Fix broadcasting in sparse dot

theano/sparse/basic.py

@@ -4008,28 +4008,34 @@ class Dot(gof.op.Op):
                 "sparse variable as inputs, but the inputs are "
                 "%s (%s) and %s (%s)." % (x, x.type, y, y.type))
 
-        if not x_is_sparse_var:
+        if x_is_sparse_var:
+            broadcast_x = (False,) * x.ndim
+        else:
             x = tensor.as_tensor_variable(x)
+            broadcast_x = x.type.broadcastable
             assert y.format in ["csr", "csc"]
             if x.ndim not in (1, 2):
                 raise TypeError(
                     'theano.sparse.Dot: input 0 (0-indexed) must have ndim of '
                     '1 or 2, %d given.' % x.ndim)
 
-        if not y_is_sparse_var:
+        if y_is_sparse_var:
+            broadcast_y = (False,) * y.ndim
+        else:
             y = tensor.as_tensor_variable(y)
+            broadcast_y = y.type.broadcastable
             assert x.format in ["csr", "csc"]
             if y.ndim not in (1, 2):
                 raise TypeError(
                     'theano.sparse.Dot: input 1 (1-indexed) must have ndim of '
                     '1 or 2, %d given.' % y.ndim)
 
-        if y.ndim == 1 or x.ndim == 1:
-            bz = (False,)
-        else:
-            bz = (False, False)
+        if len(broadcast_y) == 2:
+            broadcast_out = broadcast_x[:-1] + broadcast_y[1:]
+        elif len(broadcast_y) == 1:
+            broadcast_out = broadcast_x[:-1]
         return gof.Apply(self, [x, y], [tensor.tensor(dtype=dtype_out,
-                                                      broadcastable=bz)])
+                                                      broadcastable=broadcast_out)])
 
     def perform(self, node, inputs, out):
         x, y = inputs

theano/sparse/tests/test_basic.py

@@ -464,6 +464,12 @@ class SparseInferShapeTester(utt.InferShapeTester):
                                config.floatX, 3))],
                 Dot)
 
+    def test_dot_broadcast(self):
+        A = sp.matrix('csr')
+        b = tensor.vector()
+        bc = sp.dot(A, b[:, None]).broadcastable
+        assert bc == (False, True)
+
     def test_structured_dot(self):
         x = SparseType('csc', dtype=config.floatX)()
         y = SparseType('csc', dtype=config.floatX)()