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.