I ran into a situation where I wanted something like this. Here’s what I came up with (do note that if you’re trying to use it as an update view and it can’t find the requested object, it’ll behave as a create view rather than throwing a 404):

from django.views.generic.detail import SingleObjectTemplateResponseMixin
from django.views.generic.edit import ModelFormMixin, ProcessFormView

class CreateUpdateView(
    SingleObjectTemplateResponseMixin, ModelFormMixin, ProcessFormView
):

    def get_object(self, queryset=None):
        try:
            return super(CreateUpdateView,self).get_object(queryset)
        except AttributeError:
            return None

    def get(self, request, *args, **kwargs):
        self.object = self.get_object()
        return super(CreateUpdateView, self).get(request, *args, **kwargs)

    def post(self, request, *args, **kwargs):
        self.object = self.get_object()
        return super(CreateUpdateView, self).post(request, *args, **kwargs)

It turns out that UpdateView and CreateView inherit from exactly the same classes and mixins. The only difference is in the get/post methods. Here’s how they’re defined in the Django source (1.8.2):

class BaseCreateView(ModelFormMixin, ProcessFormView):
    """
    Base view for creating an new object instance.

    Using this base class requires subclassing to provide a response mixin.
    """
    def get(self, request, *args, **kwargs):
        self.object = None
        return super(BaseCreateView, self).get(request, *args, **kwargs)

    def post(self, request, *args, **kwargs):
        self.object = None
        return super(BaseCreateView, self).post(request, *args, **kwargs)


class CreateView(SingleObjectTemplateResponseMixin, BaseCreateView):
    """
    View for creating a new object instance,
    with a response rendered by template.
    """
    template_name_suffix = '_form'


class BaseUpdateView(ModelFormMixin, ProcessFormView):
    """
    Base view for updating an existing object.

    Using this base class requires subclassing to provide a response mixin.
    """
    def get(self, request, *args, **kwargs):
        self.object = self.get_object()
        return super(BaseUpdateView, self).get(request, *args, **kwargs)

    def post(self, request, *args, **kwargs):
        self.object = self.get_object()
        return super(BaseUpdateView, self).post(request, *args, **kwargs)


class UpdateView(SingleObjectTemplateResponseMixin, BaseUpdateView):
    """
    View for updating an object,
    with a response rendered by template.
    """
    template_name_suffix = '_form'

As you can see, the CreateView get and post methods set self.object = None while the UpdateView sets it to self.get_object(). All I’ve done is combine those two in my CreateUpdateView.get_object method which attempts to call the parent class’ get_object and returns None rather than raising an exception if there is no object.

To serve a 404 page when used as an update view, you could probably override as_view and pass it an update_only boolean argument. If update_only is True and the view can’t find the object, then raise the 404.

Like suggested by @scubabuddha I ran into a similar situation and I used his answer modified as @mario-orlandi suggested in his comment:

from django.views.generic import UpdateView


class CreateUpdateView(UpdateView):

    def get_object(self, queryset=None):
        try:
            return super().get_object(queryset)
        except AttributeError:
            return None

I used this solution with Django 1.11 but I think it can work in Django 2.0.

Update

I confirm that this solution works with Django 2.0/2.1/2.2

Simplest and basically the best solution of all link

class WorkerUpdate(UpdateView):
    form_class = WorkerForm

    def get_object(self, queryset=None):

        # get the existing object or created a new one
        obj, created = Worker.objects.get_or_create(mac=self.kwargs['mac'])

        return obj

and that’s it
thanks @chriskief

In case you don’t need to raise 404 and want all fields to be blank if the object doesn’t exists, create object when first saving and updating when it exists you can use this.

views.py

from django.views.generic import UpdateView


class CreateUpdateView(UpdateView):
    model = MyModel
    form_class = MyModelForm

    def get_object(self, queryset=None):
        return self.model.objects.filter(...).first()

forms.py

class MyModelForm(forms.ModelForm):

    class Meta:
        model = MyModel
        fields = [...]

You can also use Django Smartmin which is inspired from CBV of Django. Here is an example from the documentation : https://smartmin.readthedocs.org/en/latest/quickstart.html

To share code between your UpdateView and CreateView, instead of creating a combined class, you can use a common superclass as mixin. That way, it might be easier to separate the different concerns. And – you can re-use a lot of existing Django code.

class BookFormView(PJAXContextMixin):
    template_name = 'library/book_form.html'
    form_class = BookForm

    def form_valid(self, form):
        form.instance.owner = self.request.user
        return super().form_valid(form)

    class Meta:
        abstract = True


class BookCreateView(BookFormView, CreateView):
    pass


class FormatUpdateView(BookFormView, UpdateView):
    queryset = Book.objects

In my case it was how to adapt the provided answers for UpdateView with Singleton. That’s why I put hardcoded pk=1.
With simplified class:

class MyClass(UpdateView):
    ...
    def get_object(self):
        obj, _ = MyClass.objects.update_or_create(pk=1)
        return obj

https://docs.djangoproject.com/en/4.2/ref/models/querysets/#update-or-create