[Python-ideas] Adding "+" and "+=" operators to dict

[Chris Barker]

On Sat, Feb 14, 2015 at 11:12 AM, Ethan Furman <ethan at stoneleaf.us> wrote:

> > The current design of Python guarantees that an object always gets a
> setattr or setitem when one of its elements is assigned to. That's an
> important property, for the reasons I suggested above. So any change would
> have to preserve that property. And skipping assignment when __iadd__
> returns self would not preserve that property. So it's not just
> backward-incompatible, it's bad.
>
> --> some_var = ([1], 'abc')
> --> tmp = some_var[0]
> --> tmp += [2, 3]
> --> some_var
> ([1, 2, 3], 'abc')
>
> In that example, 'some_var' is modified without its __setitem__ ever being
> called.
>

not really -- an object in the some_var is modified -- there could be any
number of other references to that object -- so this is very much how
python works.

The fact that you can't directly use augmented assignment on an object
contained in an immutable is not a bug, but it certainly is a wart --
particuarly since it will raise an Exception AFTER it has, in fact,
performed the operation requested.

I have argued that this never would have come up if augmented assignment
were only used for in-place operations, but then we couldn't used it on
integers, which was apparently desperately wanted ;-) (and the name
"augmented assignment" would be a good name, either...).

I don't know enough about how this all works under the hood to know if it
could be made to work, but it seems the intention is clear here:

object[index] += something.


is a shorthand for:

tmp = object[index]
tmp += something

or in a specific case:

In [66]: t = ([], None)

In [67]: t[0].extend([3,4])

In [68]: t
Out[68]: ([3, 4], None)


Do others agree that this, in fact, has an unambiguous intent? And that it
would be nice if it worked? OR am I missing something?

-Chris




-- 

Christopher Barker, Ph.D.
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