bpo-34333: Fix %-formatting in Path.with_suffix() (GH-8663)

Lib/pathlib.py

@@ -813,7 +813,7 @@ class PurePath(object):
         """
         f = self._flavour
         if f.sep in suffix or f.altsep and f.altsep in suffix:
-            raise ValueError("Invalid suffix %r" % (suffix))
+            raise ValueError("Invalid suffix %r" % (suffix,))
         if suffix and not suffix.startswith('.') or suffix == '.':
             raise ValueError("Invalid suffix %r" % (suffix))
         name = self.name

Lib/test/test_pathlib.py

@@ -577,6 +577,8 @@ class _BasePurePathTest(object):
         self.assertRaises(ValueError, P('a/b').with_suffix, '.c/.d')
         self.assertRaises(ValueError, P('a/b').with_suffix, './.d')
         self.assertRaises(ValueError, P('a/b').with_suffix, '.d/.')
+        self.assertRaises(ValueError, P('a/b').with_suffix,
+                          (self.flavour.sep, 'd'))
 
     def test_relative_to_common(self):
         P = self.cls

Misc/NEWS.d/next/Library/2018-08-04-00-06-28.bpo-34333.5NHG93.rst

+Fix %-formatting in :meth:`pathlib.PurePath.with_suffix` when formatting an
+error message.