'Return a valid list of types.
`types` may be a string or a list of strings.
If `types` is not supplied, returns the default_types.'

def validate_types(self, types=None):

    types = (types or self.default_types)
    if (types is None):
        types = []
    if isinstance(types, six.string_types):
        types = [types]
    return types
