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[Python-Dev] Why does IOBase.__del__ call .close?

Benjamin Peterson benjamin at python.org
Thu Jun 12 02:54:53 CEST 2014 
On Wed, Jun 11, 2014, at 17:11, Nikolaus Rath wrote:
> MRAB <python at mrabarnett.plus.com> writes:
> > On 2014-06-11 02:30, Nikolaus Rath wrote:
> >> Hello,
> >>
> >> I recently noticed (after some rather protacted debugging) that the
> >> io.IOBase class comes with a destructor that calls self.close():
> >>
> >> [0] nikratio at vostro:~/tmp$ cat test.py
> >> import io
> >> class Foo(io.IOBase):
> >>      def close(self):
> >>          print('close called')
> >> r = Foo()
> >> del r
> >> [0] nikratio at vostro:~/tmp$ python3 test.py
> >> close called
> >>
> >> To me, this came as quite a surprise, and the best "documentation" of
> >> this feature seems to be the following note (from the io library
> >> reference):
> >>
> >> "The abstract base classes also provide default implementations of some
> >>   methods in order to help implementation of concrete stream classes. For
> >>   example, BufferedIOBase provides unoptimized implementations of
> >>   readinto() and readline()."
> >>
> >> For me, having __del__ call close() does not qualify as a reasonable
> >> default implementation unless close() is required to be idempotent
> >> (which one could deduce from the documentation if one tries to, but it's
> >> far from clear).
> >>
> >> Is this behavior an accident, or was that a deliberate decision?
> >>
> > To me, it makes sense. You want to make sure that it's closed, releasing
> > any resources it might be holding, even if you haven't done so
> > explicitly.
> 
> I agree with your intentions, but I come to the opposite conclusion:
> automatically calling close() in the destructor will hide that there's a
> problem in the code. Without that automatic cleanup, there's at least a
> good chance that a ResourceWarning will be emitted so the problem gets
> noticed. "Silently work around bugs in caller's code" doesn't seem like
> a very useful default to me...

Things which actually hold system resources (like FileIO) give
ResourceWarning if they close in __del__, so I don't understand your
point.
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