tl;dr: skipitem() in Python 3 still supports "w" and "w#", which were abandoned.  They should be removed.

--

If: 
* you have an extension that parses its arguments with
  PyArg_ParseTupleAndKeywords (or its cousins),
* you have optional positional parameters, and
* you run out of positional arguments

CPython will call skipitem() to skip over the remaining positional parameters so it can process keyword arguments.  It does this by knowing all the format units, and iterating over them and throwing away the various varargs pointers until it hits the keyword arguments part of the format string.

PyArg_ParseTuple() etc. in Python 2 supported "w" and "w#" for parsing "objects with the read-write buffer interface".  These were removed in Python 3.

skipitem() in Python 3 still supports both skipping "w" and "w#".  But in fact the only legal format unit starting with a 'w' in Python 3 is 'w*'.  So a function with a 'w*' as an optional parameter that got skipped would be misinterpreted; skipitem would see the 'w', not recognize the following '*' and leave it there, then the next function that read a character from the format string (either skipitem() or convertsimple()) would see the '*' and throw an "impossible <bad format char>" exception.

here is a patch and test case.
I added an entry to Misc/NEWS. I tried to follow the format; however, let me know if I have done this incorrectly.

Sorry it took so long to get back to this. I didn't realize this was still open. I have provided the update to the docs and moved it to the 3.6 section. I also made sure the patch still applies and the tests all pass.

bumping so that we don't forget about this.

bumping this issue

bump

I reviewed skipitems.patch.

Do you mind to convert your patch into a pull request Joe?