Make dictionary() a real constructor. Accepts at most one argument, "a

mapping object", in the same sense dict.update(x) requires of x (that x
has a keys() method and a getitem).
Questionable:  The other type constructors accept a keyword argument, so I
did that here too (e.g., dictionary(mapping={1:2}) works).  But type_call
doesn't pass the keyword args to the tp_new slot (it passes NULL), it only
passes them to the tp_init slot, so getting at them required adding a
tp_init slot to dicts.  Looks like that makes the normal case (i.e., no
args at all) a little slower (the time it takes to call dict.tp_init and
have it figure out there's nothing to do).

Lib/test/test_descr.py

 # Test descriptor-related enhancements
 
-from test_support import verify, verbose
+from test_support import verify, verbose, TestFailed
 from copy import deepcopy
 
 def testunop(a, res, expr="len(a)", meth="__len__"):
@@ -123,6 +123,55 @@ def dicts():
     verify(eval(d.__repr__(), {}) == d)
     testset2op({1:2,3:4}, 2, 3, {1:2,2:3,3:4}, "a[b]=c", "__setitem__")
 
+def dict_constructor():
+    if verbose:
+        print "Testing dictionary constructor ..."
+    d = dictionary()
+    verify(d == {})
+    d = dictionary({})
+    verify(d == {})
+    d = dictionary(mapping={})
+    verify(d == {})
+    d = dictionary({1: 2, 'a': 'b'})
+    verify(d == {1: 2, 'a': 'b'})
+    for badarg in 0, 0L, 0j, "0", [0], (0,):
+        try:
+            dictionary(badarg)
+        except TypeError:
+            pass
+        else:
+            raise TestFailed("no TypeError from dictionary(%r)" % badarg)
+    try:
+        dictionary(senseless={})
+    except TypeError:
+        pass
+    else:
+        raise TestFailed("no TypeError from dictionary(senseless={}")
+
+    try:
+        dictionary({}, {})
+    except TypeError:
+        pass
+    else:
+        raise TestFailed("no TypeError from dictionary({}, {})")
+
+    class Mapping:
+        dict = {1:2, 3:4, 'a':1j}
+
+        def __getitem__(self, i):
+            return self.dict[i]
+
+    try:
+        dictionary(Mapping())
+    except TypeError:
+        pass
+    else:
+        raise TestFailed("no TypeError from dictionary(incomplete mapping)")
+
+    Mapping.keys = lambda self: self.dict.keys()
+    d = dictionary(mapping=Mapping())
+    verify(d == Mapping.dict)
+
 binops = {
     'add': '+',
     'sub': '-',
@@ -1299,6 +1348,7 @@ def inherits():
 def all():
     lists()
     dicts()
+    dict_constructor()
     ints()
     longs()
     floats()

Objects/dictobject.c

@@ -1692,12 +1692,39 @@ dict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 	return self;
 }
 
+static int
+dict_init(PyObject *self, PyObject *args, PyObject *kwds)
+{
+	PyObject *arg = NULL;
+	static char *kwlist[] = {"mapping", 0};
+
+	if (!PyArg_ParseTupleAndKeywords(args, kwds, "|O:dictionary",
+					 kwlist, &arg))
+		return -1;
+	if (arg != NULL) {
+		if (PyDict_Merge(self, arg, 1) < 0) {
+			/* An error like "AttibuteError: keys" is too
+			   cryptic in this context. */
+			if (PyErr_ExceptionMatches(PyExc_AttributeError)) {
+				PyErr_SetString(PyExc_TypeError,
+					"argument must be of a mapping type");
+			}
+			return -1;
+		}
+	}
+	return 0;
+}
+
 static PyObject *
 dict_iter(dictobject *dict)
 {
 	return dictiter_new(dict, select_key);
 }
 
+static char dictionary_doc[] =
+"dictionary() -> new empty dictionary\n"
+"dictionary(mapping) -> new dict initialized from mapping's key+value pairs";
+
 PyTypeObject PyDict_Type = {
 	PyObject_HEAD_INIT(&PyType_Type)
 	0,
@@ -1721,7 +1748,7 @@ PyTypeObject PyDict_Type = {
 	0,					/* tp_as_buffer */
 	Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
 		Py_TPFLAGS_BASETYPE,		/* tp_flags */
-	"dictionary type",			/* tp_doc */
+	dictionary_doc,				/* tp_doc */
 	(traverseproc)dict_traverse,		/* tp_traverse */
 	(inquiry)dict_tp_clear,			/* tp_clear */
 	dict_richcompare,			/* tp_richcompare */
@@ -1736,7 +1763,7 @@ PyTypeObject PyDict_Type = {
 	0,					/* tp_descr_get */
 	0,					/* tp_descr_set */
 	0,					/* tp_dictoffset */
-	0,					/* tp_init */
+	(initproc)dict_init,			/* tp_init */
 	PyType_GenericAlloc,			/* tp_alloc */
 	dict_new,				/* tp_new */
 };