'Validate list.
Returns:
A valid value.
Raises:
BadValueError if property is not a list whose items are instances of
the item_type given to the constructor.'

def validate(self, value):

    value = super(ListProperty, self).validate(value)
    if (value is not None):
        if (not isinstance(value, list)):
            raise BadValueError(('Property    %s    must    be    a    list' % self.name))
        value = self.validate_list_contents(value)
    return value
