/_pydecimal.py

@@ -350,7 +350,7 @@ class FloatOperation(DecimalException, TypeError):
 import contextvars
 from _contextvars import _current_context_depth
 
-_current_context_var = contextvars.ContextVar('decimal_context')
+_current_context_var = contextvars.thread_inheritable('decimal_context')
 
 _context_attributes = frozenset(
     ['prec', 'Emin', 'Emax', 'capitals', 'clamp', 'rounding', 'flags', 'traps']

Lib/test/test_warnings/__init__.py

@@ -1807,12 +1807,11 @@ class PyAsyncTests(AsyncTests, unittest.TestCase):
 
 class ThreadTests(BaseTest):
     """Verifies that the catch_warnings() context manager behaves as
-    expected when used within threads.  This requires that both the
-    context_aware_warnings flag and thread_inherit_context flags are enabled.
+    expected when used within threads.  This requires the
+    context_aware_warnings flag to be enabled.
     """
 
-    ENABLE_THREAD_TESTS = (sys.flags.context_aware_warnings and
-                           sys.flags.thread_inherit_context)
+    ENABLE_THREAD_TESTS = sys.flags.context_aware_warnings
 
     def setUp(self):
         super().setUp()

Modules/_decimal/_decimal.c

@@ -7931,7 +7931,7 @@ _decimal_exec(PyObject *m)
                PyUnicode_FromString("___DECIMAL_CTX__"));
     CHECK_INT(PyModule_AddObjectRef(m, "HAVE_CONTEXTVAR", Py_False));
 #else
-    ASSIGN_PTR(state->current_context_var, PyContextVar_New("decimal_context", NULL));
+    ASSIGN_PTR(state->current_context_var, PyContextVar_NewThreadInheritable("decimal_context", NULL));
     CHECK_INT(PyModule_AddObjectRef(m, "HAVE_CONTEXTVAR", Py_True));
 #endif
     CHECK_INT(PyModule_AddObjectRef(m, "HAVE_THREADS", Py_True));

Python/_warnings.c

@@ -158,7 +158,8 @@ _PyWarnings_InitState(PyInterpreterState *interp)
     }
 
     if (st->context == NULL) {
-        st->context = PyContextVar_New("_warnings_context", NULL);
+        st->context = PyContextVar_NewThreadInheritable(
+            "_warnings_context", NULL);
         if (st->context == NULL) {
             return -1;
         }