attributeerror 3a this querydict instance is immutable django

1#views.py
2from rest_framework import generics
3
4
5class Login(generics.CreateAPIView):
6    serializer_class = MySerializerClass
7    def create(self, request, *args, **kwargs):
8        request.data._mutable = True
9        request.data['username'] = "example@mail.com"
10        request.data._mutable = False
11
12#serializes.py
13from rest_framework import serializers
14
15
16class MySerializerClass(serializers.Serializer):
17    username = serializers.CharField(required=False)
18    password = serializers.CharField(required=False)
19    class Meta:
20        fields = ('username', 'password')
21