dictionary containing a list

John Machin sjmachin at lexicon.net
Sat Oct 7 10:57:45 CEST 2006


Steve Holden wrote:
> John Machin wrote:
> > Ben wrote:
> >
> >>Hello...
> >>
> >>I have set up a dictionary into whose values I am putting a list. I
> >>loop around and around filling my list each time with new values, then
> >>dumping this list into the dictionary. Or so I thought...
> >>
> >>It would appear that what I am dumping into the dictionary value is
> >>only a pointer to the original list, so after all my iterations all I
> >>have is a dictionary whose every value is equal to that of the list the
> >>final time I looped around :-(
> >>
> >>Is there a way to acheive what I was attempting ? I have done something
> >>almost identical with classes  in a list before, and in that case a new
> >>instance was created for each list entry...
> >>
> >>
> >>I hope this makes some sense, and doesn't seem to head bangingly
> >>simple...
> >>
> >
> >
> > Do you consult your physician over a video link while wearing a ninja
> > costume down an unlit coal mine at midnight?
> >
> > Please consider the possibility that your description of what you think
> > your code might be doing is not enough for diagnosis.
> >
> > You may need to supply:
> > (1) a listing of your code
> > (2) a small amount of input data
> >    e.g. [(1, 'foo'), (42, 'bar'), (1, 'zot')]
> > (3) the output you expect from that input:
> >    e.g. {1: ['foo', 'zot'], 42: ['bar']}
> >
> One of the fascinating things about c.l.py is that sometimes a questin
> will be posted that makes almost no sense to me, and somebody else will
> casually read the OP's mind, home in on the issue and provide a useful
> and relevant answer.
>
> In this case it seems transparent to me, though probably not to you,
> that Ben's problem is rootd in the following behaviour, well-known in
> python but frequently confusing to noobs:
>
>   >>> a = [1, 2, 3]
>   >>> firstlist = a
>   >>> a.append('another element')
>   >>> firstlist
> [1, 2, 3, 'another element']
>   >>>
>

It's quite transparent to me that his symptom is caused by the one list
being used throughout the exercise, instead of one per different dict
key. What you have described is one possibility.

Here's another possibility: Making the charitable assumption that he
has an outer loop and an inner loop, maybe (as I think another poster
has already suggested) all he needs to do is move "mylist = []" inside
the outer loop. Note that he doesn't say explicitly whether the one
list that he gets is the *correct* list for the last key, or whether
it's the catenation of all the correct lists, or something else.

Yet another: Noobs do all sorts of funny things. He could be operating
on a "clean the bucket out after each use instead making a new one"
paradigm:

| >>> d= {}
| >>> L = []
| >>> L.append(1)
| >>> L.append(2)
| >>> d['a'] = L
| >>> d
| {'a': [1, 2]}
| >>> del L[:]
| >>> d
| {'a': []}
| >>> L.append(3)
| >>> L.append(4)
| >>> d['b'] = L
| >>> d
| {'a': [3, 4], 'b': [3, 4]}

Cheers,
John




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