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

[Ethan Furman]

On 02/13/2015 06:57 PM, Andrew Barnert wrote:
> On Feb 13, 2015, at 18:46, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> 
>> Andrew Barnert wrote:
>>
>>> I think it's reasonable for a target to be able to assume that it will get a
>>> setattr or setitem when one of its subobjects is assigned to. You might need
>>> to throw out cached computed properties, ...
>>
>> That's what I was thinking. But I'm not sure it would be
>> a good design,
> 
> Now I'm confused.
> 
> 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.

--
~Ethan~

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 836 bytes
Desc: OpenPGP digital signature
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20150214/882a76e3/attachment.sig>