From msg312726 on issue32880.

The call to find_good_parse_start:

    bod = y.find_good_parse_start(self.context_use_ps1, 
                         self._build_char_in_string_func(startatindex))

sends 3 parameters.  And in pyparse.find_good_parse_start(), the signature allows 3.  However, the signature is:

    def find_good_parse_start(self, is_char_in_string=None,
                              _synchre=_synchre):


This means that the `False` value in `self.use_context_ps1` is the first value instead of the function, so pyparse is always executing:

    if not is_char_in_string:
        # no clue -- make the caller pass everything
        return None


Here's the commit that changed the signature:
https://github.com/python/cpython/commit/b17544551fc8dfd1304d5679c6e444cad4d34d97