Once the underlying buffer/stream is .detach()ed from an instance of a subclass of TextIOBase or BufferedIOBase, accession of most attributes defined on TextIOBase/BufferedIOBase or the IOBase parent, as well as calling of most methods defined on TextIOBase/BufferedIOBase/IOBase, results in raising of a ValueError.

Currently, the documentation of both .detach() methods states simply:

> After the raw stream has been detached, the buffer is in an unusable state.

I propose augmenting the above to something like the following in the docs for both .detach() methods, to make this behavior more explicit:

> After the raw stream has been detached, the buffer
> is in an unusable state. As a result, accessing/calling most
> attributes/methods of either :class:`BufferedIOBase` or its
> :class:`IOBase` parent will raise :exc:`ValueError`.

I confirm that the error raised for both `BufferedReader` and `TextIOWrapper` after `.detach()` *is* ValueError in all of 3.5.7, 3.6.8, 3.7.3, and 3.8.0b1.

What is the advantage of making the doc more specific in this manner?

If we do change the doc like this, we need to add unit test ensuring that this is the behaviour, and we need are committing cpython (and all other python implementations) to behave like this in the future as well. 

I suggest this issue be rejected.

Indeed, I hadn't been thinking about the testing/maintenance burden to CPython or other implementations when I made the suggestion.

I no longer have a strong opinion about this change, so I am happy to reject/close.