* Incorporate Skip's suggestions for documentation (explain the word deque

  comes from and show the differences from lists).
* Add a rotate() method.

Doc/lib/libcollections.tex

 \section{\module{collections} ---
-         High-performance datatypes}
+         High-performance container datatypes}
 
 \declaremodule{standard}{collections}
 \modulesynopsis{High-performance datatypes}
@@ -8,7 +8,7 @@
 \versionadded{2.4}
 
 
-This module implements high-performance datatypes.  Currently, the
+This module implements high-performance container datatypes.  Currently, the
 only datatype is a deque.  Future additions may include B-trees
 and Fibonacci heaps.
 
@@ -17,9 +17,15 @@ and Fibonacci heaps.
   \method{append()}) with data from \var{iterable}.  If \var{iterable}
   is not specified, the new deque is empty.
 
-  Deques are a generalization of stacks and queues.  They support
-  thread-safe, memory efficient appends and pops from either side of the
-  deque with approximately the same performance in either direction.
+  Deques are a generalization of stacks and queues (the name is pronounced
+  ``deck'' and is short for ``double-ended queue'').  Deques support
+  thread-safe, memory efficient appends and pops from either side of the deque
+  with approximately the same \code{O(1)} performance in either direction.
+
+  Though \class{list} objects support similar operations, they are optimized
+  for fast fixed-length operations and incur \code{O(n)} memory movement costs
+  for \samp{pop(0)} and \samp{insert(0, v)} operations which change both the
+  size and position of the underlying data representation.
   \versionadded{2.4}
 \end{funcdesc}
 
@@ -58,48 +64,62 @@ Deque objects support the following methods:
    If no elements are present, raises a \exception{LookupError}.   
 \end{methoddesc}
 
-In addition to the above, deques support iteration, membership testing
-using the \keyword{in} operator, \samp{len(d)}, \samp{copy.copy(d)},
-\samp{copy.deepcopy(d)}, \samp{reversed(d)} and pickling.
+\begin{methoddesc}{rotate}{n}
+   Rotate the deque \var{n} steps to the right.  If \var{n} is
+   negative, rotate to the left.  Rotating one step to the right
+   is equivalent to:  \samp{d.appendleft(d.pop())}.
+\end{methoddesc}
+
+In addition to the above, deques support iteration, pickling, \samp{len(d)},
+\samp{reversed(d)}, \samp{copy.copy(d)}, \samp{copy.deepcopy(d)}, and
+membership testing with the \keyword{in} operator.
 
 Example:
 
 \begin{verbatim}
 >>> from collections import deque
->>> d = deque('ghi')        # make a new deque with three items
->>> for elem in d:          # iterate over the deque's elements
+>>> d = deque('ghi')                 # make a new deque with three items
+>>> for elem in d:                   # iterate over the deque's elements
 	print elem.upper()
 
 	
 G
 H
 I
->>> d.append('j')           # add a new entry to the right side
->>> d.appendleft('f')       # add a new entry to the left side
->>> d                       # show the representation of the deque
+>>> d.append('j')                    # add a new entry to the right side
+>>> d.appendleft('f')                # add a new entry to the left side
+>>> d                                # show the representation of the deque
 deque(['f', 'g', 'h', 'i', 'j'])
->>> d.pop()                 # return and remove the rightmost item
+>>> d.pop()                          # return and remove the rightmost item
 'j'
->>> d.popleft()             # return and remove the leftmost item
+>>> d.popleft()                      # return and remove the leftmost item
 'f'
->>> list(d)                 # list the contents of the deque
+>>> list(d)                          # list the contents of the deque
 ['g', 'h', 'i']
->>> list(reversed(d))       # list the contents of a deque in reverse
+>>> list(reversed(d))                # list the contents of a deque in reverse
 ['i', 'h', 'g']
->>> 'h' in d                # search the deque
+>>> 'h' in d                         # search the deque
 True
->>> d.extend('jkl')         # extend() will append many elements at once
+>>> d.extend('jkl')                  # add multiple elements at once
+>>> d
+deque(['g', 'h', 'i', 'j', 'k', 'l'])
+>>> d.rotate(1)                      # right rotation
+>>> d
+deque(['l', 'g', 'h', 'i', 'j', 'k'])
+>>> d.rotate(-1)                     # left rotation
 >>> d
 deque(['g', 'h', 'i', 'j', 'k', 'l'])
->>> d.clear()               # empty the deque
->>> d.pop()                 # cannot pop from an empty deque
+>>> deque(reversed(d))               # make a new deque in reverse order
+deque(['l', 'k', 'j', 'i', 'h', 'g'])
+>>> d.clear()                        # empty the deque
+>>> d.pop()                          # cannot pop from an empty deque
 
 Traceback (most recent call last):
   File "<pyshell#6>", line 1, in -toplevel-
     d.pop()
 LookupError: pop from an empty deque
 
->>> d.extendleft('abc')     # extendleft() reverses the element order
+>>> d.extendleft('abc')              # extendleft() reverses the input order
 >>> d
 deque(['c', 'b', 'a'])
 

Lib/test/test_deque.py

@@ -40,6 +40,18 @@ class TestBasic(unittest.TestCase):
         d.extendleft('bcd')
         self.assertEqual(list(d), list(reversed('abcd')))
 
+    def test_rotate(self):
+        s = 'abcde'
+        d = deque(s)
+        d.rotate(2)
+        self.assertEqual(''.join(d), 'deabc')
+        d.rotate(3)
+        self.assertEqual(''.join(d), s)
+        d.rotate(-3)
+        self.assertEqual(''.join(d), 'deabc')
+        d.rotate(-15)
+        self.assertEqual(''.join(d), 'deabc')
+
     def test_len(self):
         d = deque('ab')
         self.assertEqual(len(d), 2)

Modules/collectionsmodule.c

@@ -244,6 +244,44 @@ deque_extendleft(dequeobject *deque, PyObject *iterable)
 PyDoc_STRVAR(extendleft_doc, 
 "Extend the left side of the deque with elements from the iterable");
 
+static PyObject *
+deque_rotate(dequeobject *deque, PyObject *args)
+{
+	int i, n;
+	PyObject *item, *rv;
+
+	if (!PyArg_ParseTuple(args, "i:rotate", &n))
+		return NULL;
+
+	if (n == 0  ||  deque->len == 0)
+		Py_RETURN_NONE;
+
+	for (i=0 ; i<n ; i++) {
+		item = deque_pop(deque, NULL);
+		if (item == NULL)
+			return NULL;
+		rv = deque_appendleft(deque, item);
+		Py_DECREF(item);
+		if (rv == NULL)
+			return NULL;
+		Py_DECREF(rv);
+	}
+	for (i=0 ; i>n ; i--) {
+		item = deque_popleft(deque, NULL);
+		if (item == NULL)
+			return NULL;
+		rv = deque_append(deque, item);
+		Py_DECREF(item);
+		if (rv == NULL)
+			return NULL;
+		Py_DECREF(rv);
+	}
+	Py_RETURN_NONE;
+}
+
+PyDoc_STRVAR(rotate_doc, 
+"Rotate the deque n steps to the right.  If n is negative, rotates left.");
+
 static int
 deque_len(dequeobject *deque)
 {
@@ -461,6 +499,10 @@ static PyMethodDef deque_methods[] = {
 		METH_NOARGS,	 clear_doc},
 	{"__copy__",		(PyCFunction)deque_copy,	
 		METH_NOARGS,	 copy_doc},
+	{"extend",		(PyCFunction)deque_extend,	
+		METH_O,		 extend_doc},
+	{"extendleft",	(PyCFunction)deque_extendleft,	
+		METH_O,		 extendleft_doc},
 	{"pop",			(PyCFunction)deque_pop,	
 		METH_NOARGS,	 pop_doc},
 	{"popleft",		(PyCFunction)deque_popleft,	
@@ -469,10 +511,8 @@ static PyMethodDef deque_methods[] = {
 		METH_NOARGS,	 reduce_doc},
 	{"__reversed__",	(PyCFunction)deque_reviter,	
 		METH_NOARGS,	 reversed_doc},
-	{"extend",		(PyCFunction)deque_extend,	
-		METH_O,		 extend_doc},
-	{"extendleft",	(PyCFunction)deque_extendleft,	
-		METH_O,		 extendleft_doc},
+	{"rotate",		(PyCFunction)deque_rotate,	
+		METH_VARARGS,	rotate_doc},
 	{NULL,		NULL}	/* sentinel */
 };