docs: def validate_param(self, method, param, value):
          """Validates the parameter according to the configurations in the
          _validations dictionary of either the method or the instance. This
          dictionaries are written by the decorator
          :func:`wsgiservice.decorators.validate`.
  
          .. todo:: Allow validation by type (e.g. header, post, query, etc.)
  
          :param method: A function to get the validation information from (done
                         using :func:`_get_validation`).
          :type method: Python function
          :param param: Name of the parameter to validate the value for.
          :type param: str
          :param value: Value passed in for the given parameter.
          :type value: Any valid Python value
  
          :raises: :class:`wsgiservice.exceptions.ValidationException` if the
              value is invalid for the given method and parameter.
          """
          rules = self._get_validation(method, param)
          if not rules:
              return
          if value is None or (isinstance(value, basestring) and len(value) == 0):
              raise ValidationException(
                  "Value for {0} must not be empty.".format(param))
          elif rules.get('re'):
              if not re.search('^' + rules['re'] + '$', value):
                  raise ValidationException(
                      "{0} value {1} does not validate.".format(param, value))