> I don't like _dealloc_warn() at all. It looks as a dirty hack and doesn't work with custom file-like objects.

I tried to support emitting a ResourceWarning when the close() method is overriden in a subclass, but then I saw that io doesn't support this use case neither:
---
import io
import os

class MyFile(io.FileIO):
    def close(self):
        os.close(self.fileno())

f = MyFile('/etc/issue')
f = None
---
=> no warning

Ok, now I have to read again Martin's patches since I didn't understand them at the first read :-)