dictionary containing a list

Steve Holden steve at holdenweb.com
Sat Oct 7 09:18:48 CEST 2006

John Machin wrote:
> Ben wrote:
>>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
> 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']

Ben probably needs to look at creating copies using the list() type:

  >>> a = [1, 2, 3]
  >>> firstlist = list(a)
  >>> a.append('another element')
  >>> firstlist
[1, 2, 3]

or perhaps, in omore complex circumstances, using the copy module.

Steve Holden       +44 150 684 7255  +1 800 494 3119
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Skype: holdenweb       http://holdenweb.blogspot.com
Recent Ramblings     http://del.icio.us/steve.holden

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