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It looks like logging a warning at Python exit always works on Python 2.7:
---
import _warnings
class Bla:
 def __del__(self, w=_warnings):
 w.warn_explicit('message', DeprecationWarning, 'x.py', 5)
bla = Bla()
---
It looks like it uses the Python implementation and linecache.getline() always works. Maybe the warning is emitted earlier that the Python 3 code emitting ResourceWarning. I see that warnings.py of Python 2 imports linecache at top level, whereas the Python 3 code imports the module at the first use in showwarning(). I recall that imports at top level are avoided to get faster startup time.