numpy · mattip · Sep 8, 2019 · Aug 28, 2019 · Aug 31, 2019 · Aug 31, 2019
diff --git a/doc/release/upcoming_changes/14393.c_api.rst b/doc/release/upcoming_changes/14393.c_api.rst
@@ -0,0 +1,5 @@
+PyDataType_ISUNSIZED(descr) now returns False for structured datatypes
+----------------------------------------------------------------------
+Previously this returned True for any datatype of itemsize 0, but now this
+returns false for the non-flexible datatype with itemsize 0, ``np.dtype([])``.
+
diff --git a/doc/source/reference/c-api/array.rst b/doc/source/reference/c-api/array.rst
@@ -997,6 +997,10 @@ argument must be a :c:type:`PyObject *<PyObject>` that can be directly interpret
     called on flexible dtypes. Types that are attached to an array will always
     be sized, hence the array form of this macro not existing.
 
+    .. versionchanged:: 1.18
+
+    For structured datatypes with no fields this function now returns False.
+
 .. c:function:: PyTypeNum_ISUSERDEF(num)
 
 .. c:function:: PyDataType_ISUSERDEF(descr)

diff --git a/numpy/core/include/numpy/ndarraytypes.h b/numpy/core/include/numpy/ndarraytypes.h
@@ -1695,7 +1695,8 @@ PyArray_CLEARFLAGS(PyArrayObject *arr, int flags)
 #define PyDataType_ISOBJECT(obj) PyTypeNum_ISOBJECT(((PyArray_Descr*)(obj))->type_num)
 #define PyDataType_HASFIELDS(obj) (((PyArray_Descr *)(obj))->names != NULL)
 #define PyDataType_HASSUBARRAY(dtype) ((dtype)->subarray != NULL)
-#define PyDataType_ISUNSIZED(dtype) ((dtype)->elsize == 0)
+#define PyDataType_ISUNSIZED(dtype) ((dtype)->elsize == 0 && \
+                                      !PyDataType_HASFIELDS(dtype))
 #define PyDataType_MAKEUNSIZED(dtype) ((dtype)->elsize = 0)
 
 #define PyArray_ISBOOL(obj) PyTypeNum_ISBOOL(PyArray_TYPE(obj))

diff --git a/numpy/core/src/multiarray/arrayobject.c b/numpy/core/src/multiarray/arrayobject.c
@@ -1200,15 +1200,28 @@ _void_compare(PyArrayObject *self, PyArrayObject *other, int cmp_op)
             }
         }
         if (res == NULL && !PyErr_Occurred()) {
-            PyErr_SetString(PyExc_ValueError, "No fields found.");
+            /* these dtypes had no fields. Use a MultiIter to broadcast them
+             * to an output array, and fill with True (for EQ)*/
+            PyArrayMultiIterObject *mit = (PyArrayMultiIterObject *)
+                                          PyArray_MultiIterNew(2, self, other);
+            if (mit == NULL) {
+                return NULL;
+            }
+
+            res = PyArray_NewFromDescr(&PyArray_Type,
+                                       PyArray_DescrFromType(NPY_BOOL),
+                                       mit->nd, mit->dimensions,
+                                       NULL, NULL, 0, NULL);
+            Py_DECREF(mit);
+            if (res) {
+                 PyArray_FILLWBYTE((PyArrayObject *)res,
+                                   cmp_op == Py_EQ ? 1 : 0);
+            }
         }
         return res;
     }
     else {
-        /*
-         * compare as a string. Assumes self and
-         * other have same descr->type
-         */
+        /* compare as a string. Assumes self and other have same descr->type */
         return _strings_richcompare(self, other, cmp_op, 0);
     }
 }

diff --git a/numpy/core/src/multiarray/ctors.c b/numpy/core/src/multiarray/ctors.c
@@ -3843,7 +3843,13 @@ PyArray_FromBuffer(PyObject *buf, PyArray_Descr *type,
     s = (npy_intp)ts - offset;
     n = (npy_intp)count;
     itemsize = type->elsize;
-    if (n < 0 ) {
+    if (n < 0) {
+        if (itemsize == 0) {
+            PyErr_SetString(PyExc_ValueError,
+                            "cannot determine count if itemsize is 0");
+            Py_DECREF(type);
+            return NULL;
+        }
         if (s % itemsize != 0) {
             PyErr_SetString(PyExc_ValueError,
                             "buffer size must be a multiple"\
@@ -4036,7 +4042,7 @@ PyArray_FromIter(PyObject *obj, PyArray_Descr *dtype, npy_intp count)
     }
     for (i = 0; (i < count || count == -1) &&
              (value = PyIter_Next(iter)); i++) {
-        if (i >= elcount) {
+        if (i >= elcount && elsize != 0) {
             npy_intp nbytes;
             /*
               Grow PyArray_DATA(ret):

diff --git a/numpy/core/src/multiarray/methods.c b/numpy/core/src/multiarray/methods.c
@@ -1861,7 +1861,7 @@ array_reduce_ex(PyArrayObject *self, PyObject *args)
         PyDataType_FLAGCHK(descr, NPY_ITEM_HASOBJECT) ||
         (PyType_IsSubtype(((PyObject*)self)->ob_type, &PyArray_Type) &&
          ((PyObject*)self)->ob_type != &PyArray_Type) ||
-        PyDataType_ISUNSIZED(descr)) {
+        descr->elsize == 0) {
         /* The PickleBuffer class from version 5 of the pickle protocol
          * can only be used for arrays backed by a contiguous data buffer.
          * For all other cases we fallback to the generic array_reduce

diff --git a/numpy/core/tests/test_dtype.py b/numpy/core/tests/test_dtype.py
@@ -419,6 +419,31 @@ def test_partial_dict(self):
         assert_raises(ValueError, np.dtype,
                 {'formats': ['i4', 'i4'], 'f0': ('i4', 0), 'f1':('i4', 4)})
 
+    def test_fieldless_views(self):
+        a = np.zeros(2, dtype={'names':[], 'formats':[], 'offsets':[], 
+                               'itemsize':8})
+        assert_raises(ValueError, a.view, np.dtype([]))
+
+        d = np.dtype((np.dtype([]), 10))
+        assert_equal(d.shape, (10,))
+        assert_equal(d.itemsize, 0)
+        assert_equal(d.base, np.dtype([]))
+
+        arr = np.fromiter((() for i in range(10)), [])
+        assert_equal(arr.dtype, np.dtype([]))
+        assert_raises(ValueError, np.frombuffer, b'', dtype=[])
+        assert_equal(np.frombuffer(b'', dtype=[], count=2),
+                     np.empty(2, dtype=[]))
+
+        assert_raises(ValueError, np.dtype, ([], 'f8'))
+        assert_raises(ValueError, np.zeros(1, dtype='i4').view, [])
+
+        assert_equal(np.zeros(2, dtype=[]) == np.zeros(2, dtype=[]),
+                     np.ones(2, dtype=bool))
+
+        assert_equal(np.zeros((1, 2), dtype=[]) == a,
+                     np.ones((1, 2), dtype=bool))
+
 
 class TestSubarray(object):
     def test_single_subarray(self):