bpo-29894: Deprecate returning an instance of complex subclass from __complex__

Lib/test/test_complex.py

@@ -383,8 +383,9 @@ class complex2(complex):
             def __complex__(self):
                 return None
 
-        self.assertAlmostEqual(complex(complex0(1j)), 42j)
-        self.assertAlmostEqual(complex(complex1(1j)), 2j)
+        self.assertEqual(complex(complex0(1j)), 42j)
+        with self.assertWarns(DeprecationWarning):
+            self.assertEqual(complex(complex1(1j)), 2j)
         self.assertRaises(TypeError, complex, complex2(1j))
 
     @support.requires_IEEE_754

Lib/test/test_getargs2.py

@@ -408,7 +408,8 @@ def test_D(self):
         self.assertEqual(getargs_D(ComplexSubclass(7.5+0.25j)), 7.5+0.25j)
         self.assertEqual(getargs_D(ComplexSubclass2(7.5+0.25j)), 7.5+0.25j)
         self.assertRaises(TypeError, getargs_D, BadComplex())
-        self.assertEqual(getargs_D(BadComplex2()), 4.25+0.5j)
+        with self.assertWarns(DeprecationWarning):
+            self.assertEqual(getargs_D(BadComplex2()), 4.25+0.5j)
         self.assertEqual(getargs_D(BadComplex3(7.5+0.25j)), 7.5+0.25j)
 
         for x in (DBL_MIN, -DBL_MIN, DBL_MAX, -DBL_MAX, INF, -INF):

Misc/NEWS

@@ -10,6 +10,10 @@ What's New in Python 3.7.0 alpha 1?
 Core and Builtins
 -----------------
 
+- bpo-29894: The deprecation warning is emitted if __complex__ returns an
+  instance of a strict subclass of complex.  In a future versions of Python
+  this can be an error.
+
 - bpo-29859: Show correct error messages when any of the pthread_* calls in
   thread_pthread.h fails.
 

Objects/complexobject.c

@@ -274,17 +274,31 @@ PyComplex_ImagAsDouble(PyObject *op)
 }
 
 static PyObject *
-try_complex_special_method(PyObject *op) {
+try_complex_special_method(PyObject *op)
+{
     PyObject *f;
     _Py_IDENTIFIER(__complex__);
 
     f = _PyObject_LookupSpecial(op, &PyId___complex__);
     if (f) {
         PyObject *res = _PyObject_CallNoArg(f);
         Py_DECREF(f);
-        if (res != NULL && !PyComplex_Check(res)) {
-            PyErr_SetString(PyExc_TypeError,
-                "__complex__ should return a complex object");
+        if (!res || PyComplex_CheckExact(res)) {
+            return res;
+        }
+        if (!PyComplex_Check(res)) {
+            PyErr_Format(PyExc_TypeError,
+                "__complex__ returned non-complex (type %.200s)",
+                res->ob_type->tp_name);
+            Py_DECREF(res);
+            return NULL;
+        }
+        /* Issue #29894: warn if 'res' not of exact type complex. */
+        if (PyErr_WarnFormat(PyExc_DeprecationWarning, 1,
+                "__complex__ returned non-complex (type %.200s).  "
+                "The ability to return an instance of a strict subclass of complex "
+                "is deprecated, and may be removed in a future version of Python.",
+                res->ob_type->tp_name)) {
             Py_DECREF(res);
             return NULL;
         }
@@ -1030,12 +1044,7 @@ complex_new_impl(PyTypeObject *type, PyObject *r, PyObject *i)
         }
         if (tmp == NULL)
             return NULL;
-        if (!PyFloat_Check(tmp)) {
-            PyErr_SetString(PyExc_TypeError,
-                            "float(r) didn't return a float");
-            Py_DECREF(tmp);
-            return NULL;
-        }
+        assert(PyFloat_Check(tmp));
         cr.real = PyFloat_AsDouble(tmp);
         cr.imag = 0.0;
         Py_DECREF(tmp);