django.contrib.auth.login() function not returning any user as logged in

You have to set the default auth class as session authenticate class in DRF settings. Read more about it here [1].

Session auth uses session id to identify the user. So you have to send the cookie based session id in the request. Read about session auth here [2].

for example:

REST_FRAMEWORK = {
    'DEFAULT_AUTHENTICATION_CLASSES': [
        'rest_framework.authentication.SessionAuthentication', # <-- set this class
    ]
}

Use this code:

def post(self, request, format = None):
    serializer = UserLoginSerializer(data=request.data)
    if serializer.is_valid():
        user = authenticate(username=serializer.validated_data["username"], password=serializer.validated_data["password"])
        if user:
            return Response(UserSerializer(user).data, status=status.HTTP_201_CREATED)
        return Response("Invalid username/password", status=status.HTTP_401_UNAUTHORIZED)
    return Response(serializer.errors, status=status.HTTP_401_UNAUTHORIZED)

But my recommendation is to use Token auth [3].

To use token auth 2 things will change:

1. The default auth class in DRF settings
2. When sending a request to any DRF API view you to send the Auth header as Token <token-value>

Your post method and API views code will remain same.

[1] https://www.django-rest-framework.org/api-guide/authentication/#setting-the-authentication-scheme

[2] https://www.django-rest-framework.org/api-guide/authentication/#sessionauthentication

[3] https://www.django-rest-framework.org/api-guide/authentication/#tokenauthentication

I am using DRF for login. You can check this serializers and login view.

serializers.py:

    class LoginSerializer(serializers.Serializer):
              username = serializers.CharField()
              password = serializers.CharField()

    def validate(self, data):
         user = authenticate(**data)
         if user and user.is_active:
               return user
    raise serializers.ValidationError("Incorrect Credentials")

login view:

    class LoginAPIView(generics.GenericAPIView):
          queryset = User.objects.all()
          permission_classes = [AllowAny]
          serializer_class = LoginSerializer

 def post(self, request, *args, **kwargs):
     serializer = self.get_serializer(data=request.data)
     serializer.is_valid(raise_exception=True)
     user = serializer.validated_data
     return Response({
        "user": UserSerializer(user, context=self.get_serializer_context()).data,
        "token": AuthToken.objects.create(user)[1]
    }) 

settings.py rest framework classes :

REST_FRAMEWORK = {

    'DEFAULT_AUTHENTICATION_CLASSES': [
    # 'rest_framework.authentication.BasicAuthentication',
    # 'rest_framework.authentication.SessionAuthentication',
    'knox.auth.TokenAuthentication',
],
    
'DEFAULT_PERMISSION_CLASSES': [
    'rest_framework.permissions.DjangoModelPermissionsOrAnonReadOnly'
],

'DEFAULT_FILTER_BACKENDS': ['django_filters.rest_framework.DjangoFilterBackend']

}