Issue #16290: __complex__ must now always return an instance of complex.

Lib/test/test_complex.py

@@ -221,6 +221,8 @@ class ComplexTest(unittest.TestCase):
         self.assertRaises(TypeError, complex, OS(None))
         self.assertRaises(TypeError, complex, NS(None))
         self.assertRaises(TypeError, complex, {})
+        self.assertRaises(TypeError, complex, NS(1.5))
+        self.assertRaises(TypeError, complex, NS(1))
 
         self.assertAlmostEqual(complex("1+10j"), 1+10j)
         self.assertAlmostEqual(complex(10), 10+0j)

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.4.0 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #16290: A float return value from the __complex__ special method is no
+  longer accepted in the complex() constructor.
+
 - Issue #16416: On Mac OS X, operating system data are now always
   encoded/decoded to/from UTF-8/surrogateescape, instead of the locale encoding
   (which may be ASCII if no locale environment variable is set), to avoid

Objects/complexobject.c

@@ -271,6 +271,12 @@ try_complex_special_method(PyObject *op) {
     if (f) {
         PyObject *res = PyObject_CallFunctionObjArgs(f, NULL);
         Py_DECREF(f);
+        if (res != NULL && !PyComplex_Check(res)) {
+            PyErr_SetString(PyExc_TypeError,
+                "__complex__ should return a complex object");
+            Py_DECREF(res);
+            return NULL;
+        }
         return res;
     }
     return NULL;
@@ -296,12 +302,6 @@ PyComplex_AsCComplex(PyObject *op)
     newop = try_complex_special_method(op);
 
     if (newop) {
-        if (!PyComplex_Check(newop)) {
-            PyErr_SetString(PyExc_TypeError,
-                "__complex__ should return a complex object");
-            Py_DECREF(newop);
-            return cv;
-        }
         cv = ((PyComplexObject *)newop)->cval;
         Py_DECREF(newop);
         return cv;