我记得5年前来搜狐面试时一个问题就是关于装饰器如何保持函数签名的问题。

由来

xadmin通过实现自己的BaseAdminView（继承自Django的View）来完成xadmin后台界面的处理。在解决一个csrf的问题时，翻了下xadmin BaseAdminView和Django的View部分的代码，关键点少了一条 update_wrapper 使用。导致我的小伙伴调试了半天。

还是看代码

可以对比看下:

## xadmin代码
@classonlymethod
def as_view(cls):
    def view(request, *args, **kwargs):
        self = cls(request, *args, **kwargs)

        if hasattr(self, 'get') and not hasattr(self, 'head'):
            self.head = self.get

        if self.request_method in self.http_method_names:
            handler = getattr(
                self, self.request_method, self.http_method_not_allowed)
        else:
            handler = self.http_method_not_allowed

        return handler(request, *args, **kwargs)

    # take name and docstring from class
    update_wrapper(view, cls, updated=())
    view.need_site_permission = cls.need_site_permission

    return view

### django/views/generic/base.py:class View中的代码
@classonlymethod
def as_view(cls, **initkwargs):
    """Main entry point for a request-response process."""
    for key in initkwargs:
        if key in cls.http_method_names:
            raise TypeError("You tried to pass in the %s method name as a "
                            "keyword argument to %s(). Don't do that."
                            % (key, cls.__name__))
        if not hasattr(cls, key):
            raise TypeError("%s() received an invalid keyword %r. as_view "
                            "only accepts arguments that are already "
                            "attributes of the class." % (cls.__name__, key))

    def view(request, *args, **kwargs):
        self = cls(**initkwargs)
        if hasattr(self, 'get') and not hasattr(self, 'head'):
            self.head = self.get
        self.request = request
        self.args = args
        self.kwargs = kwargs
        return self.dispatch(request, *args, **kwargs)
    view.view_class = cls
    view.view_initkwargs = initkwargs

    # take name and docstring from class
    update_wrapper(view, cls, updated=())

    # and possible attributes set by decorators
    # like csrf_exempt from dispatch
    update_wrapper(view, cls.dispatch, assigned=())    ## 差了这个啊！同学们
    return view

xadmin是直接把dispatch的代码放到内部的view中了，这样看起来直观，但是缺少了对外可配置(通过重写增加装饰器）的dispatch函数。导致无法对view进行csrf_exempt装饰(其实是dispatch上装饰@csrf_exempt时，装饰器返回的inner上会设置csrf_exempt = True的属性)。

update_wrapper的用法

上述代码应该挺明显了，update_wrapper的作用就是把cls.dispatch上的所有属性全部赋值到装饰函数上，也就是代码中的 view。

在Python中有几个库是“居家旅行”必备的，functools就是之一，其中的partial也十分有用，用法参考这里python中functools宝库下的partial。

参考

• https://docs.python.org/2/library/functools.html#functools.update_wrapper