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.

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))