BUG: Respect itershape=() in nditer

Previously this was treated as itershape=None, which meant something else.

The docstring was added only a handful of commits ago, so it shouldn't matter that the documented default "changed".

3 files changed, 12 insertions, 9 deletions

diff --git a/numpy/core/_add_newdocs.py b/numpy/core/_add_newdocs.py
index 68902d25a..b963f32b8 100644
--- a/numpy/core/_add_newdocs.py
+++ b/numpy/core/_add_newdocs.py
@@ -152,7 +152,7 @@ add_newdoc('numpy.core', 'flatiter', ('copy',
 
 add_newdoc('numpy.core', 'nditer',
     """
-    nditer(op, flags=None, op_flags=None, op_dtypes=None, order='K', casting='safe', op_axes=None, itershape=(), buffersize=0)
+    nditer(op, flags=None, op_flags=None, op_dtypes=None, order='K', casting='safe', op_axes=None, itershape=None, buffersize=0)
 
     Efficient multi-dimensional iterator object to iterate over arrays.
     To get started using this object, see the
diff --git a/numpy/core/src/multiarray/nditer_pywrap.c b/numpy/core/src/multiarray/nditer_pywrap.c
index add40f460..505c5a841 100644
--- a/numpy/core/src/multiarray/nditer_pywrap.c
+++ b/numpy/core/src/multiarray/nditer_pywrap.c
@@ -17,6 +17,7 @@
 #include "npy_pycompat.h"
 #include "alloc.h"
 #include "common.h"
+#include "conversion_utils.h"
 #include "ctors.h"
 
 /* Functions not part of the public NumPy C API */
@@ -748,7 +749,7 @@ npyiter_init(NewNpyArrayIterObject *self, PyObject *args, PyObject *kwds)
     int oa_ndim = -1;
     int op_axes_arrays[NPY_MAXARGS][NPY_MAXDIMS];
     int *op_axes[NPY_MAXARGS];
-    PyArray_Dims itershape = {NULL, 0};
+    PyArray_Dims itershape = {NULL, -1};
     int buffersize = 0;
 
     if (self->iter != NULL) {
@@ -765,7 +766,7 @@ npyiter_init(NewNpyArrayIterObject *self, PyObject *args, PyObject *kwds)
                     npyiter_order_converter, &order,
                     PyArray_CastingConverter, &casting,
                     &op_axes_in,
-                    PyArray_IntpConverter, &itershape,
+                    PyArray_OptionalIntpConverter, &itershape,
                     &buffersize)) {
         npy_free_cache_dim_obj(itershape);
         return -1;
@@ -800,7 +801,7 @@ npyiter_init(NewNpyArrayIterObject *self, PyObject *args, PyObject *kwds)
         }
     }
 
-    if (itershape.len > 0) {
+    if (itershape.len != -1) {
         if (oa_ndim == -1) {
             oa_ndim = itershape.len;
             memset(op_axes, 0, sizeof(op_axes[0]) * nop);
@@ -812,10 +813,6 @@ npyiter_init(NewNpyArrayIterObject *self, PyObject *args, PyObject *kwds)
             goto fail;
         }
     }
-    else if (itershape.ptr != NULL) {
-        npy_free_cache_dim_obj(itershape);
-        itershape.ptr = NULL;
-    }
 
     self->iter = NpyIter_AdvancedNew(nop, op, flags, order, casting, op_flags,
                                   op_request_dtypes,
diff --git a/numpy/core/tests/test_nditer.py b/numpy/core/tests/test_nditer.py
index 24272bb0d..7eca9ec3a 100644
--- a/numpy/core/tests/test_nditer.py
+++ b/numpy/core/tests/test_nditer.py
@@ -2688,7 +2688,13 @@ def test_0d_iter():
     i = nditer(np.arange(5), ['multi_index'], [['readonly']], op_axes=[()])
     assert_equal(i.ndim, 0)
     assert_equal(len(i), 1)
-    # note that itershape=(), still behaves like None due to the conversions
+
+    i = nditer(np.arange(5), ['multi_index'], [['readonly']], op_axes=[()], itershape=())
+    assert_equal(i.ndim, 0)
+    assert_equal(len(i), 1)
+
+    # passing an itershape alone is not enough, the op_axes are also needed
+    assert_raises(ValueError, nditer, np.arange(5), ['multi_index'], [['readonly']], itershape=())
 
     # Test a more complex buffered casting case (same as another test above)
     sdt = [('a', 'f4'), ('b', 'i8'), ('c', 'c8', (2, 3)), ('d', 'O')]