According to the the documentation, BufferedReader.read() and BufferedWriter.write() should raise io.BlockingIOError if the file is in non-blocking mode and the operation cannot succeed without blocking.
However, BufferedReader.read() returns None (which is what RawIOBase.read() is documented as doing), and BufferedWriter.write() raises IOError with a message like
raw write() returned invalid length -1 (should have been
between 0 and 5904)
I tested this on Linux with Python 2.6, 2.7 and 3.x.
Attached is a unit test.