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

[Nikolaus Rath]

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?


Best,
-Nikolaus

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