Example test case I think illustrates the issue:
https://github.com/mathspace/django-rest-framework/compare/validation-race

If I call self.data inside a serializer.validate, the existing stale instance data ends up cached on serializer._data. The workaround that passed my tests was to access serializer.instance instead.

Curious as to thoughts on this, e.g.

• Should serializer.data be accessible inside the validate methods, i.e. while the serializer is essentially still under construction?
• Or should the _data cache be appropriately flushed so the test can pass?

I would lean towards the former only because the serializer.data by being partly constructed could be quite misleading.