以下是Python 2.7中collections
模块的源代码。我对OrderedDict
初始化其__root
变量的方式感到困惑。为什么使用try
和except
,是否有必要?为什么不能只使用
self.__root = root = [] # sentinel node
root[:] = [root, root, None]
self.__map = {}
self.__update(*args, **kwds)
初始化self.__root
?
非常感谢...
class OrderedDict(dict):
'Dictionary that remembers insertion order'
# An inherited dict maps keys to values.
# The inherited dict provides __getitem__, __len__, __contains__, and get.
# The remaining methods are order-aware.
# Big-O running times for all methods are the same as regular dictionaries.
# The internal self.__map dict 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).
# Each link is stored as a list of length three: [PREV, NEXT, KEY].
def __init__(self, *args, **kwds):
'''Initialize an ordered dictionary. The signature is the same as
regular dictionaries, but keyword arguments are not recommended because
their insertion order is arbitrary.
'''
if len(args) > 1:
raise TypeError('expected at most 1 arguments, got %d' % len(args))
try:
self.__root
except AttributeError:
self.__root = root = [] # sentinel node
root[:] = [root, root, None]
self.__map = {}
self.__update(*args, **kwds)
答案 0 :(得分:6)
我找到了一个讨论here(值得注意的是Raymond Hettinger是一个python核心开发人员)。
基本上,对于用户第二次拨打__init__
(而不是update
)的情况,这似乎是一种预防措施:
In [1]: from collections import OrderedDict
In [2]: od = OrderedDict([(1,2), (3,4)])
In [3]: od
Out[3]: OrderedDict([(1, 2), (3, 4)])
In [4]: od.__init__([(5,6), (7,8)])
In [5]: od
Out[5]: OrderedDict([(1, 2), (3, 4), (5, 6), (7, 8)])
虽然非常罕见,但这对于dict.__init__
的一致性非常重要,dict.update
也可以第二次而不是In [6]: d = {1:2, 3:4}
In [7]: d.__init__([(5,6), (7,8)])
In [8]: d
Out[8]: {1: 2, 3: 4, 5: 6, 7: 8} # warning: don't rely on this order!
调用:
{{1}}