BUG: core: richcompare must indicate undefined comparison via Py_NotImplemented (fixes #1491)

The Python documentation indicates that the result from comparisons
should be Py_NotImplemented when undefined, and there is no reason why
not follow that also in array_richcompare.

2 files changed, 21 insertions, 15 deletions

diff --git a/numpy/core/src/multiarray/arrayobject.c b/numpy/core/src/multiarray/arrayobject.c
index ab1e824be..587e45d26 100644
--- a/numpy/core/src/multiarray/arrayobject.c
+++ b/numpy/core/src/multiarray/arrayobject.c
@@ -907,16 +907,15 @@ array_richcompare(PyArrayObject *self, PyObject *other, int cmp_op)
             array_other = PyArray_FromObject(other,
                     typenum, 0, 0);
             /*
-             * If not successful, then return False. This fixes code
-             * that used to allow equality comparisons between arrays
-             * and other objects which would give a result of False.
+             * If not successful, indicate that the items cannot be compared
+             * this way.
              */
             if ((array_other == NULL) ||
                     (array_other == Py_None)) {
                 Py_XDECREF(array_other);
                 PyErr_Clear();
-                Py_INCREF(Py_False);
-                return Py_False;
+                Py_INCREF(Py_NotImplemented);
+                return Py_NotImplemented;
             }
         }
         else {
@@ -952,14 +951,14 @@ array_richcompare(PyArrayObject *self, PyObject *other, int cmp_op)
         }
         /*
          * If the comparison results in NULL, then the
-         * two array objects can not be compared together so
-         * return zero
+         * two array objects can not be compared together;
+         * indicate that
          */
         Py_DECREF(array_other);
         if (result == NULL) {
             PyErr_Clear();
-            Py_INCREF(Py_False);
-            return Py_False;
+            Py_INCREF(Py_NotImplemented);
+            return Py_NotImplemented;
         }
         break;
     case Py_NE:
@@ -976,14 +975,13 @@ array_richcompare(PyArrayObject *self, PyObject *other, int cmp_op)
             array_other = PyArray_FromObject(other, typenum, 0, 0);
             /*
              * If not successful, then objects cannot be
-             * compared and cannot be equal, therefore,
-             * return True;
+             * compared this way
              */
             if ((array_other == NULL) || (array_other == Py_None)) {
                 Py_XDECREF(array_other);
                 PyErr_Clear();
-                Py_INCREF(Py_True);
-                return Py_True;
+                Py_INCREF(Py_NotImplemented);
+                return Py_NotImplemented;
             }
         }
         else {
@@ -1021,8 +1019,8 @@ array_richcompare(PyArrayObject *self, PyObject *other, int cmp_op)
         Py_DECREF(array_other);
         if (result == NULL) {
             PyErr_Clear();
-            Py_INCREF(Py_True);
-            return Py_True;
+            Py_INCREF(Py_NotImplemented);
+            return Py_NotImplemented;
         }
         break;
     case Py_GT:
diff --git a/numpy/core/tests/test_regression.py b/numpy/core/tests/test_regression.py
index 52548ca1a..12fe089e6 100644
--- a/numpy/core/tests/test_regression.py
+++ b/numpy/core/tests/test_regression.py
@@ -1426,5 +1426,13 @@ class TestRegression(TestCase):
         rep = repr(ra) # should not cause a segmentation fault
         assert_raises(ValueError, setattr, ra.dtype, 'names', ('f1', 'f1'))
 
+    def test_eq_string_and_object_array(self):
+        # From e-mail thread "__eq__ with str and object" (Keith Goodman)
+        a1 = np.array(['a', 'b'], dtype=object)
+        a2 = np.array(['a', 'c'])
+        assert_array_equal(a1 == a2, [True, False])
+        assert_array_equal(a2 == a1, [True, False])
+
+
 if __name__ == "__main__":
     run_module_suite()