[Tutor] list of dict question

Francesco Loffredo fal at libero.it
Mon Oct 11 11:07:38 CEST 2010

Thank you, Alan and Dave, for your spotting this weak point in my 
understanding of Python!

On 11/10/2010 2.11, Dave Angel wrote:
> On 2:59 PM, Alan Gauld wrote:
>> "Francesco Loffredo" <fal at libero.it> wrote
>>> did, Roelof's code would work perfectly, and you could store in a list
>>> all the subsequent changes of a dictionary without calling them with
>>> different names.
>> You don;'t need dfifferent names. Provided the name creates a
>> new object inside the loop you can reuse the same name.
>> Roeloff's problem was that he only created one object, outside
>> his loop.
>> lst = []
>> for n in range(3):
>> obj = {}
I didn't know that this creates a new obj if obj already exists, I 
thought it would just update it. That's my mistake.
Does this mean that if I write:
obj = {}
obj = {}
obj = {}
then I'll create 3 different dictionaries, with the name obj referring 
to the third?

>> obj[n] = str(n)
>> lst.append(obj)
>> Creats a list of 3 distinct dictionaries but only uses one name - obj.
Ok, this does not lose too much in elegance.

>>> I understand that if .append() stored a copy of the dict in the list,
>>> you will end up with lots of copies and a huge amount of memory used by
>>> your list, but that's exactly what will happen if you make those copies
>>> yourself. But you wouldn't have to devise a way to generate a new name
>>> for the dictionary every time you need to update it.
>> Python uses references throughout, what you are suggesting would
>> be a change to the normal way that Python uses names.
I needed a hint about what is "the normal way that Python uses names", 
thank you.

> Probably more importantly, if a language only implemented copies, you
> couldn't have references without some different syntax. On the other
> hand, with Python, everything's a reference, and if you want a copy, you
> make one when you need it. You never do for immutables, and only you
> know when you need it for mutable objects.
> DaveA
Thank you too, Dave. I thought that to make a copy you would have to 
create a different name, and this sounded too cumbersome for me...

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