[Python-ideas] PEP: Dict addition and subtraction

[Neil Girdhar]

I think that sequence should be fixed.

On Fri., Mar. 1, 2019, 7:12 p.m. Brandt Bucher, <brandtbucher at gmail.com>
wrote:

> While working through my implementation, I've come across a couple of
> inconsistencies with the current proposal:
>
> > The merge operator will have the same relationship to the dict.update
> method as the list concatenation operator has to list.extend, with dict
> difference being defined analogously.
>
> I like this premise. += for lists *behaves* like extend, and += for dicts
> *behaves* like update.
>
> However, later in the PEP it says:
>
> > Augmented assignment will just call the update method. This is
> analogous to the way list += calls the extend method, which accepts any
> iterable, not just lists.
>
> In your Python implementation samples from the PEP, dict subclasses will
> behave differently from how list subclasses do. List subclasses, without
> overrides, return *list* objects for bare "+" operations (and "+=" won't
> call an overridden "extend" method). So a more analogous
> pseudo-implementation (if that's what we seek) would look like:
>
> def __add__(self, other):
>     if isinstance(other, dict):
>         new = dict.copy(self)
>         dict.update(new, other)
>         return new
>     return NotImplemented
>
>  def __radd__(self, other):
>     if isinstance(other, dict):
>         new = dict.copy(other)
>         dict.update(other, self)
>         return new
>     return NotImplemented
>
> def __iadd__(self, other):
>     if isinstance(other, dict):
>         dict.update(self, other)
>         return self
>     return NotImplemented
>
> This is what my C looks like right now. We can choose to update these semantics to be "nicer" to subclasses, but I don't see any precedent for it (lists, sets, strings, etc.).
>
> Brandt
>
>
> On Fri, Mar 1, 2019 at 11:41 AM Brett Cannon <brett at python.org> wrote:
>
>>
>>
>> On Fri, Mar 1, 2019 at 8:50 AM Brandt Bucher <brandtbucher at gmail.com>
>> wrote:
>>
>>> I’ve never been part of this process before, but I’m interested in
>>> learning and helping any way I can.
>>>
>>
>> Thanks!
>>
>>
>>>
>>> My addition implementation is attached to the bpo, and I’m working today
>>> on bringing it in line with the PEP in its current form (specifically,
>>> subtraction operations).
>>>
>>> https://github.com/python/cpython/pull/12088
>>>
>>
>> When your proposed patch is complete, Brandt, just ask Steven to update
>> the PEP to mention that there's a proposed implementation attached to the
>> issue tracking the idea.
>>
>> -Brett
>>
>>
>>>
>>>
>>> Brandt
>>>
>>> On Mar 1, 2019, at 08:26, Steven D'Aprano <steve at pearwood.info> wrote:
>>>
>>> Attached is a draft PEP on adding + and - operators to dict for
>>> discussion.
>>>
>>> This should probably go here:
>>>
>>> https://github.com/python/peps
>>>
>>> but due to technical difficulties at my end, I'm very limited in what I
>>> can do on Github (at least for now). If there's anyone who would like to
>>> co-author and/or help with the process, that will be appreciated.
>>>
>>>
>>> --
>>> Steven
>>>
>>> <dict_addition_pep.txt>
>>>
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