Why is form_valid not updating the object but creating a new record?

Question

Django

Please tell my friends how to save instance correctly after form validation.

#forms.py
from django import forms
from clinic.models import Doctor

class DoctorForm(forms.ModelForm):
    class Meta:
        model = Doctor
       fields = [_all_]
#views.py
class DoctorDetailView(FormMixin, DetailView):
    form_class = DoctorForm
    model = Doctor

    def get_success_url(self):
        return reverse('doctor_detail', kwargs={'pk': self.object.pk})

    def get_context_data(self, **kwargs):
        #Получаем непосредственно сам объект
        objectDoctor = Doctor.objects.get(pk =self.object.pk)
        context = super(DoctorDetailView, self).get_context_data(**kwargs)
        #Передаем инстанс объекта в форму
        context['form'] = self.form_class(instance = objectDoctor)
        return context

    def post(self, request, *args, **kwargs):
        self.object = self.get_object()
        form_class = self.get_form_class()

        form = self.get_form(form_class)
        if form.is_valid():
            return self.form_valid(form)
        else:
            return self.form_invalid(form)

    def form_valid(self, form):
        #Сохраняем данные полученные из POST
        # self.object = form.save(commit = False)
        instance = form.save()
        instance.save() 
        return super(DoctorDetailView, self).form_valid(form)

After entering the data, the current object is not updated for me, but a new object with form data is formed.

Answer 1

look here:
UpdateView
CreateView
Look at the differences between Create and Update View, namely self.get_object() and self.get_form_kwargs().
The get_form_kwargs function returns a dictionary with the keys 'initial', 'instance', 'data', 'files', and 'instance' defined in the ModelFormMixin (and in your case you don't use it). When get_form_kwargs value is 'instance' the model object - the form updates the object, when None - creates a new one.
Addition:
Yes, in ModelFormMixin instance is already passed to the form from self.object. Open ModelFormMixin and expand "def get_form_kwargs(self):" and what's in it. ModelFormMixin inherits FormMixin, calls super() in the overridden get_form_kwargs method,

def get_form_kwargs(self):
    """
    Returns the keyword arguments for instantiating the form.
    """
    kwargs = super(ModelFormMixin, self).get_form_kwargs()
    if hasattr(self, 'object'):
        kwargs.update({'instance': self.object})
    return kwargs

Answer 2

Read what is inside the FormMixin
https://github.com/django/django/blob/master/djang...
Or go through the debugger
Then try to remove the post
Then look at the ModelFormMixin