Question

In the base class inside generic views django creates a method view at runtime , attaches it to the generic view class and then calls the dispatch method on that class

I did not understand the purpose of this approach , why did not the dispatch method was called directly from the as_view method ?

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

    http_method_names = ['get', 'post', 'put', '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.iteritems():
            setattr(self, key, value)

    @classonlymethod
    def as_view(cls, **initkwargs):
        """
        Main entry point for a request-response process.
        """
        # sanitize keyword arguments
        for key in initkwargs:
            if key in cls.http_method_names:
                raise TypeError(u"You tried to pass in the %s method name as a "
                                u"keyword argument to %s(). Don't do that."
                                % (key, cls.__name__))
            if not hasattr(cls, key):
                raise TypeError(u"%s() received an invalid keyword %r" % (
                    cls.__name__, key))

        def view(request, *args, **kwargs):
            self = cls(**initkwargs)
            return self.dispatch(request, *args, **kwargs)

        # 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 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
        self.request = request
        self.args = args
        self.kwargs = kwargs
        return handler(request, *args, **kwargs)

    def http_method_not_allowed(self, request, *args, **kwargs):
        allowed_methods = [m for m in self.http_method_names if hasattr(self, m)]
        logger.warning('Method Not Allowed (%s): %s' % (request.method, request.path),
            extra={
                'status_code': 405,
                'request': self.request
            }
        )
        return http.HttpResponseNotAllowed(allowed_methods)

Answer 1

In class based generic views, any request should have it’s own instance of the MyView class.

Here is our urls.py:

from foo.views import AboutView
....
(r'^about/', AboutView.as_view()),

urls.py is imported once per django thread. This means that calling as_view does not create an instance of AboutView.

When a request is processed by the urlconf, the view() method is called and only then an AboutView instance is created, passing and populating it with all the relevant data needed for this particular request.

Using :

(r'^about/', AboutView().dispatch), #WRONG!!!!

will cause all requests to share the same instance of the view, including possible properties that should not be re used by different requests.