理解Django中基于类的视图-视图基类

基于类的视图或CBV是Django中争议最大的功能之一。与基于功能的视图（FBV）相比，CBV似乎更令人困惑和难以理解。在本系列的Django中的《理解基于类的视图》中，我们将详细介绍CBV，以了解它们的工作方式和使用方法。

首先，我们将介绍View基类，如何在URLconf中使用CBV以及View类如何路由从其继承的其他类的视图逻辑。

先决条件
本文针对那些以前可能尝试使用CBV并希望了解其工作原理的人。如果您满足以下条件，您将从本文中获得最大收益：

之前已经用Django建立了一个项目
尝试至少使用一次CBV
对Python中的类有基本的了解
本文包含许多不完整的代码段，用于说明目的。如果您以前使用过Django，那么了解这些片段在项目中的适合位置就不难了。我尝试添加尽可能多的上下文，但是省略了许多支持代码，以使文章的长度易于理解。

CBV的调用方式
让我们开始看看与FBV相比，我们如何在URLconf中使用CBV。假设在项目的urls.py文件中有一个名为MyView的CBV和一个名为my_view的FBV，我们在path（）函数中将它们用作urlpatterns的一部分。

# Class-Based View
path('new-cbv/', MyView.as_view(), name='new_cbv')

# Function-Based View
path('new-fbv/', my_view, name='new_fbv')

# Class-Based View

path('new-cbv/', MyView.as_view(), name='new_cbv')

# Function-Based View

path('new-fbv/', my_view, name='new_fbv')

Django希望path（）的第二个参数是一个函数。这意味着我们可以直接将FBV提供给path（）。我们提供my_view而不是my_view（），因为我们不想调用该函数。 Django稍后会调用它并适当地使用它。

使用CBV是不同的。最初，我们可能希望可以将MyView类直接传递给path（）。

# Wrong way to use a CBV in the URLconf
path('new-cbv/', MyView, name='new_cbv')

1 2	# Wrong way to use a CBV in the URLconf path('new-cbv/', MyView, name='new_cbv')

但是，这将不起作用，因为path（）不希望将类用作视图的参数。我们需要以某种方式从类中获取一个函数。我们通过在MyView上调用.as_view（）来实现。但是使用MyView.as_view（）时，我们正在调用一个函数，而不是像使用FBV那样直接传递一个函数。我们尚不知道.as_view（）返回什么，但是，如果path（）需要一个函数，则.as_view（）必须返回一个。为了找出返回的结果，我们必须深入研究Django的内置View基类。

深入View类
所有CBV都从View继承为其基类。 CBV可能继承自许多不同的类和mixin，但它们都以View开头。让我们直接从Django源代码中查看View背后的代码。

class View:
        """
        Intentionally simple parent class for all views. Only implements
        dispatch-by-method and simple sanity checking.
        """

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

        def __init__(self, **kwargs):
            """
            Constructor. Called in the URLconf; can contain helpful extra
            keyword arguments, and other things.
            """
            # Go through keyword arguments, and either save their values to our
            # instance, or raise an error.
            for key, value in kwargs.items():
                setattr(self, key, value)

        @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(
                        'The method name %s is not accepted as a keyword argument '
                        'to %s().' % (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)
                self.setup(request, *args, **kwargs)
                if not hasattr(self, 'request'):
                    raise AttributeError(
                        "%s instance has no 'request' attribute. Did you override "
                        "setup() and forget to call super()?" % cls.__name__
                    )
                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

        def setup(self, request, *args, **kwargs):
            """Initialize attributes shared by all view methods."""
            if hasattr(self, 'get') and not hasattr(self, 'head'):
                self.head = self.get
            self.request = request
            self.args = args
            self.kwargs = kwargs

        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)

        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 HttpResponseNotAllowed(self._allowed_methods())

        def options(self, request, *args, **kwargs):
            """Handle responding to requests for the OPTIONS HTTP verb."""
            response = HttpResponse()
            response['Allow'] = ', '.join(self._allowed_methods())
            response['Content-Length'] = '0'
            return response

        def _allowed_methods(self):
            return [m.upper() for m in self.http_method_names if hasattr(self, m)]

class View:

"""

Intentionally simple parent class for all views. Only implements

dispatch-by-method and simple sanity checking.

"""

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

def __init__(self, **kwargs):

"""

Constructor. Called in the URLconf; can contain helpful extra

keyword arguments, and other things.

"""

# Go through keyword arguments, and either save their values to our

# instance, or raise an error.

for key, value in kwargs.items():

setattr(self, key, value)

@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(

'The method name %s is not accepted as a keyword argument '

'to %s().' % (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)

self.setup(request, *args, **kwargs)

if not hasattr(self, 'request'):

raise AttributeError(

"%s instance has no 'request' attribute. Did you override "

"setup() and forget to call super()?" % cls.__name__

)

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

def setup(self, request, *args, **kwargs):

"""Initialize attributes shared by all view methods."""

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

self.head = self.get

self.request = request

self.args = args

self.kwargs = kwargs

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)

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 HttpResponseNotAllowed(self._allowed_methods())

def options(self, request, *args, **kwargs):

"""Handle responding to requests for the OPTIONS HTTP verb."""

response = HttpResponse()

response['Allow'] = ', '.join(self._allowed_methods())

response['Content-Length'] = '0'

return response

def _allowed_methods(self):

return [m.upper() for m in self.http_method_names if hasattr(self, m)]

这里有很多事情要做，但是我们将以Django从.as_view（）开始的方式来完成它。

as_view（）方法
由于我们在URLconf中调用.as_view（），因此是在创建类的实例之前调用View的第一部分。

@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(
                'The method name %s is not accepted as a keyword argument '
                'to %s().' % (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)
        self.setup(request, *args, **kwargs)
        if not hasattr(self, 'request'):
            raise AttributeError(
                "%s instance has no 'request' attribute. Did you override "
                "setup() and forget to call super()?" % cls.__name__
            )
        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

@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(

'The method name %s is not accepted as a keyword argument '

'to %s().' % (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)

self.setup(request, *args, **kwargs)

if not hasattr(self, 'request'):

raise AttributeError(

"%s instance has no 'request' attribute. Did you override "

"setup() and forget to call super()?" % cls.__name__

)

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

@classonlymethod装饰器与.as_view（）一起使用，以确保未在类的实例上调用它，而是仅在类上直接调用它。现在是指出使用CBV时不直接创建类实例的好时机。

new_view = View() # You won't do this

1	new_view = View() # You won't do this

取而代之的是，稍后会由于.as_view（）而创建一个实例。

.as_view（）有两个参数：cls，即类.as_view（）被调用并自动传递给该方法，以及** initkwargs。 ** initkwargs是我们在调用.as_view（）时传递给它的任何关键字参数，而在最终创建该类的实例时可能需要这些参数。我们很快就会看到另一组关键字参数，因此请记住，在类实例化期间使用** initkwargs。如果确实有关键字参数，则可以像这样在URLconf中传递它们：

# Passing example keyword arguments to .as_view
path('new-cbv/', MyView.as_view(kwarg1=new_kwarg_1, kwarg2=new_kwarg_2), name='new_cbv')

1 2	# Passing example keyword arguments to .as_view path('new-cbv/', MyView.as_view(kwarg1=new_kwarg_1, kwarg2=new_kwarg_2), name='new_cbv')

调用.as_view（）时执行的第一个代码是遍历initkwargs并执行两次检查。

for key in initkwargs:
      if key in cls.http_method_names:
          raise TypeError(
              'The method name %s is not accepted as a keyword argument '
              'to %s().' % (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))

for key in initkwargs:

if key in cls.http_method_names:

raise TypeError(

'The method name %s is not accepted as a keyword argument '

'to %s().' % (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))

首先对照包含HTTP verbs列表的View类的http_method_names属性检查initkwargs中的每个键。

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

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

如果我们尝试将HTTP动词作为.as_view（）的参数传递，则会出现错误，因为它可能导致视图逻辑的执行出现问题。

还检查每个键以确保它与该类的现有属性匹配。 View类在http_method_names旁边没有类属性，因此让我们快速看一下Django内置的RedirectView CBV的前几行。

class RedirectView(View):
        """Provide a redirect on any GET request."""
        permanent = False
        url = None
        pattern_name = None
        query_string = False

class RedirectView(View):

"""Provide a redirect on any GET request."""

permanent = False

url = None

pattern_name = None

query_string = False

RedirectView具有多个类属性，我们可以通过.as_view（）上的关键字参数来设置。但是，如果我们尝试传递不是列出的关键字参数之一的关键字参数，则会收到错误消息。

检查initkwargs之后，我们进入.as_view（）部分，将我们与最初的目标联系起来，以查明.as_view（）返回什么，并希望它是一个函数，因为URLconf中的path（）需要一个函数。

在.as_view（）中定义了一个view（）函数，如果我们跳到.as_view（）的底部，我们可以看到它返回了此函数。

所以现在我们知道

path('new-cbv/', MyView.as_view(), name='new_cbv')

1	path('new-cbv/', MyView.as_view(), name='new_cbv')

看起来像

path('new-cbv/', view, name='new_cbv')

1	path('new-cbv/', view, name='new_cbv')

一旦对MyView.as_view（）进行了评估，则该外观与使用FBV时的外观相同。这意味着view（）将接受与Django调用时FBV相同的参数。我们不会介绍Django如何调用视图以及如何执行视图，但是您可以在Django文档中了解更多信息。

在详细介绍view（）之前，还有MyView.as_view（）的其余部分。首先，将view.view_class和view.view_initkwargs的视图上的类属性分别设置为view和initkwargs的类。

最后，对update_wrapper（）进行了两次调用。这将从类和dispatch（）方法复制元数据进行查看。

完成.as_view（）之后，我们现在可以查看view（）函数及其功能。

使用view（）创建类实例
调用view（）时究竟会做什么？简而言之，它启动了执行视图逻辑并最终返回响应的事件链。让我们看看它是如何做到的。

def view(request, *args, **kwargs):
        self = cls(**initkwargs)
        self.setup(request, *args, **kwargs)
        if not hasattr(self, 'request'):
            raise AttributeError(
                "%s instance has no 'request' attribute. Did you override "
                "setup() and forget to call super()?" % cls.__name__
            )
        return self.dispatch(request, *args, **kwargs)

def view(request, *args, **kwargs):

self = cls(**initkwargs)

self.setup(request, *args, **kwargs)

if not hasattr(self, 'request'):

raise AttributeError(

"%s instance has no 'request' attribute. Did you override "

"setup() and forget to call super()?" % cls.__name__

)

return self.dispatch(request, *args, **kwargs)

view（）具有三个参数：request，* args和** kwargs。这些由Django在调用带有URL模式的* args和** kwargs的视图时提供。重要的是不要将** kwargs与我们之前看到的initkwargs混淆。 ** initkwargs来自对as_view（）的调用，而** kwargs来自URL中匹配的模式。

view（）要做的第一件事是通过传递类** initkwargs并将该实例分配给self来创建该类的实例。如果我们回到View类的开头，可以看到__init __（）方法中设置了类属性，其中** initkwargs在本地称为** kwargs。

def __init__(self, **kwargs):
    """
    Constructor. Called in the URLconf; can contain helpful extra
    keyword arguments, and other things.
    """
    # Go through keyword arguments, and either save their values to our
    # instance, or raise an error.
    for key, value in kwargs.items():
        setattr(self, key, value)

def __init__(self, **kwargs):

"""

Constructor. Called in the URLconf; can contain helpful extra

keyword arguments, and other things.

"""

# Go through keyword arguments, and either save their values to our

# instance, or raise an error.

for key, value in kwargs.items():

setattr(self, key, value)

创建类实例后，将调用setup（）方法，该方法采用传递给view（）的相同三个参数。它将参数保存到类实例，这使它们可用于视图的所有后续方法。

def setup(self, request, *args, **kwargs):
        """Initialize attributes shared by all view methods."""
        if hasattr(self, 'get') and not hasattr(self, 'head'):
            self.head = self.get
        self.request = request
        self.args = args
        self.kwargs = kwargs

def setup(self, request, *args, **kwargs):

"""Initialize attributes shared by all view methods."""

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

self.head = self.get

self.request = request

self.args = args

self.kwargs = kwargs

它还检查self是否具有get属性和head属性。如果它有get但没有head，它将创建head并为其分配get。

调用setup（）之后，在view（）函数中还要进行一项检查，以确保self具有request属性。

if not hasattr(self, 'request'):
        raise AttributeError(
            "%s instance has no 'request' attribute. Did you override "
            "setup() and forget to call super()?" % cls.__name__
        )

if not hasattr(self, 'request'):

raise AttributeError(

"%s instance has no 'request' attribute. Did you override "

"setup() and forget to call super()?" % cls.__name__

)

如果没有请求属性，将显示的错误消息很好地说明了为什么需要进行此检查的原因-可能会覆盖setup（）方法，从而可能不会在实例上创建请求属性。我们将在以后的文章中保留重写类方法的“为什么”和“如何”，但是在创建自己的CBV或修改Django内置的通用CBV时，这很常见。

完成此检查后，view（）最终通过调用dispatch（）方法返回。

使用dispatch（）路由视图逻辑
dispatch（）传递了与view（）相同的三个参数，并且确实执行了其名称所暗示的功能-它根据请求类型将视图分派到正确的逻辑分支。

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)

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)

它采用请求的HTTP方法，检查以确保它在self.http_method_names中存储的允许方法列表中，然后从self中获取相同名称的对应方法，并将其分配给处理程序。例如，如果发出GET请求，则将为处理程序分配该类的get（）方法。或者，如果发出POST请求，则处理程序将是该类的post（）方法。如果请求方法不在self.http_method_names中，或者self没有相应的方法，则为处理程序分配http_method_not_allowed方法，该方法将返回HttpResponseNotAllowed。 dispatch（）通过调用带有请求，* args和** kwargs参数的分配给处理程序的任何方法来返回。

如果回头查看View类的完整代码，我们会注意到没有get（）或post（）方法，或者大多数其他HTTP方法的方法。它仅包含方法options（）来处理HTTP OPTIONS方法。

那么当我们需要处理GET请求时会发生什么呢？

至此，我们已经达到了View类的极限。视图并不意味着是独立的CBV。它用作其他CBV继承的基类。从View继承的类将定义get（），post（）和其他处理请求所需的方法。如果再次查看RedirectView，我们将看到它如何为所有HTTP方法名称定义方法。

# Inside RedirectView
def get(self, request, *args, **kwargs):
        url = self.get_redirect_url(*args, **kwargs)
        if url:
            if self.permanent:
                return HttpResponsePermanentRedirect(url)
            else:
                return HttpResponseRedirect(url)
        else:
            logger.warning(
                'Gone: %s', request.path,
                extra={'status_code': 410, 'request': request}
            )
            return HttpResponseGone()

def head(self, request, *args, **kwargs):
    return self.get(request, *args, **kwargs)

def post(self, request, *args, **kwargs):
    return self.get(request, *args, **kwargs)

def options(self, request, *args, **kwargs):
    return self.get(request, *args, **kwargs)

def delete(self, request, *args, **kwargs):
    return self.get(request, *args, **kwargs)

def put(self, request, *args, **kwargs):
    return self.get(request, *args, **kwargs)

def patch(self, request, *args, **kwargs):
    return self.get(request, *args, **kwargs)

# Inside RedirectView

def get(self, request, *args, **kwargs):

url = self.get_redirect_url(*args, **kwargs)

if url:

if self.permanent:

return HttpResponsePermanentRedirect(url)

else:

return HttpResponseRedirect(url)

else:

logger.warning(

'Gone: %s', request.path,

extra={'status_code': 410, 'request': request}

)

return HttpResponseGone()

def head(self, request, *args, **kwargs):