bpo-35942: Improve the error message if __fspath__ returns invalid types in path_converter (GH-11831)

The error message emitted when returning invalid types from __fspath__ in interfaces that allow passing PathLike objects has been improved and now it does explain the origin of the error.
(cherry picked from commit 09fbcd6085e18b534fd4161844ff39f77eb4a082)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

1 files changed, 15 insertions, 8 deletions

diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index e587ed23c2..e7a1f987de 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -979,28 +979,35 @@ path_converter(PyObject *o, void *p)
     if (!is_index && !is_buffer && !is_unicode && !is_bytes) {
         /* Inline PyOS_FSPath() for better error messages. */
         _Py_IDENTIFIER(__fspath__);
-        PyObject *func = NULL;
+        PyObject *func, *res;
 
         func = _PyObject_LookupSpecial(o, &PyId___fspath__);
         if (NULL == func) {
             goto error_format;
         }
-        /* still owns a reference to the original object */
-        Py_DECREF(o);
-        o = _PyObject_CallNoArg(func);
+        res = _PyObject_CallNoArg(func);
         Py_DECREF(func);
-        if (NULL == o) {
+        if (NULL == res) {
             goto error_exit;
         }
-        else if (PyUnicode_Check(o)) {
+        else if (PyUnicode_Check(res)) {
             is_unicode = 1;
         }
-        else if (PyBytes_Check(o)) {
+        else if (PyBytes_Check(res)) {
             is_bytes = 1;
         }
         else {
-            goto error_format;
+            PyErr_Format(PyExc_TypeError,
+                 "expected %.200s.__fspath__() to return str or bytes, "
+                 "not %.200s", Py_TYPE(o)->tp_name,
+                 Py_TYPE(res)->tp_name);
+            Py_DECREF(res);
+            goto error_exit;
         }
+
+        /* still owns a reference to the original object */
+        Py_DECREF(o);
+        o = res;
     }
 
     if (is_unicode) {