Subclassing Expression with non-hard-coded value_shape

Is it possible to subclass Expression without hard-coding the value_shape? For example, I would like to do something like

class ExpSubClass(Expression):

    def __init__(self, V, **kwargs):
 self.ndims = kwargs['ndims']

    def eval_cell(self, values, x, ufc_cell):
        values[:] = whatever....

    def value_shape(self):
        return self.ndims, # instead of "return 3," for example

But if I do this, I get the following error:

Traceback (most recent call last):
....
  File "/home/s/steinman/krisvs/dorsal-trunk/Work/FEniCS/lib/python2.7/site-packages/dolfin/functions/expression.py", line 177, in __init__
    element = _auto_select_element_from_shape(self.value_shape(),
  File "/scratch/s/steinman/jmynard/compflow/problems/pipewave.py", line 59, in value_shape
    return self.ndims,
AttributeError: 'ExpSubClass' object has no attribute 'ndims'

After some testing, it would appear that value_shape gets called before __init__ . I have read the documentation about subclassing Expression which says **kwargs should be used, but I'm not sure if I have understood the "magic behind the scenes" correctly :)

Thanks.