<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def lookup_field_class(self, field, obj=None, default=None):
 """ Looks up any additional class we should include when rendering this field """

        css = ""

        # is there a class specified for this field
        if field in self.field_config and 'class' in self.field_config[field]:
            css = self.field_config[field]['class']

        # if we were given a default, use that
        elif default:
            css = default

        return css