Where does the self.get_serializer method come from in the Django REST Framework?

CreateModelMixin along with all other mixin classes (Eg. ListModelMixin, UpdateModelMixin etc) are defined in rest_framework/mixins.py file.

These mixin classes provide all the basic CRUD operations on a model. You just need to define a serializer_class and queryset in your generic view to perform all these operations. DRF has separated out these common functionality in separate mixin classes so that they can be injected/mixed-in in a view and used as and when required.

In the DRF source code, there’s a get_serializer method. It wasn’t
inherited from object and it’s not a method in the CreateModelMixin
class. Where does this method come from?

In the GenericAPIView, get_serializer method is defined. The combination of different mixin classes along with GenericAPIView class provide us different generic views for different use cases.

class GenericAPIView(views.APIView):
    """
    Base class for all other generic views.
    """

    def get_serializer(self, *args, **kwargs):
        """
        Return the serializer instance that should be used for validating and
        deserializing input, and for serializing output.
        """
        serializer_class = self.get_serializer_class()
        kwargs['context'] = self.get_serializer_context()
        return serializer_class(*args, **kwargs)

Other generic views then inherit the relevant mixin along with GenericAPIView.

Eg. CreateAPIView inherit the CreateModelMixin along with GenericAPIView to provide create-only endpoints.

# rest_framework/generics.py
class CreateAPIView(mixins.CreateModelMixin,
                    GenericAPIView):
    ...