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The I/O ABC documentation has a blanket disclaimer at the top:
 "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()."
Which effectively means that to subclass one of them requires digging into Lib/_pyio.py to figure out which methods provide useful implementations.
It would be handy to have a table, similar to the one for the collections ABCs [1], that spells out which methods are provided.
[1]: http://docs.python.org/py3k/library/collections.html#abcs-abstract-base-classes
I hope to have a patch ready sometime within the next week.

+io and doc people
Attached is a simple patch to add a table to the documentation summarizing the I/O ABCs.

What does "unsupported" mean? "Abstract" would look more exact.

Not sure what "unsupported methods" means. E.g. "readinto" is listed as provided by RawIOBase in the doc text.

> What does "unsupported" mean? "Abstract" would look more exact.
It means they raise io.UnsupportedOperation when called (unless the subclass overrides them to do something else). 
They are not marked with @abstractmethod, so "Abstract" would be wrong.
It should be a bit more clear in context. Here's a link to the point in the documentation where the table would fit:
http://docs.python.org/dev/py3k/library/io.html#class-hierarchy
The most relevant bit is: "implementations are allowed to raise UnsupportedOperation if they do not support a given operation."
> E.g. "readinto" is listed as provided by RawIOBase in the doc text.
The doc text describes the API. That is, it describes what the readinto method should do if a subclass of RawIOBase decides to implement it. The existing text doesn't contain any clues as to which methods contain useful implementations versus which will raise io.UnsupportedOperation if not overridden. Hence the need for the table.

> > What does "unsupported" mean? "Abstract" would look more exact.
> 
> It means they raise io.UnsupportedOperation when called (unless the
> subclass overrides them to do something else). 
> 
> They are not marked with @abstractmethod, so "Abstract" would be
> wrong.
Except that "unsupported" is even more wrong, because it makes it look
like that these methods are not supported by the respective ABCs.
Actually, what happens is that the default implementations are stubs. If
those methods weren't "supported" at all they wouldn't be defined in the
first place.

Other suggestions for a better name for that column are certainly welcome. :-)
"Stub Methods"?

> Other suggestions for a better name for that column are certainly welcome. :-)
> 
> "Stub Methods"?
Fine with me.

Updated the patch. Is it ok now?

Fixed in a58204570a7c, 3cb85250a0a3, 93742b046519.