Django admin: exclude field on change form only

Question

If there a way to detect if information in a model is being added or changed.

If there is can this information be used to exclude fields.

Some pseudocode to

Answer 1

class SubSectionAdmin(admin.ModelAdmin):
    # ...
    def change_view(self, request, object_id, extra_context=None):       
        self.exclude = ('field', )
        return super(SubSectionAdmin, self).change_view(request, object_id, extra_context)