function call questions

[MRAB]

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.
>
You call the .setdefault method with a key and a default value.

The .setdefault method does this: if the key is in the dict, it returns 
the associated value, else it puts the key and the default into the dict 
and then returns the default.