Allow mixed ndarray-sparse ops where the shape doesn't change.

sparse/coo/umath.py

@@ -400,7 +400,6 @@ def __init__(self, func, *args, **kwargs):
         kwargs : dict
             Extra arguments to pass to the function.
         """
-
         from .core import COO
         from ..sparse_array import SparseArray
 
@@ -426,14 +425,18 @@ def __init__(self, func, *args, **kwargs):
         self.kwargs = kwargs
         self.cache = {}
 
-        self._get_fill_value()
         self._check_broadcast()
+        self._get_fill_value()
 
     def get_result(self):
         from .core import COO
         if self.args is None:
             return NotImplemented
 
+        if self.shape == self.ndarray_shape:
+            args = [a.todense() if isinstance(a, COO) else a for a in self.args]
+            return self.func(*args, **self.kwargs)
+
         if any(s == 0 for s in self.shape):
             data = np.empty((0,), dtype=self.fill_value.dtype)
             coords = np.empty((0, len(self.shape)), dtype=np.intp)
@@ -487,7 +490,7 @@ def _get_fill_value(self):
                 arg.fill_value if isinstance(arg, COO) else _zero_of_dtype(arg.dtype) for arg in self.args)
             fill_value = self.func(*zero_args, **self.kwargs)[()]
 
-        if not equivalent(fill_value, fill_value_array).all():
+        if not equivalent(fill_value, fill_value_array).all() and self.shape != self.ndarray_shape:
             raise ValueError('Performing a mixed sparse-dense operation that would result in a dense array. '
                              'Please make sure that func(sparse_fill_values, ndarrays) is a constant array.')
 
@@ -512,12 +515,17 @@ def _check_broadcast(self):
         non_ndarray_shape = _get_nary_broadcast_shape(
             *tuple(arg.shape for arg in self.args if isinstance(arg, COO))
         )
+        ndarray_shape = _get_nary_broadcast_shape(
+            *tuple(arg.shape for arg in self.args if isinstance(arg, np.ndarray))
+        )
 
         if full_shape != non_ndarray_shape:
             raise ValueError('Please make sure that the broadcast shape of just the sparse arrays is '
                              'the same as the broadcast shape of all the operands.')
 
         self.shape = full_shape
+        self.ndarray_shape = ndarray_shape
+        self.non_ndarray_shape = non_ndarray_shape
 
     def _get_func_coords_data(self, mask):
         """

tests/test_coo.py

@@ -840,7 +840,7 @@ def test_sparsearray_elemwise(format):
 
 
 def test_ndarray_densification_fails():
-    xs = sparse.random((3, 4), density=0.5)
+    xs = sparse.random((2, 3, 4), density=0.5)
     y = np.random.rand(3, 4)
 
     with pytest.raises(ValueError):
@@ -1536,7 +1536,7 @@ def test_cache_csr():
 def test_empty_shape():
     x = COO(np.empty((0, 1), dtype=np.int8), [1.0])
     assert x.shape == ()
-    assert ((2 * x).todense() == np.array(2.0)).all()
+    assert_eq(2 * x, np.float_(2.0))
 
 
 def test_single_dimension():