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From: Steven D'Aprano on 4 Jan 2010 21:35
On Tue, 05 Jan 2010 00:52:56 +0000, r0g wrote:

> I'd be strongly inclined to think the result would be the sequence on
> the left with the data from the second sequence appended to it. What's
> wrong with a little duck typing here eh?

That's not the existing behaviour. List concatenation doesn't mutate the
left hand list, it creates a new list:


>>> L = [1, 2, 3]
>>> L2 = L + [4, 5, 6]
>>> L
[1, 2, 3]
>>> L2
[1, 2, 3, 4, 5, 6]


But if you insist on in-place modification, why do you prefer appending
the right hand sequence to the left instead of prepending the left hand
sequence to the right?



--
Steven
From: r0g on 4 Jan 2010 22:01
Steven D'Aprano wrote:
> On Tue, 05 Jan 2010 00:52:56 +0000, r0g wrote:
>
>> I'd be strongly inclined to think the result would be the sequence on
>> the left with the data from the second sequence appended to it. What's
>> wrong with a little duck typing here eh?


OK, I hadn't read all the other responses when I posted, some of which
make fair points why this wouldn't be wise. Fair enough.



>
> That's not the existing behaviour. List concatenation doesn't mutate the
> left hand list, it creates a new list:
>


I would expect it to append. That's my prejudice though, as I do that
far more often :/



>
>>>> L = [1, 2, 3]
>>>> L2 = L + [4, 5, 6]
>>>> L
> [1, 2, 3]
>>>> L2
> [1, 2, 3, 4, 5, 6]
>
>
> But if you insist on in-place modification, why do you prefer appending
> the right hand sequence to the left instead of prepending the left hand
> sequence to the right?
>
>
>



In-place seems more natural for a mutable type. I admit the left right
thing is my prejudice though, western cultural bias I suppose. Its not
entirely unprecedented though, the parser reads left to right and the
leftmost terms take precedent in lazy logic evaluation.

Still, the responses to this have convinced me the + operator shouldn't
make assumptions, I'm more open to how += works though as it implies
in-place and the left over right precedent quite nicely.


Roger.
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