[Python-checkins] r70475 - python/trunk/Lib/collections.py

Fri Mar 20 00:12:42 CET 2009

Author: raymond.hettinger
Date: Fri Mar 20 00:12:41 2009
New Revision: 70475
Log:
* Add implementation notes.
* Re-order methods so that those touching the underlying data
 structure come first and the derived methods come last.
Modified:
 python/trunk/Lib/collections.py
Modified: python/trunk/Lib/collections.py
==============================================================================

--- python/trunk/Lib/collections.py	(original)
+++ python/trunk/Lib/collections.py	Fri Mar 20 00:12:41 2009
@@ -18,6 +18,18 @@
 ################################################################################
 
 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 cause 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.
+ # The inherited dict provides __getitem__, __len__, __contains__, and get.
+ # The remaining methods are order-aware.
+ # Big-Oh running times for all methods is the same as for regular dictionaries.
 
 def __init__(self, *args, **kwds):
 if len(args) > 1:
@@ -49,24 +61,17 @@
 
 def __iter__(self):
 end = self.__end
- curr = end[2]
+ curr = end[2] # start at first link
 while curr is not end:
- yield curr[0]
- curr = curr[2]
+ yield curr[0] # yield KEY for each link
+ curr = curr[2] # goto next link
 
 def __reversed__(self):
 end = self.__end
- curr = end[1]
+ curr = end[1] # start at last link
 while curr is not end:
- yield curr[0]
- curr = curr[1]
-
- def popitem(self, last=True):
- if not self:
- raise KeyError('dictionary is empty')
- key = next(reversed(self)) if last else next(iter(self))
- value = self.pop(key)
- return key, value
+ yield curr[0] # yield KEY for each link
+ curr = curr[1] # goto prev link
 
 def __reduce__(self):
 items = [[k, self[k]] for k in self]
@@ -89,6 +94,13 @@
 iteritems = MutableMapping.iteritems
 __ne__ = MutableMapping.__ne__
 
+ def popitem(self, last=True):
+ if not self:
+ raise KeyError('dictionary is empty')
+ key = next(reversed(self)) if last else next(iter(self))
+ value = self.pop(key)
+ return key, value
+
 def __repr__(self):
 if not self:
 return '%s()' % (self.__class__.__name__,)
