Accept Jim Jewett's api suggestion to use None instead of -1 to indicate unbounded deques.

Doc/library/collections.rst

@@ -51,7 +51,7 @@ ordered dictionaries.
 
    .. versionadded:: 2.4
 
-   If *maxlen* is not specified or is *-1*, deques may grow to an
+   If *maxlen* is not specified or is *None*, deques may grow to an
    arbitrary length.  Otherwise, the deque is bounded to the specified maximum
    length.  Once a bounded length deque is full, when new items are added, a
    corresponding number of items are discarded from the opposite end.  Bounded

Lib/test/test_deque.py

@@ -48,6 +48,7 @@ class TestBasic(unittest.TestCase):
         self.assertEqual(list(d), range(50, 150))
 
     def test_maxlen(self):
+        self.assertRaises(ValueError, deque, 'abc', -1)
         self.assertRaises(ValueError, deque, 'abc', -2)
         d = deque(range(10), maxlen=3)
         self.assertEqual(repr(d), 'deque([7, 8, 9], maxlen=3)')
@@ -73,7 +74,7 @@ class TestBasic(unittest.TestCase):
             fo.close()
             os.remove(test_support.TESTFN)
 
-        d = deque(range(10), maxlen=-1)
+        d = deque(range(10), maxlen=None)
         self.assertEqual(repr(d), 'deque([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])')
         try:
             fo = open(test_support.TESTFN, "wb")
@@ -489,6 +490,22 @@ class TestSubclass(unittest.TestCase):
         self.assertEqual(type(d), type(e))
         self.assertEqual(list(d), list(e))
 
+        d = Deque('abcde', maxlen=4)
+
+        e = d.__copy__()
+        self.assertEqual(type(d), type(e))
+        self.assertEqual(list(d), list(e))
+
+        e = Deque(d)
+        self.assertEqual(type(d), type(e))
+        self.assertEqual(list(d), list(e))
+
+        s = pickle.dumps(d)
+        e = pickle.loads(s)
+        self.assertNotEqual(id(d), id(e))
+        self.assertEqual(type(d), type(e))
+        self.assertEqual(list(d), list(e))
+
 ##    def test_pickle(self):
 ##        d = Deque('abc')
 ##        d.append(d)

Modules/_collectionsmodule.c

@@ -598,8 +598,11 @@ deque_nohash(PyObject *self)
 static PyObject *
 deque_copy(PyObject *deque)
 {
-	return PyObject_CallFunction((PyObject *)(Py_Type(deque)), "Oi",
-		deque, ((dequeobject *)deque)->maxlen, NULL);
+	if (((dequeobject *)deque)->maxlen == -1)
+		return PyObject_CallFunction((PyObject *)(Py_Type(deque)), "O", deque, NULL);
+	else
+		return PyObject_CallFunction((PyObject *)(Py_Type(deque)), "Oi",
+			deque, ((dequeobject *)deque)->maxlen, NULL);
 }
 
 PyDoc_STRVAR(copy_doc, "Return a shallow copy of a deque.");
@@ -617,10 +620,17 @@ deque_reduce(dequeobject *deque)
 		Py_XDECREF(dict);
 		return NULL;
 	}
-	if (dict == NULL)
-		result = Py_BuildValue("O(Oi)", Py_Type(deque), aslist, deque->maxlen);
-	else
-		result = Py_BuildValue("O(Oi)O", Py_Type(deque), aslist, deque->maxlen, dict);
+	if (dict == NULL) {
+		if (deque->maxlen == -1)
+			result = Py_BuildValue("O(O)", Py_Type(deque), aslist);
+		else
+			result = Py_BuildValue("O(Oi)", Py_Type(deque), aslist, deque->maxlen);
+	} else {
+		if (deque->maxlen == -1)
+			result = Py_BuildValue("O(OO)O", Py_Type(deque), aslist, Py_None, dict);
+		else
+			result = Py_BuildValue("O(Oi)O", Py_Type(deque), aslist, deque->maxlen, dict);
+	}
 	Py_XDECREF(dict);
 	Py_DECREF(aslist);
 	return result;
@@ -797,14 +807,20 @@ static int
 deque_init(dequeobject *deque, PyObject *args, PyObject *kwdargs)
 {
 	PyObject *iterable = NULL;
+	PyObject *maxlenobj = NULL;
 	int maxlen = -1;
 	char *kwlist[] = {"iterable", "maxlen", 0};
 
-	if (!PyArg_ParseTupleAndKeywords(args, kwdargs, "|Oi:deque", kwlist, &iterable, &maxlen))
-		return -1;
-	if (maxlen < -1) {
-		PyErr_SetString(PyExc_ValueError, "maxlen must be -1 or greater");
+	if (!PyArg_ParseTupleAndKeywords(args, kwdargs, "|OO:deque", kwlist, &iterable, &maxlenobj))
 		return -1;
+	if (maxlenobj != NULL && maxlenobj != Py_None) {
+		maxlen = PyInt_AsLong(maxlenobj);
+		if (maxlen == -1 && PyErr_Occurred())
+			return -1;
+		if (maxlen < 0) {
+			PyErr_SetString(PyExc_ValueError, "maxlen must be non-negative");
+			return -1;
+		}
 	}
 	deque->maxlen = maxlen;
 	if (iterable != NULL) {