data._mutable= True in Django rest framework

Question

I have seen this many times and looked it everywhere but couldn't figure out what it actually means and is it mandatory? I have not used this data._mutable = True or False in my code before and I am not sure whether I should be using it or not.

The code snippet looks somewhat like this.

def update(self, request, *args, **kwargs):
        instance = self.get_object()
        data = request.data
        if data.get("something") == "null":
            data._mutable = True
            data["something"] = None
            data._mutable = False

Why do we need to assign True or False to the private attribute _mutable of data object.??

Answer 1

If you use as parser a FormParser ^[drf-doc] or MultiPartParser ^[drf-doc], or another parser that parses to a QueryDict ^[Django-doc], then the QueryDict is by default immutable. That means that it will reject any changes to it, so you can not add, remove or edit key-value pairs.

By setting the ._mutable attribute, you can prevent this from raising errors, and thus mutate the QueryDict. But it is not good practice, since it is not documented that you can make the QueryDict mutable by setting the ._mutable attribute to True. Usually you work with .copy() ^[Django-doc] which will return a mutable deep copy, so:

def update(self, request, *args, **kwargs):
        instance = self.get_object()
        data = request.data
        if data.get('something') == 'null':
            data = data.copy()
            data['something'] = None
        # …