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Ya, this is well known, although it may not be documented.  
pythonw's purpose in life is *not* to create (or inherit) a 
console window (a "DOS box").  Therefore stdin, stdout, and 
stderr aren't attached to anything usable.  Microsoft's C 
runtime seems to attach them to buffers that aren't 
connected to anything, so they complain if you ever exceed 
the buffer size.

The short course is that stdin, stdout and stderr are useless 
in programs without a console window, so you shouldn't use 
them.  Or you should you install your own file-like objects, 
and make them do something useful to you.

I think it would be helpful if pythonw did something fancier 
(e.g., pop up a window containing attempted output), but 
that's in new-feature terrority, and nobody has contributed 
code for it anyway.