ENH: Allow all allocated operands in nditer/NpyIter

This allows all operands to be allocated,  it is necessary to
provide the dtype in this case, if missing the error changes
from ``ValueError`` to ``TypeError``

Closes gh-13934, gh-15140

2 files changed, 13 insertions, 19 deletions

diff --git a/numpy/core/src/multiarray/nditer_constr.c b/numpy/core/src/multiarray/nditer_constr.c
index b6acce570..9134e9f04 100644
--- a/numpy/core/src/multiarray/nditer_constr.c
+++ b/numpy/core/src/multiarray/nditer_constr.c
@@ -1225,22 +1225,6 @@ npyiter_prepare_operands(int nop, PyArrayObject **op_in,
         }
     }
 
-    /* If all the operands were NULL, it's an error */
-    if (op[0] == NULL) {
-        int all_null = 1;
-        for (iop = 1; iop < nop; ++iop) {
-            if (op[iop] != NULL) {
-                all_null = 0;
-                break;
-            }
-        }
-        if (all_null) {
-            PyErr_SetString(PyExc_ValueError,
-                    "At least one iterator operand must be non-NULL");
-            goto fail_nop;
-        }
-    }
-
     if (any_writemasked_ops && maskop < 0) {
         PyErr_SetString(PyExc_ValueError,
                 "An iterator operand was flagged as WRITEMASKED, "
diff --git a/numpy/core/tests/test_nditer.py b/numpy/core/tests/test_nditer.py
index b43bc50e9..473ea9754 100644
--- a/numpy/core/tests/test_nditer.py
+++ b/numpy/core/tests/test_nditer.py
@@ -1594,11 +1594,12 @@ def test_iter_allocate_output_errors():
     # Allocated output can't have buffering without delayed bufalloc
     assert_raises(ValueError, nditer, [a, None], ['buffered'],
                                             ['allocate', 'readwrite'])
-    # Must specify at least one input
-    assert_raises(ValueError, nditer, [None, None], [],
+    # Must specify dtype if there are no inputs (cannot promote existing ones;
+    # maybe this should use the 'f4' here, but it does not historically.)
+    assert_raises(TypeError, nditer, [None, None], [],
                         [['writeonly', 'allocate'],
                          ['writeonly', 'allocate']],
-                        op_dtypes=[np.dtype('f4'), np.dtype('f4')])
+                        op_dtypes=[None, np.dtype('f4')])
     # If using op_axes, must specify all the axes
     a = arange(24, dtype='i4').reshape(2, 3, 4)
     assert_raises(ValueError, nditer, [a, None], [],
@@ -1623,6 +1624,15 @@ def test_iter_allocate_output_errors():
                         op_dtypes=[None, np.dtype('f4')],
                         op_axes=[None, [0, np.newaxis, 2]])
 
+def test_all_allocated():
+    # When no output and no shape is given, `()` is used as shape.
+    i = np.nditer([None], op_dtypes=["int64"])
+    assert i.operands[0].shape == ()
+    assert i.dtypes == (np.dtype("int64"),)
+
+    i = np.nditer([None], op_dtypes=["int64"], itershape=(2, 3, 4))
+    assert i.operands[0].shape == (2, 3, 4)
+
 def test_iter_remove_axis():
     a = arange(24).reshape(2, 3, 4)