Python rest-framework 中类的继承关系(as_view)

一. 背景

最近几天一直在学习restful framework的源代码，用户请求的流程，在路由系统这块遇到一个疑问，关于类的继承关系，当请求进来到路由这块，执行as_view()方法的时候，为什么会运行父类View的as_view()方法再执行到APIView的dispatch方法呢？这里记录一下一遍后面方便自己查阅

二. 代码示例

1. 路由

from django.conf.urls import url
from api import views as api_view

urlpatterns = [
    url(r'^index/', api_view.IndexView.as_view()),
]

2.  视图类

from rest_framework.response import  Response
from rest_framework.views import APIView

class IndexView(APIView):
    def get(self,request):
        return Response('...')

三. 源代码分析

a. APIView的as_view方法

class APIView(View):

    @classmethod
    def as_view(cls, **initkwargs):

        if isinstance(getattr(cls, 'queryset', None), models.query.QuerySet):
            def force_evaluation():
                raise RuntimeError(
                    'Do not evaluate the `.queryset` attribute directly, '
                    'as the result will be cached and reused between requests. '
                    'Use `.all()` or call `.get_queryset()` instead.'
                )
            cls.queryset._fetch_all = force_evaluation

        view = super(APIView, cls).as_view(**initkwargs)
        # 这里是继承了父类的方法as_view()

        view.cls = cls
        view.initkwargs = initkwargs

        return csrf_exempt(view)

# 代码太多只截取部分代码

b. super(APIView,cls).as_view(**initkwargs)执行了什么操作

# super(APIView,self) 首先找到 APIView的父类（就是类 View），然后把View类的as_view属性 转换为类 APIView的属性
# 相当于将View中的as_view()中的代码复制到API_View中的as_view中

所以最终运行 super(APIView,cls).as_view(**initkwargs)执行了View中的as_view方法

class View(object):

    http_method_names = ['get', 'post', 'put', 'patch', 'delete', 'head', 'options', 'trace']

    def __init__(self, **kwargs):

        for key, value in six.iteritems(kwargs):
            setattr(self, key, value)

    @classonlymethod
    def as_view(cls, **initkwargs):

        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)
            # 运行这里吧最后执行了self.dispatch
        view.view_class = cls
        view.view_initkwargs = initkwargs

        update_wrapper(view, cls, updated=())

        update_wrapper(view, cls.dispatch, assigned=())
        return view       # 返回了view函数