Making PyString_FromFormatV() build an empty string on 2.7 will fail and raise the following error:SystemError:Objects/stringobject.c:3903: bad argument to internal functionIt does not matter if format is empty or not (e.g., both PyString_FromFormat("") and PyString_FromFormat("%s", "")).The exception is raised from _PyString_Resize() (called at the end of PyString_FromFormatV()), because the string given to that method has a ref count different than 1. This is expected for empty strings, because PyString_FromStringAndSize() returns a reference to <nullstring> when <size> is 0.A possible fix would be to prevent the call to _PyString_Resize() from PyString_FromFormatV() if <string> is equal to <nullstring>, or if there is no need to resize the string.Python 3 versions before 3.6 might be impacted as well through PyBytes_FromFormatV() (cannot check).The following code can be used to trigger the bug:    static int dobug(const char *fmt, ...) {        va_list args;        va_start(args, fmt);            PyObject *str = PyString_FromFormatV(fmt, args);        va_end(args);            if(str == NULL) {            fprintf(stderr, "Error: PyString_FromFormatV(%s) returned NULL\n", fmt);            return -1;        }            Py_DECREF(str);            return 0;    }    static PyObject* bug(PyObject *self) {        fprintf(stderr, "dobug(\"\") => %d\n", dobug(""));        fprintf(stderr, "dobug(\"%%s\", \"\") => %d\n", dobug("%s", ""));            if(PyErr_Occurred()) return NULL;        Py_RETURN_NONE;    }

For the record, it does not fail on 3.x because _PyBytes_Resize() checks if the "string" needs to be resized (and returns if not) before checking its ref count. Maybe something similar should be done in _PyString_Resize() for 2.x.

BTW, problem does appear in 3.4 as it's only been fixed in 3.5+ as a fix for issue#25270

New changeset44cc4822bb3799858201e61294c5863f93ec12e2 by Serhiy Storchaka in branch 'master':bpo-33817: Fix _PyBytes_Resize() for empty bytes object. (GH-11516)https://github.com/python/cpython/commit/44cc4822bb3799858201e61294c5863f93ec12e2

New changeset08a81df05004147ee174ece645679576ab867860 by Serhiy Storchaka in branch '2.7':bpo-33817: Fix _PyString_Resize() and _PyUnicode_Resize() for empty strings. (GH-11515)https://github.com/python/cpython/commit/08a81df05004147ee174ece645679576ab867860

New changesetd39c19255910b9dce08c595f511597e98b09e91f by Miss Islington (bot) in branch '3.7':bpo-33817: Fix _PyBytes_Resize() for empty bytes object. (GH-11516)https://github.com/python/cpython/commit/d39c19255910b9dce08c595f511597e98b09e91f