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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Verbose and flexible args and kwargs syntax
Date: Mon, 12 Dec 2011 16:58:33 -0500
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On 12/12/2011 12:12 PM, Eelco wrote:

> Im not sure if this is a genuine understanding, or trollish
> obtuseness.

If you are referring to what I write, it is based on genuine 
understanding of Python.

> Yes, the target can be anywhere in the sequence. And yes, the
> resulting list can contain objects of any type, so its very flexible
> in that regard too.
>
> But to relate it to the topic of this thread: no, the syntax does not
> allow one to select the type of the resulting sequence. It always
> constructs a list.

One use case of *target is to ignore the stuff collected in the target 
because one only wants a few end values from the iterable. Another is to 
pull stuff out because one wants to iterate through the rest. For both 
uses, a list is as good as anything.

> Yes, we can cast the list to be whatever we want on the next line,

Convert. For the very few cases one wants to do this, it is quite adequate.

 > but the question is whether this language design can be improved upon.

Not easily.

> The choice of a list feels arbitrary,

On the contrary, a list is precisely what is needed to collect an 
indefinite number of leftovers.

 > adding another line to cast it to
> something else would be even more verbose, and whats more, there would
> be serious performance implications if one should seek to apply this
> pattern to a deque/linkedlist; it would make taking off the head/tail
> of the list from a constant to a linear operation.

For a linked list, no *target and no copying is needed:

head, tail = llist

>>>> head, deque(tail) = somedeque
>
> Is better in every way I can think of (readability, consistence,
> performance) than:

>>>> head, *tail = somedeque
>>>> tail = deque(tail)

But your suggestion is much worse in each way than

head = somedeque.popleft()

To repeat, there is no reason to copy even once. If one does not want to 
mutate the deque, then one mutates an iterator view of the deque.

-- 
Terry Jan Reedy