This fix gh-847.

theano/sparse/basic.py

@@ -267,6 +267,9 @@ class _sparse_py_operators:
     T = property(lambda self: transpose(self),
                  doc="Return aliased transpose of self (read-only)")
 
+    def astype(self, dtype):
+        return cast(self, dtype)
+
     def __neg__(self):
         return neg(self)
 
@@ -1031,6 +1034,9 @@ ccast = Cast('complex64')
 zcast = Cast('complex128')
 
 
+def cast(variable, dtype):
+    return Cast(dtype)(variable)
+
 #
 # Conversion
 #

theano/sparse/tests/test_basic.py

@@ -1951,7 +1951,7 @@ class TestCast(utt.InferShapeTester):
 
     def test_cast(self):
         cast_csc = dict([
-            (x, [theano.function([x], Cast(t)(x))
+            (x, [theano.function([x], x.astype(t))
                  for t in self.compatible_types])
             for x in self.x_csc])