Copy of Antoine Pitrou's email (sent in 2012 ;-):
https://mail.python.org/pipermail/python-dev/2012-August/121396.html

Hello,

I was considering a FileIO.writelines() implementation based on
writev() and I noticed that the current RawIO.writelines()
implementation is broken: RawIO.write() can return a partial write but
writelines() ignores the result and happily proceeds to the next
iterator item (and None is returned at the end).

(it's probably broken with non-blocking streams too, for the same
reason)

In the spirit of RawIO.write(), I think RawIO.writelines() could return
the number of bytes written (allowing for partial writes).

Regards

Antoine.

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