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class PostUpdate(View):
    # why:为什么这样能实现博客的修改而非新增?
    form_class = PostForm
    model = Post
    template_name = 'blog/post_form_update.html'

    def get(self, request, year, month, slug):
        post = get_object_or_404(self.model, 
                                 pub_date__year=year, pub_date__month=month, slug=slug)
        return render(request,
                      self.template_name,
                      {'form': self.form_class(instance=post), 'post': post})

    def post(self, request, year, month, slug):
        post = get_object_or_404(self.model, 
                                 pub_date__year=year, pub_date__month=month, slug=slug)
        # 这里的instance=post是实现博客的修改而非新增的关键
        bound_form = self.form_class(request.POST, instance=post)
        if bound_form.is_valid():
            new_post = bound_form.save()
            return redirect(new_post)
        else:
            return render(request,
                          self.template_name,
                          {'form': bound_form, 'post': post})