'Returns the TransformerData associated with the given
key.  If not found, creates and returns a new TransformerData
and maps it to the given key.'

def get_or_create(self, key):

    try:
        return self[key]
    except KeyError:
        new_transformer_data = TransformerData()
        self[key] = new_transformer_data
        return new_transformer_data
