CBV源码分析

Posted 2021-11-07 kp1995

FBV和CBV

FBV(function base views) : 在视图层中使用函数处理请求

CBV(class base views): 在视图层中使用类处理请求

Python是一个面向对象的编程语言, 面向对象的优点(继承,封装,多态), 使用CBV,用类写view,这样的做的优点:

• 提高代码的服用性,可以使用面向对象的技术,比如Mixin(多继承)
• 可以用不同的函数针对不同的HTTP方法处理,而不是用过if判断,提高代码的可读性

CBV简单示例

# 在urls.py中进行路由配置
urlpatterns = [
    # (正则表达式,  函数内存地址)
    url(r'^admin/', admin.site.urls),
    url(r'^books/$', views.Books.as_view()),
]

# views视图中
from django.views import  View
class Books(View):
    def get(self,request, *args, **kwargs):
        print("get")
        return HttpResponse("ok")

    def dispatch(self, request, *args, **kwargs):
        print("dispatch")
        ret = super(Books,self).dispatch(request, *args, **kwargs)
        print("ret",ret)
        return  HttpResponse(ret)
      
'''
输出结果为:
dispatch
get
ret <HttpResponse status_code=200, "text/html; charset=utf-8">
'''

源码分析

# 从views.Books.as_view()入手, 先了解as_view方法

class View(object):
    http_method_names = ['get', 'post', 'put', 'patch', 'delete', 'head', 'options', 'trace']

    def __init__(self, **kwargs):
        "省略"

    @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=())
        return view

由于配置的views.IndexView.as_view()参数为null,所以在for key in initkwargs会直接跳过，如果不为null,就去判断执行接下来的代码，大概意思就是，if key in cls.http_method_names:和if not hasattr(cls, key):，如果传递过来的字典中某键包含在 http_method_names列表中列表中和本类中没有该键属性都会跑出异常

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

as_view函数下内置一个方法def view(request, *args, **kwargs):

                
  # 函数可以当做对象来使用, 一切皆对象
  # 为view函数添加了属性
            view.view_class = cls
        view.view_initkwargs = initkwargs

      
  # 再次对view函数添加属性
        # 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=())

# 点进 update_wrapper

WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__qualname__', '__doc__',
                       '__annotations__')
WRAPPER_UPDATES = ('__dict__',)
def update_wrapper(wrapper,
                   wrapped,
                   assigned = WRAPPER_ASSIGNMENTS,
                   updated = WRAPPER_UPDATES):
    """Update a wrapper function to look like the wrapped function

       wrapper is the function to be updated
       wrapped is the original function
       assigned is a tuple naming the attributes assigned directly
       from the wrapped function to the wrapper function (defaults to
       functools.WRAPPER_ASSIGNMENTS)
       updated is a tuple naming the attributes of the wrapper that
       are updated with the corresponding attribute from the wrapped
       function (defaults to functools.WRAPPER_UPDATES)
    """
    for attr in assigned:
        try:
            value = getattr(wrapped, attr)
        except AttributeError:
            pass
        else:
            setattr(wrapper, attr, value)
    for attr in updated:
        getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
    # Issue #17482: set __wrapped__ last so we don't inadvertently copy it
    # from the wrapped function when updating __dict__
    wrapper.__wrapped__ = wrapped
    # Return the wrapper so this can be used as a decorator via partial()
    return wrapper

为view函数添加了一些属性，用来更新某些操作，以及可以获取某些数据操作，比如WRAPPER_UPDATES = (‘__dict__‘,)

# view函数
  
            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

为本类实例对象赋值request、args、kwargs，最后执行return self.dispatch(request, *args, **kwargs) (相当于一个装饰器的功能, 可以自定义内容) , 所以views.Books.as_view()中的url配置最后会返回view方法

view方法体再次对本类对象进行了一些封装,也是面向对象最主要的特征.比如:self.request = request, self.args = args, self.kwargs = kwargs

# dispatch函数

    def dispatch(self, request, *args, **kwargs):
        # Try to dispatch to the right method; if a method doesn't exist,
        # defer to the error handler. Also defer to the error handler if the
        # request method isn't on the approved list.
        if request.method.lower() in self.http_method_names:
            handler = getattr(self, request.method.lower(), self.http_method_not_allowed)
        else:
            handler = self.http_method_not_allowed
        return handler(request, *args, **kwargs)

通过判断request的请求方式，通过反射的方式，判断是否在http_method_names列表中，没有的话进行相关处理，赋值操作handler，handler = self.http_method_not_allowed

如果没有找到, 就会构造一个默认的

    def http_method_not_allowed(self, request, *args, **kwargs):
        logger.warning(
            'Method Not Allowed (%s): %s', request.method, request.path,
            extra={'status_code': 405, 'request': request}
        )
        return http.HttpResponseNotAllowed(self._allowed_methods())

class HttpResponseNotAllowed(HttpResponse):
    status_code = 405

    def __init__(self, permitted_methods, *args, **kwargs):
        super(HttpResponseNotAllowed, self).__init__(*args, **kwargs)
        self['Allow'] = ', '.join(permitted_methods)

    def __repr__(self):
        return '<%(cls)s [%(methods)s] status_code=%(status_code)d%(content_type)s>' % {
            'cls': self.__class__.__name__,
            'status_code': self.status_code,
            'content_type': self._content_type_for_repr,
            'methods': self['Allow'],
        }

handler(request, *args, **kwargs)，其实就是执行代码中CBV类Books的get方法