Python version of CAReduce now uses scalar_op.identity when reducing a 0-length array.

theano/tensor/elemwise.py

@@ -822,7 +822,14 @@ class CAReduce(Op):
         to_reduce = reversed(sorted(axis))
         if to_reduce:
             for dimension in to_reduce:
-                variable = self.ufunc.reduce(variable, dimension)
+                # If it's a zero-size array, use scalar_op.identity if available
+                if variable.shape[dimension] == 0:
+                    if hasattr(self.scalar_op, 'identity'):
+                        variable = self.scalar_op.identity
+                    else:
+                        raise ValueError("Input (%s) has zero-size on axis %s, but self.scalar_op (%s) has no attribute 'identity'" % (variable, dimension, self.scalar_op))
+                else:
+                    variable = self.ufunc.reduce(variable, dimension)
             output[0] = theano._asarray(variable, dtype = node.outputs[0].type.dtype)
         else:
             output[0] = numpy.copy(variable)