The third arg of BlockingIOError is used in two quite different ways.
In write(s) it indicates the number of bytes of s which have been "consumed" (ie written to the raw file or buffered).
But in flush() and flush_unlocked() (in _pyio) it indicates the number of bytes from the internal buffer which have been written to the raw file.
I think this explains the following comment in write():
# We're full, so let's pre-flush the buffer
try:
self._flush_unlocked()
except BlockingIOError as e:
# We can't accept anything else.
# XXX Why not just let the exception pass through?
raise BlockingIOError(e.errno, e.strerror, 0)
I don't think flush() should try to tell us how many bytes were flushed: we only need to know whether we need to try again.