Forward port 70475: Add implementation notes. Put methods in more readable order.
diff --git a/Lib/collections.py b/Lib/collections.py
index f1b62c7..bb5991c 100644
--- a/Lib/collections.py
+++ b/Lib/collections.py
@@ -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]
@@ -85,6 +90,13 @@
values = MutableMapping.values
items = MutableMapping.items
+ 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__,)