Hi All,
Thanks for explaining it so nice way. I got it now. What protocols I need to
learn, to define a custom immutable class ?
Thanks,
Arup Rakshit
a...@zeit.io
> On 16-Apr-2019, at 10:54 PM, Dennis Lee Bieber <wlfr...@ix.netcom.com> wrote:
>
> On Tue, 16 Apr 2019 13:13:18 +0530, Arup Rakshit <a...@zeit.io> declaimed the
> following:
>
>>
>> But what I don’t understand here is that what he said about the immutable
>> container objects. Why after changing the value of internal mutable objects
>> we still say the container object as mutable? Any examples to define what
>> the author meant will be helpful.
>>
>
>
> immTuple = ([], {})
>
> A tuple holding a list and a dictionary. You can not change which list
> or which dictionary -- that is fixed (immutable). But the way Python object
> bindings work is that the list and dictionary are not really IN the tuple
> -- the tuple, in the run time level, has references to the list and
> dictionary, with the those objects being stored somewhere else.
>
> Think of it as the tuple having strings that are tied to the content
> objects. You can not change the strings in the tuple -- they always lead
> from the tuple to the "contained" object. The list and dictionary are also
> containers -- you can add or remove objects (mutation) inside those
> containers, but they remain the same containers.
>
> To really understand this requires understanding Name Binding...
>
> aName = something
>
> binds "aName" to the object "something" (it does not copy something to a
> named variable -- unlike many classic languages; in C, FORTRAN, etc. that
> statement copies the contents of the memory address of "something" to the
> memory address of "aName"). Binding is more like writing "aName" on a
> post-it note, attaching that note to a string, and attaching the other end
> of the string to the object "something" (if "something" is another name,
> you follow the string to the actual object, and bind to the object).
>
> immTuple[0]
>
> accesses the first element (the list) of the tuple -- you can consider it
> to be a "name" bound to the first element..
>
> immTuple[0] = anything
>
> fails because tuples are immutable, and that statement says "change the
> string in the first element of the tuple to connect to a different object"
>
> immTuple[0].append(anything)
>
> works because it follows the string /to/ the list object, and that object
> is a container that allows modification of what it contains.
>
>
> --
> Wulfraed Dennis Lee Bieber AF6VN
> wlfr...@ix.netcom.com
>
> --
> https://mail.python.org/mailman/listinfo/python-list
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