according to the docs (https://docs.python.org/3.7/c-api/arg.html?highlight=pyarg_parsetuple#c.PyArg_ParseTuple), PyArg_ParseTuple
returns true for success or false for failure. I also looked at the
implementation in Python/getargs.c, and it seems that indeed PyArg_ParseTuple
can return only 0 or 1.

however, in some places in the codebase, there are checks whether
PyArg_ParseTuple returned a negative int.
I found a bunch in Modules/_testcapimodule.c, and one in Modules/_io/textio.c
in textiowrapper_read_chunk. (hopefully i haven't missed other places.)

this code crashes because of the check in textiowrapper_read_chunk:
import codecs
import _io
class MyDec():
    def getstate(self):
        return 420
class MyDecWrapper():
    def __call__(self, blabla):
        return MyDec()
quopri = codecs.lookup("quopri")
quopri._is_text_encoding = True
quopri.incrementaldecoder = MyDecWrapper()
t = _io.TextIOWrapper(_io.BytesIO(b'aaaaaa'),
                      newline='\n', encoding="quopri")
t.read(42)


I guess all of these checks should be changed to check whether PyArg_ParseTuple
returned 0.

also, before this specific call to PyArg_ParseTuple in textiowrapper_read_chunk,
we should check whether 'state' is a tuple.
Moreover, this call shouldn't produce a wrong error message, as explained in
#28261.

Do you want to provide a PR Oren?

yes, soon.
(just wanted to hear your opinion before doing that.)

New changeset ba7d7365215d791025d1efd25393c91404f2cfc8 by Serhiy Storchaka (Oren Milman) in branch 'master':
bpo-31243: Fixed PyArg_ParseTuple failure checks. (#3171)
https://github.com/python/cpython/commit/ba7d7365215d791025d1efd25393c91404f2cfc8

New changeset c7750c2a3af014a5b742d0159d8957ea95b6c221 by Serhiy Storchaka (Oren Milman) in branch '3.6':
[3.6] bpo-31243: Fixed PyArg_ParseTuple failure checks. (GH-3171) (#3233)
https://github.com/python/cpython/commit/c7750c2a3af014a5b742d0159d8957ea95b6c221

New changeset 20958e6d91d11a80d6c664ce6346791259b1d193 by Serhiy Storchaka (Oren Milman) in branch '2.7':
[2.7] bpo-31243: Fixed PyArg_ParseTuple failure checks. (GH-3171) (#3235)
https://github.com/python/cpython/commit/20958e6d91d11a80d6c664ce6346791259b1d193

Thank you for your contribution Oren!