在 2016年10月22日星期六 UTC+8下午9:15:06，Frank Millman写道： > wrote in message > news:9c91a4cf-1f3e-43b3-b75c-afc96b0b4...@googlegroups.com... > > > I have read Anssi's post already before I sent the post. To be frankly, I > can't understand why he got the right answer. I'm sorry for my silly. "So > when we assign to r again, it's the empty dict inside t (the one accessed > by key 'a')". I do can't understand why this happens. that is the reason why > I have asked for this once again and again. There must be some import point > I missed but I don't what is it. > > Let's try this - > > >>> t = {} > >>> r = t > >>> r = r.setdefault('a', {}) > >>> t > {'a': {}} > > I think you are happy up to this point. > > We now have three objects - > > "t" is a dictionary > 'a' is a key in the dictionary > {} is the value associated with the key 'a' in "t" > > I think you are happy up to this point. > > The question is, what is "r"? > > Before the assignment, "r" was a reference to the dictionary referenced by > "t". > > After the assignment, "r" no longer refers to "t". It is now a reference to > the > third object listed above, the {} that is the value associated with the key > 'a'. > > >>> t > {'a': {}} > >>> t['a'] > {} > >>> r > {} > >>> t['a] is r > True > > Keep looking at this until it sinks in. "r" and "t['a']" are *the same > object*. We just have two ways of accessing it. > > Try adding some key/values to the empty dictionary - > > >>> r['x'] = 99 > >>> r > {'x': 99} > >>> t['a'] > {'x': 99} > >>> t > {'a': {'x': 99}} > > I will pause at this point, and give you a moment to absorb that. > > Hopefully, the penny will drop and everything will become clear. > > If not, let us know which of the above steps you do not understand. > > Good luck - keep plugging away, and you will get there :-) > > Frank > > P.S. I assume you understand that the lines prefixed with '>>>' are to be > entered while in the python interpreter. It is really important that you > type these lines in yourself and examine the results.
Hi Frank, I got it this time. Thanks very much for your help. I thought r is an empty dictionary without any connection to t before. Now I know that happened. Thanks. regards skyworld -- https://mail.python.org/mailman/listinfo/python-list
