The following snippet behaves differently in the C IO implementation than in the Python IO implementation:

  import sys
  sys.stdout.write('unicode ')
  sys.stdout.buffer.write(b'bytes ')

To test this, I have created two scripts, testpyio.py (using _pyio) and testio.py (using _io).  The output is as follows:

% python3 testpyio.py
unicode bytes
% python3 testio.py
bytes unicode
%

In my opinion, the behavior exhibited by _pyio is more correct.  It appears that to get the C implementation to print the lines in the correct order, there must be a flush in between the statements.  This extra flush would create a lot of overhead.

I am attaching the two test scripts.

The C implementation prints the output in the correct order if each write ends with a newline.