[Python-Dev] Why does IOBase.del call .close? (original) (raw)

Benjamin Peterson benjamin at python.org
Sat Jun 14 05:26:02 CEST 2014

On Fri, Jun 13, 2014, at 20:04, Nikolaus Rath wrote:

Benjamin Peterson <benjamin at python.org> writes: > On Thu, Jun 12, 2014, at 18:06, Nikolaus Rath wrote: >> Consider this simple example: >> >> $ cat test.py >> import io >> import warnings >> >> class StridedStream(io.IOBase): >> def init(self, name, stride=2): >> super().init() >> self.fh = open(name, 'rb') >> self.stride = stride >> >> def read(self, len): >> return self.fh.read(self.stride*len)[::self.stride] >> >> def close(self): >> self.fh.close() >> >> class FixedStridedStream(StridedStream): >> def del(self): >> # Prevent IOBase.del frombeing called. >> pass >> >> warnings.resetwarnings() >> warnings.simplefilter('error') >> >> print('Creating & loosing StridedStream..') >> r = StridedStream('/dev/zero') >> del r >> >> print('Creating & loosing FixedStridedStream..') >> r = FixedStridedStream('/dev/zero') >> del r >> >> $ python3 test.py >> Creating & loosing StridedStream.. >> Creating & loosing FixedStridedStream.. >> Exception ignored in: <io.FileIO name='/dev/zero' mode='rb'> >> ResourceWarning: unclosed file <io.BufferedReader name='/dev/zero'> >> >> In the first case, the destructor inherited from IOBase actually >> prevents the ResourceWarning from being emitted. > > Ah, I see. I don't see any good ways to fix it, though, besides setting > some flag if close() is called from del.

How about not having IOBase.del call self.close()? Any resources acquired by the derived class would still clean up after themselves when they are garbage collected.

Well, yes, but that's probably a backwards compat problem.

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