restframework custom EXCEPTION_HANDLER not effect

In a new project custom exception_handler is working，but to my old project it’s not working, i follow the doc custom exception handler

to writing. And I set print in my exception_handler, but it’s not show any thing.(exception will be catch but using the default handler.)

# myapp.exception.py
from rest_framework.views import exception_handler, Response
from rest_framework.exceptions import APIException


def custom_exception(exc, context):
    response = exception_handler(exc, context)
    print(exc)
    if response is not None:
        response.data['exception'] = 'h'
    else:
        response = Response(data={'abc':123}, status=500)
    return  response

# settings.py

REST_FRAMEWORK = {
    'EXCEPTION_HANDLER': 'basuser.exception.custom_exception'
}
INSTALLED_APPS = [
'...',
'...',
'rest_framework'
]

#views.py
class CustomException(APIException):
    status_code = 401
    default_detail = 'detail exception'
    default_code = 'custom_exception'

class TestPage(APIView):


    def get(self, request,format=None):
        return Response({'data':100})

    def post(self, request):
        raise CustomException()

# out put
Unauthorized: /testpage/
[03/Feb/2022 03:33:21] "POST /testpage/ HTTP/1.1" 401 21

# json response
{
    "detail": "detail exception"
}

It’s so confused…