[Tutor] lists, name semantics

[Bill Allen]

Everyone that responded,

Thanks very much for the excellent explanations!  The distinction between a
reference to an object and a seperate copy of the object is quite clear now.

--Bill
On Apr 18, 2015 1:44 AM, "Alan Gauld" <alan.gauld at btinternet.com> wrote:

> On 18/04/15 04:16, Bill Allen wrote:
>
>> If I have a list defined as my_list = ['a','b','c'], what is the is
>> differnce between refering to it as my_list or my_list[:]?   These seem
>> equivalent to me.  Is that the case?  Is there any nuance I am missing
>> here?   Situations where one form should be used as opposed to the other?
>>
>
> Others have already given some good explanations.
> I'll add a slightly different take.
>
> Your confusion starts with your first statement:
>
> > I have a list defined as my_list = ['a','b','c']
>
> What you should be saying is
>
> I have a list defined as ['a', 'b', 'c']
>
> Thats the list object that you are working with. The object is completely
> separate from the name that you choose to associate
> with it.
>
> You then bound that list to a name: my_list.
> You could bind it to any number of names but
> there would still only be one object:
>
> foo = my_list
> bar = foo
> baz = my_list
>
> Now I have 4 names all referring to the same list object.
>
> The next source of confusion comes from another mist-statement:
>
> > differnce between refering to it as my_list or my_list[:]
>
> The [:] at the end is an operator that returns a copy of the list.
> So when you use it you are NOT referring to the original list
> at all. You are creating a new copy.
>
> So if we now take one of our previous names, say foo, and do:
>
> foo = my_list[:]
>
> foo no longer refers to your original list ['a','b','c']
> but to a completely new copy of that list.
>
> If you modify my_list the changes will show up when you look
> at bar and baz as well. But foo will be unchanged
>
> my_list[0] = 'z'
> print baz   -> prints ['z','b','c'] - the same list as my_list
> print foo   -> prints ['a','b','c'] - a different list object
>
> Understanding the separation of names from objects in Python is essential
> to understanding how it works. It is different to many
> other languages in this respect.
>
> And understanding the difference between identity and value is also
> important. Two completely different objects can have the same value
> and so appear the same but they are in fact entirely different.
> Think about two drivers who both buy the exact same model of car.
> They may look identical, but they are two separate cars.
>
> HTH
> --
> Alan G
> Author of the Learn to Program web site
> http://www.alan-g.me.uk/
> http://www.amazon.com/author/alan_gauld
> Follow my photo-blog on Flickr at:
> http://www.flickr.com/photos/alangauldphotos
>
>
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