function call questions

[chenyong20000 at gmail.com]

在 2016年10月20日星期四 UTC+8下午1:32:18，Frank Millman写道：
> wrote in message 
> news:5506e4d8-bd1d-4e56-8d1b-f71fa8293393 at googlegroups.com...
> 
> 在 2016年10月19日星期三 UTC+8下午3:17:18，Peter Otten写道：
> > chenyong20000 at 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, chenyong20000 at 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

Hi Frank,

thanks very much for your kind help. Your reply is clear. But I'm hindered by those you've not explained.

I'm always confused by "r = r.setdefault('a', {})". when the first loop finished, as what you have pointed out,
> >>> t
> {'a': {}}
> >>> r
> {}

Then next "r = r.setdefault('b', {})" will run again. Here what is "r" in "r.setdefault('b',{})"? According to final result, it should be "t['a']", which I can't understand. I thought the command is r.setdefault, so it should still be last "r", i.e., empty {}. Could you please let me know what I missed? thanks.