python类missing()的实例源码

def _func2method(func, method_name):
    def method(self, name, default=ma.missing, subcast=None, **kwargs):
        name = self._prefix + name if self._prefix else name
        parsed_key, raw_value = _get_from_environ(name, default)
        if raw_value is ma.missing:
            raise EnvError('Environment variable "{}" not set'.format(parsed_key))
        value = func(raw_value, **kwargs)
        self._fields[name] = ma.fields.Field(**kwargs)
        self._values[name] = value
        return value
    method.__name__ = str(method_name)  # cast to str for Py2 compat
    return method