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news:5506e4d8-bd1d-4e56-8d1b-f71fa8293...@googlegroups.com...
在 2016年10月19日星期三 UTC+8下午3:17:18，Peter Otten写道：
chenyong20...@gmail.com wrote:
> 在 2016年10月19日星期三 UTC+8上午11:46:28，MRAB写道：
>> On 2016-10-19 03:15, chenyong20...@gmail.com wrote:
>> > Thanks Peter and Anssi for your kind help. Now I'm ok with the first
>> > question. But the second question still confused me. Why "it seems
>> > that
>> > after root = root.setdefault(ch,{}) tree['a'] and root are the same
>> > object" and follows tree['a']['b']? Thanks.
>> >
please forgive my stupid. I still can't follow this.
Let's see if I can explain. I am using 't' and 'r' instead of 'tree' and
'root', but otherwise it is the same as your original example.
t = {}
r = t
id(t)
2542235910088
id(r)
2542235910088
At this point, t and r are both references to the same empty dictionary.
r = r.setdefault('a', {})
This has done two things.
It has inserted the key 'a' into the dictionary, and set its value to {}.
t
{'a': {}}
id(t)
2542235910088
It has also rebound 'r' so that it now references the new empty dictionary
that has been inserted.
r
{}
id(r)
2542234429896
t['a']
{}
id(t['a'])
2542234429896
Now continue this process with r = r.setdefault('b', {}), and watch what
happens.
Hopefully this will help you to understand. Feel free to ask further if not
sure.
Frank Millman
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