ENH: Allow removal of 0-sized axes from nditer and empty gufunc calls

The caller should take of not using pointers given in such a case

3 files changed, 18 insertions, 43 deletions

diff --git a/doc/release/1.13.0-notes.rst b/doc/release/1.13.0-notes.rst
index bf4f1c815..f802b4c12 100644
--- a/doc/release/1.13.0-notes.rst
+++ b/doc/release/1.13.0-notes.rst
@@ -99,6 +99,24 @@ now issue a DeprecationWarning - ``.__getitem__(slice(start, end))`` should be
 used instead.
 
 
+C API
+-----
+
+GUfuncs on empty arrays and NpyIter axis removal
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It is now allowed to remove a zero-sized axis from NpyIter. Which may mean
+that code removing axes from NpyIter has to add an additional check when
+accessing the removed dimensions later on.
+
+The largest followup change is that gufuncs are now allowed to have zero-sized
+inner dimensions. This means that a gufunc now has to anticipate an empty inner
+dimension, while this was never possible and an error raised instead.
+
+For most gufuncs no change should be necessary. However, it is now possible
+for gufuncs with a signature such as ``(..., N, M) -> (..., M)`` to return
+a valid result if ``N=0`` without further wrapping code.
+
+
 New Features
 ============
 
diff --git a/numpy/core/src/multiarray/nditer_api.c b/numpy/core/src/multiarray/nditer_api.c
index da3c85ae0..f2bc23715 100644
--- a/numpy/core/src/multiarray/nditer_api.c
+++ b/numpy/core/src/multiarray/nditer_api.c
@@ -106,12 +106,6 @@ NpyIter_RemoveAxis(NpyIter *iter, int axis)
         return NPY_FAIL;
     }
 
-    if (NAD_SHAPE(axisdata_del) == 0) {
-        PyErr_SetString(PyExc_ValueError,
-                "cannot remove a zero-sized axis from an iterator");
-        return NPY_FAIL;
-    }
-
     /* Adjust the permutation */
     for (idim = 0; idim < ndim-1; ++idim) {
         npy_int8 p = (idim < xdim) ? perm[idim] : perm[idim+1];
diff --git a/numpy/core/src/umath/ufunc_object.c b/numpy/core/src/umath/ufunc_object.c
index caa36bd2d..52c2a6454 100644
--- a/numpy/core/src/umath/ufunc_object.c
+++ b/numpy/core/src/umath/ufunc_object.c
@@ -3269,14 +3269,6 @@ PyUFunc_Accumulate(PyUFuncObject *ufunc, PyArrayObject *arr, PyArrayObject *out,
         op[0] = NpyIter_GetOperandArray(iter)[0];
         op[1] = NpyIter_GetOperandArray(iter)[1];
 
-        if (PyArray_SIZE(op[0]) == 0) {
-            if (out == NULL) {
-                out = op[0];
-                Py_INCREF(out);
-            }
-            goto finish;
-        }
-
         if (NpyIter_RemoveAxis(iter, axis) != NPY_SUCCEED) {
             goto fail;
         }
@@ -3623,35 +3615,6 @@ PyUFunc_Reduceat(PyUFuncObject *ufunc, PyArrayObject *arr, PyArrayObject *ind,
         need_outer_iterator = 1;
     }
 
-    /* Special case when the index array's size is zero */
-    if (ind_size == 0) {
-        if (out == NULL) {
-            npy_intp out_shape[NPY_MAXDIMS];
-            memcpy(out_shape, PyArray_SHAPE(arr),
-                            PyArray_NDIM(arr) * NPY_SIZEOF_INTP);
-            out_shape[axis] = 0;
-            Py_INCREF(op_dtypes[0]);
-            op[0] = out = (PyArrayObject *)PyArray_NewFromDescr(
-                                        &PyArray_Type, op_dtypes[0],
-                                        PyArray_NDIM(arr), out_shape, NULL, NULL,
-                                        0, NULL);
-            if (out == NULL) {
-                goto fail;
-            }
-        }
-        else {
-            /* Allow any zero-sized output array in this case */
-            if (PyArray_SIZE(out) != 0) {
-                PyErr_SetString(PyExc_ValueError,
-                        "output operand shape for reduceat is "
-                        "incompatible with index array of shape (0,)");
-                goto fail;
-            }
-        }
-
-        goto finish;
-    }
-
     if (need_outer_iterator) {
         npy_uint32 flags = NPY_ITER_ZEROSIZE_OK|
                            NPY_ITER_REFS_OK|