OrderedDict KeyError | 易学教程

问题

import collections

d = collections.defaultdict(dict)

d["i"]["a"] = "111"
d["i"]["b"] = "222"

print d

od = collections.OrderedDict()

od["i"]["a"] = "111"
od["i"]["b"] = "222"

print od

Output:

defaultdict(<type 'dict'>, {'i': {'a': '111', 'b': '222'}})
Traceback (most recent call last):
  File "app_main.py", line 51, in run_toplevel
  File "/Users/adam/Desktop/collections.py", line 12, in <module>
    od["i"]["a"] = "111"
KeyError: 'i'

Why the key error with OrderedDict and what I can do about it?

Thanks.

回答1:

An OrderedDict is not also a defaultdict. You'd have to do something like this:

import collections
od = collections.OrderedDict()
od["i"] = collections.OrderedDict()
od["i"]["a"] = "111"
od["i"]["b"] = "222"
print od

Output:

OrderedDict([('i', OrderedDict([('a', '111'), ('b', '222')]))])

See this answer for a potential ordered defaultdict implementation.

回答2:

Thats the main advantage of defaultdict especially, it will capture the Keyerror and call the function passed as the argument to defaultdict. But OrderedDict is for different purpose.

If a new dataStrcture combines both functionality would be beneficial. Also, using userDict() it must be possible to implement such a functionality.

You can refer my article on Python Collections

https://techietweak.wordpress.com/2015/11/11/python-collections/

Hope this helps.

来源：https://stackoverflow.com/questions/15283578/ordereddict-keyerror

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python