use the stricter PyMapping_Check (closes #15801)

Lib/test/string_tests.py

@@ -1142,6 +1142,9 @@ 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 range(100):

Misc/NEWS

@@ -12,6 +12,9 @@ Core and Builtins
 
 - Issue #15761: Fix crash when PYTHONEXECUTABLE is set on Mac OS X.
 
+- Issue #15801: Make sure mappings passed to '%' formatting are actually
+  subscriptable.
+
 - Issue #15726: Fix incorrect bounds checking in PyState_FindModule.
   Patch by Robin Schreiber.
 

Objects/unicodeobject.c

@@ -9504,8 +9504,7 @@ PyObject *PyUnicode_Format(PyObject *format,
         arglen = -1;
         argidx = -2;
     }
-    if (Py_TYPE(args)->tp_as_mapping && !PyTuple_Check(args) &&
-        !PyUnicode_Check(args))
+    if (PyMapping_Check(args) && !PyTuple_Check(args) && !PyUnicode_Check(args))
         dict = args;
 
     while (--fmtcnt >= 0) {