tl;dr: skipitem() inPython/getsargs.c doesn't know about the 'w*' format unit, which could lead to a crash.  It should know about it.--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 argumentsCPython 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.Since nobody ever explained about the "w*" format unit to it, that means when it sees 'w*' it will misinterpret it.In both 2.7 and 3.x, it'll interpret it as 'w', throw away one "char *" item from varargs, then the next thing that tries to parse a format unit (skipitem or convertsimple) will see the '*' and throw an 'impossible<bad format char>' exception.(It's another bug that skipitem() supports 'w' and 'w#' in Python 3, but I filed that separately as#23926 .)

New changeset504373c59b48f1ea12132d515459022730db6047 by Serhiy Storchaka in branch 'master':bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192)https://github.com/python/cpython/commit/504373c59b48f1ea12132d515459022730db6047

New changesetef19fd200d0768919f1658466f8b6080b191fba0 by Serhiy Storchaka in branch '2.7':[2.7]bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192). (GH-8255)https://github.com/python/cpython/commit/ef19fd200d0768919f1658466f8b6080b191fba0

New changeset48d2aeb009f8e58b1efae00ee0e225951b58737e by Miss Islington (bot) in branch '3.7':bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192)https://github.com/python/cpython/commit/48d2aeb009f8e58b1efae00ee0e225951b58737e

New changesetec7562068fd123969ab4d27e11f4070271c72e16 by Miss Islington (bot) in branch '3.6':bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192)https://github.com/python/cpython/commit/ec7562068fd123969ab4d27e11f4070271c72e16