Conditional field requirement with DjangoRestFramework serializer

You can modify the fields in the __init__ method of BookingSerializer. After initialization the fields are accessible in self.fields

class BookingSerializer(Serializer):
    reservation_history_id = serializers.IntegerField()
    credit_card = 'api.v1.serializer.AddCreditCardSerializer()'
    save_credit_card = serializers.BooleanField()
    promo_code = serializers.BooleanField(required=False)

    def __init__(self, *args, **kwargs):
        # initialize fields
        super(BookingSerializer, self).__init__(*args, **kwargs)
        # now modify the creditcard field
        self.fields['credit_card'].required = yourBackendMethod()