[issue5700] io.FileIO calls flush() after file closed

[Antoine Pitrou]

Antoine Pitrou <pitrou at free.fr> added the comment:

> It is necessary to make MI work. With out it the inheritance graph looks
> like this (using _pyio):
> 
> 
> io.IOBase    _pyio.IOBase
>     |              |
> io.FileIO       MyMixin
>     |              |
>      \            /
>       \          /
>        \        /
>          MyClass

MyMixin doesn't need to inherit from IOBase, since precisely it is a
mixin. It's just a piece of functionality that you drop into an already
existing inheritance tree.

> But it seems like this is the right way to fix this problem anyway - as
> a user, I would expect isinstance(FileIO(...), IOBase) but that is not
> currently the case with _pyio.

That's because the reference ABCs are defined in the io module, not in
_pyio:

True
>>> issubclass(_pyio.FileIO, io.IOBase)
True
>>> issubclass(io.TextIOWrapper, io.IOBase)
True
>>> issubclass(_pyio.TextIOWrapper, io.IOBase)
True

_pyio.IOBase and friends are just concrete implementations of those
ABCs:

True
>>> issubclass(_pyio.TextIOBase, io.TextIOBase)
True

I know it looks a bit non-intuitive at first, but the point is that the
ABCs are unified, even there are two different implementations. I think
it's better than having two different sets of ABCs depending on whether
you import the pure-Python version or the C version.

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<http://bugs.python.org/issue5700>
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