Allow to extend serializer context using kwargs of get_serializer method

[Olerdrive]

Fixes #6955

Description

As I mentioned in the #6955 it is impossible to pass any extra context to the serializer from the generic Viewset as it will be overwritten with the default value in the get_serializer method.

My pull request fixes this allowing the default context to be extended with the provided one.

[rpkilby]

Hi @Olerdrive. I'd argue that you shouldn't leverage the context here. Instead, context_key should be an argument for initializing the serializer. If you do want to use the context though, the correct thing to do would be to override the get_serializer_context method.

def get_serializer_context(self):
    context = super().get_serializer_context()
    context['context_key'] = 'value'
    return context

[Pithikos]

@rpkilby there are cases where actually this would make things much simpler and make code more consistent.

In my case for example I need to pass some metadata when uploading a file (e.g. reading the headers of an MPEG), but I still want to keep all other views to return just 'request' and 'view'.

The solution you mention, does not work in my case since the context comes from a specific view - create. Overiding get_serializer_context would require from me to refactor all other existing views like 'delete' where that context doesn't make much sense.

[rpkilby]

@Pithikos you could inspect the viewset properties, such as action, to determine if you want to add additional context. e.g.,

def get_serializer_context(self):
    context = super().get_serializer_context()
    if self.action == 'list':
        context['context_key'] = 'value'
    return context

[rpkilby]

I'd agree that it would be preferable to provide the context explicitly in the call to get_serializer, as opposed to introspecting details about the view in get_serializer_context, however the original PR proposed merging the contents of the default context with the provided context, which is what I objected to. It might be reasonable to change get_serializer to do:

    def get_serializer(self, *args, **kwargs):
        """
        Return the serializer instance that should be used for validating and
        deserializing input, and for serializing output.
        """
        serializer_class = self.get_serializer_class()
        kwargs.setdefault('context', self.get_serializer_context())
        return serializer_class(*args, **kwargs)

The above allows the user to override the context, without merging them.

[rpkilby]

@Pithikos see #7298. You would still need to pass the full context as necessary (view, request).