Replace BadInternalCall with TypeError. Add a test case. Fix whitespace.

Just van Rossum showed a weird, but clever way for pure python code to
trigger the BadInternalCall.  The C code had assumed that calling a class
constructor would return an instance of that class; however, classes that
abuse __new__ can invalidate that assumption.

Lib/test/test_types.py

@@ -558,6 +558,13 @@ if type(dictlike.fromkeys('a')) is not dictlike:
     raise TestFailed, 'dictsubclass.fromkeys created wrong type'
 if type(dictlike().fromkeys('a')) is not dictlike:
     raise TestFailed, 'dictsubclass.fromkeys created wrong type'
+from UserDict import UserDict
+class mydict(dict):
+    def __new__(cls, *args, **kwargs):
+        return UserDict(*args, **kwargs)
+try: mydict.fromkeys('a b c'.split())
+except TypeError: pass
+else: raise TestFailed, 'dict.fromkeys() failed to detect non-dict class.'
 # dict.copy()
 d = {1:1, 2:2, 3:3}
 if d.copy() != {1:1, 2:2, 3:3}: raise TestFailed, 'dict copy'

Objects/dictobject.c

@@ -973,15 +973,16 @@ dict_fromkeys(PyObject *mp, PyObject *args)
 	PyObject *cls;
 	int status;
 
-        if (!PyArg_ParseTuple(args, "OO|O:fromkeys", &cls, &seq, &value))
+	if (!PyArg_ParseTuple(args, "OO|O:fromkeys", &cls, &seq, &value))
 		return NULL;
 
 	d = PyObject_CallObject(cls, NULL);
 	if (d == NULL)
 		return NULL;
 	if (!PyDict_Check(d)) {
-		PyErr_BadInternalCall();
 		Py_DECREF(d);
+		PyErr_SetString(PyExc_TypeError,
+			"class constructor must return a subclass of dict");
 		return NULL;
 	}