code: def validate(self):
          
          super(NumericFilterBase, self).validate()
  
          self.not_null = False
  
          if self._value.startswith('+'):
              self._cmp = operator.gt
              self._rt_cmp = 'greater'
              self._value = self._value[1:]
          elif self._value.startswith('-'):
              self._cmp = operator.lt
              self._rt_cmp = 'less'
              self._value = self._value[1:]
          else:
              self._cmp = operator.eq
              self._rt_cmp = 'equal'
docs: Validate filter condition (template method).