use the stricter PyMapping_Check (closes #15801)

Lib/test/string_tests.py

@@ -1113,6 +1113,9 @@ class MixinStrUnicodeUserStringTest:
         self.checkraises(TypeError, '%10.*f', '__mod__', ('foo', 42.))
         self.checkraises(ValueError, '%10', '__mod__', (42,))
 
+        class X(object): pass
+        self.checkraises(TypeError, 'abc', '__mod__', X())
+
     def test_floatformatting(self):
         # float formatting
         for prec in xrange(100):

Misc/NEWS

@@ -9,6 +9,9 @@ What's New in Python 2.7.4
 Core and Builtins
 -----------------
 
+- Issue #15801: Make sure mappings passed to '%' formatting are actually
+  subscriptable.
+
 - Issue #15604: Update uses of PyObject_IsTrue() to check for and handle
   errors correctly.  Patch by Serhiy Storchaka.
 

Objects/stringobject.c

@@ -4254,7 +4254,7 @@ PyString_Format(PyObject *format, PyObject *args)
         arglen = -1;
         argidx = -2;
     }
-    if (Py_TYPE(args)->tp_as_mapping && !PyTuple_Check(args) &&
+    if (PyMapping_Check(args) && !PyTuple_Check(args) &&
         !PyObject_TypeCheck(args, &PyBaseString_Type))
         dict = args;
     while (--fmtcnt >= 0) {

Objects/unicodeobject.c

@@ -8275,7 +8275,7 @@ PyObject *PyUnicode_Format(PyObject *format,
         arglen = -1;
         argidx = -2;
     }
-    if (Py_TYPE(args)->tp_as_mapping && !PyTuple_Check(args) &&
+    if (PyMapping_Check(args) && !PyTuple_Check(args) &&
         !PyObject_TypeCheck(args, &PyBaseString_Type))
         dict = args;