Closes #12579. Positional fields with str.format_map() now raise a ValueError instead of SystemError.

Lib/test/test_unicode.py

@@ -736,6 +736,11 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertRaises(TypeError, '{a'.format_map)
         self.assertRaises(TypeError, '}a'.format_map)
 
+        # issue #12579: can't supply positional params to format_map
+        self.assertRaises(ValueError, '{}'.format_map, {'a' : 2})
+        self.assertRaises(ValueError, '{}'.format_map, 'a')
+        self.assertRaises(ValueError, '{a} {}'.format_map, {"a" : 2, "b" : 1})
+
     def test_format_auto_numbering(self):
         class C:
             def __init__(self, x=100):

Misc/ACKS

@@ -78,6 +78,7 @@ Eli Bendersky
 Andrew Bennetts
 Andy Bensky
 Michel Van den Bergh
+Julian Berman
 Eric Beser
 Steven Bethard
 Stephen Bevan

Misc/NEWS

@@ -10,6 +10,10 @@ What's New in Python 3.2.2?
 Core and Builtins
 -----------------
 
+- Issue #12579: str.format_map() now raises a ValueError if used on a
+  format string that contains positional fields. Initial patch by
+  Julian Berman.
+
 - Issue #11627: Fix segfault when __new__ on a exception returns a non-exception
   class.
 

Objects/stringlib/string_format.h

@@ -511,6 +511,16 @@ get_field_object(SubString *input, PyObject *args, PyObject *kwargs,
         Py_DECREF(key);
     }
     else {
+        /* If args is NULL, we have a format string with a positional field
+           with only kwargs to retrieve it from. This can only happen when
+           used with format_map(), where positional arguments are not
+           allowed. */
+        if (args == NULL) {
+            PyErr_SetString(PyExc_ValueError, "Format string contains "
+                            "positional fields");
+            goto error;
+        }
+
         /* look up in args */
         obj = PySequence_GetItem(args, index);
         if (obj == NULL)