bpo-34392: Add sys. _is_interned()

Doc/library/sys.rst

@@ -865,6 +865,18 @@ always available.
    value of :func:`intern` around to benefit from it.
 
 
+.. function:: _is_interned(string)
+
+   Return :const:`True` if the given string is "interned", :const:`False`
+   otherwise.
+
+   .. versionadded:: 3.8
+
+   .. impl-detail::
+
+      It is not guaranteed to exist in all implementations of Python.
+
+
 .. function:: is_finalizing()
 
    Return :const:`True` if the Python interpreter is

Lib/test/test_sys.py

@@ -504,11 +504,25 @@ def test_43581(self):
     def test_intern(self):
         global numruns
         numruns += 1
+        has_is_interned = (test.support.check_impl_detail(cpython=True)
+                           or hasattr(sys, '_is_interned'))
         self.assertRaises(TypeError, sys.intern)
+        self.assertRaises(TypeError, sys.intern, b'abc')
+        if has_is_interned:
+            self.assertRaises(TypeError, sys._is_interned)
+            self.assertRaises(TypeError, sys._is_interned, b'abc')
         s = "never interned before" + str(numruns)
+        if has_is_interned:
+            self.assertIs(sys._is_interned(s), False)
         self.assertTrue(sys.intern(s) is s)
+        if has_is_interned:
+            self.assertIs(sys._is_interned(s), True)
         s2 = s.swapcase().swapcase()
+        if has_is_interned:
+            self.assertIs(sys._is_interned(s2), False)
         self.assertTrue(sys.intern(s2) is s)
+        if has_is_interned:
+            self.assertIs(sys._is_interned(s2), False)
 
         # Subclasses of string can't be interned, because they
         # provide too much opportunity for insane things to happen.
@@ -520,6 +534,8 @@ def __hash__(self):
                 return 123
 
         self.assertRaises(TypeError, sys.intern, S("abc"))
+        if has_is_interned:
+            self.assertIs(sys._is_interned(S("abc")), False)
 
     def test_sys_flags(self):
         self.assertTrue(sys.flags)

Misc/NEWS.d/next/Core and Builtins/2018-08-13-13-25-15.bpo-34392.9kIlMF.rst

@@ -0,0 +1 @@
+Added :func:`sys.isinterned`.

Python/sysmodule.c

@@ -447,6 +447,23 @@ Return the string itself or the previously interned string object with the\n\
 same value.");
 
 
+/*[clinic input]
+sys._is_interned -> bool
+
+  string: unicode
+  /
+
+Return True if the given string is "interned".
+[clinic start generated code]*/
+
+static int
+sys__is_interned_impl(PyObject *module, PyObject *string)
+/*[clinic end generated code: output=c3678267b4e9d7ed input=039843e17883b606]*/
+{
+    return PyUnicode_CHECK_INTERNED(string);
+}
+
+
 /*
  * Cached interned string objects used for calling the profile and
  * trace functions.  Initialized by trace_init().
@@ -1557,6 +1574,7 @@ static PyMethodDef sys_methods[] = {
      METH_NOARGS, enablelegacywindowsfsencoding_doc },
 #endif /* MS_WINDOWS */
     {"intern",          sys_intern,     METH_VARARGS, intern_doc},
+    SYS__IS_INTERNED_METHODDEF
     {"is_finalizing",   sys_is_finalizing, METH_NOARGS, is_finalizing_doc},
 #ifdef USE_MALLOPT
     {"mdebug",          sys_mdebug, METH_VARARGS},