Enable py::ellipsis tests on Python 2 and mention Ellipsis in the docs

Author: YannickJadoul

docs/advanced/pycpp/numpy.rst

@@ -371,6 +371,8 @@ Ellipsis
 Python 3 provides a convenient ``...`` ellipsis notation that is often used to
 slice multidimensional arrays. For instance, the following snippet extracts the
 middle dimensions of a tensor with the first and last index set to zero.
+In Python 2, the syntactic sugar ``...`` is not available, but the singleton
+``Ellipsis`` (of type ``ellispsis``) can still be used directly.
 
 .. code-block:: python
 

tests/test_numpy_array.cpp

@@ -382,9 +382,7 @@ TEST_SUBMODULE(numpy_array, sm) {
         return a;
     });
 
-#if PY_MAJOR_VERSION >= 3
-        sm.def("index_using_ellipsis", [](py::array a) {
-            return a[py::make_tuple(0, py::ellipsis(), 0)];
-        });
-#endif
+    sm.def("index_using_ellipsis", [](py::array a) {
+        return a[py::make_tuple(0, py::ellipsis(), 0)];
+    });
 }

tests/test_numpy_array.py

@@ -431,7 +431,6 @@ def test_array_create_and_resize(msg):
     assert(np.all(a == 42.))
 
 
-@pytest.unsupported_on_py2
 def test_index_using_ellipsis():
     a = m.index_using_ellipsis(np.zeros((5, 6, 7)))
     assert a.shape == (6,)