Add more comments. Improve variable names.

Make links clearer by using a Link object
instead of a list.  Use proxy links to avoid
circular references.

Lib/collections.py

@@ -10,6 +10,7 @@ from operator import itemgetter as _itemgetter, eq as _eq
 from keyword import iskeyword as _iskeyword
 import sys as _sys
 import heapq as _heapq
+from weakref import proxy
 from itertools import repeat as _repeat, chain as _chain, starmap as _starmap, \
                       ifilter as _ifilter, imap as _imap, izip as _izip
 
@@ -17,68 +18,79 @@ from itertools import repeat as _repeat, chain as _chain, starmap as _starmap, \
 ### OrderedDict
 ################################################################################
 
+class _Link(object):
+    __slots__ = 'prev', 'next', 'key', '__weakref__'
+
 class OrderedDict(dict, MutableMapping):
     'Dictionary that remembers insertion order'
-    # The inherited dict maps keys to values.
-    # The internal self.__map dictionary maps keys to links in a doubly linked list.
-    # The doubly linked list starts and ends with a sentinel element.
-    # The sentinel element never gets deleted.  It simplifies the algorithm.
-    # Setting a new item causes a new link to append to the doubly linked list.
-    # Deleting an item uses self.__map to find the link, which is then removed.
-    # Iteration follows the linked list in order.
-    # Reverse iteration follows the links backwards.
+    # An inherited dict maps keys to values.
     # The inherited dict provides __getitem__, __len__, __contains__, and get.
     # The remaining methods are order-aware.
-    # Big-Oh running times for all methods are the same as for regular dictionaries.
+    # Big-O running times for all methods are the same as for regular dictionaries.
+
+    # The internal self.__map dictionary maps keys to links in a doubly linked list.
+    # The circular doubly linked list starts and ends with a sentinel element.
+    # The sentinel element never gets deleted (this simplifies the algorithm).
+    # The prev/next links are weakref proxies (to prevent circular references).
+    # Individual links are kept alive by the hard reference in self.__map.
+    # Those hard references disappear when a key is deleted from an OrderedDict.
 
     def __init__(self, *args, **kwds):
         if len(args) > 1:
             raise TypeError('expected at most 1 arguments, got %d' % len(args))
         try:
-            self.__end
+            self.__root
         except AttributeError:
             self.clear()
         self.update(*args, **kwds)
 
     def clear(self):
-        self.__end = end = []
-        end += [None, end, end]         # sentinel node for doubly linked list
-        self.__map = {}                 # key --> [key, prev, next]
+        self.__root = root = _Link()    # sentinel node for the doubly linked list
+        root.prev = root.next = root
+        self.__map = {}
         dict.clear(self)
 
     def __setitem__(self, key, value):
+        # Setting a new item creates a new link which goes at the end of the linked
+        # list, and the inherited dictionary is updated with the new key/value pair.
         if key not in self:
-            end = self.__end
-            curr = end[1]
-            curr[2] = end[1] = self.__map[key] = [key, curr, end]
+            self.__map[key] = link = _Link()
+            root = self.__root
+            last = root.prev
+            link.prev, link.next, link.key = last, root, key
+            last.next = root.prev = proxy(link)
         dict.__setitem__(self, key, value)
 
     def __delitem__(self, key):
+        # Deleting an existing item uses self.__map to find the link which is
+        # then removed by updating the links in the predecessor and successor nodes.
         dict.__delitem__(self, key)
-        key, prev, next = self.__map.pop(key)
-        prev[2] = next
-        next[1] = prev
+        link = self.__map.pop(key)
+        link.prev.next = link.next
+        link.next.prev = link.prev
 
     def __iter__(self):
-        end = self.__end
-        curr = end[2]                   # start at first link
-        while curr is not end:
-            yield curr[0]               # yield KEY for each link
-            curr = curr[2]              # goto next link
+        # Traverse the linked list in order.
+        root = self.__root
+        curr = root.next
+        while curr is not root:
+            yield curr.key
+            curr = curr.next
 
     def __reversed__(self):
-        end = self.__end
-        curr = end[1]                   # start at last link
-        while curr is not end:
-            yield curr[0]               # yield KEY for each link
-            curr = curr[1]              # goto prev link
+        # Traverse the linked list in reverse order.
+        root = self.__root
+        curr = root.prev
+        while curr is not root:
+            yield curr.key
+            curr = curr.prev
 
     def __reduce__(self):
         items = [[k, self[k]] for k in self]
-        tmp = self.__map, self.__end
-        del self.__map, self.__end
+        tmp = self.__map, self.__root
+        del self.__map, self.__root
         inst_dict = vars(self).copy()
-        self.__map, self.__end = tmp
+        self.__map, self.__root = tmp
         if inst_dict:
             return (self.__class__, (items,), inst_dict)
         return self.__class__, (items,)