[Python-ideas] adding dictionaries

[Ron Adam]

On 07/28/2014 11:04 AM, Steven D'Aprano wrote:
> On Mon, Jul 28, 2014 at 03:33:06PM +0000,dw+python-ideas at hmmz.org  wrote:
>> >On Tue, Jul 29, 2014 at 12:59:51AM +1000, Steven D'Aprano wrote:
>> >
>>>> > > >One good reason is that people are still convinced "dict(A, **B)"
>>>> > > >makes some kind of sense.
>>> > >
>>> > >Explain please. dict(A, **B) makes perfect sense to me, and it works
>>> > >perfectly too. It's a normal constructor call, using the same syntax
>>> > >as any other function or method call. Are you suggesting that it does
>>> > >not make sense?
>> >
>> >It worked in Python 2, but Python 3 added code to explicitly prevent the
>> >kwargs mechanism from being abused by passing non-string keys.
> /face-palm
>
> Ah of course! You're right, using dict(A, **B) isn't general enough.
  and make the language easier to write and use
> I'm still inclined to prefer allowing update() to accept multiple
> arguments:
>
> a.update(b, c, d)

To me, the constructor and update method should be as near alike as possible.

So I think if it's done in the update method, it should also work in the 
constructor.  And other type constructors, such as list, should work in 
similar ways as well.  I'm not sure that going in this direction would be 
good in the long term.


> rather than a += b + c + d
>
> which suggests that maybe there ought to be an updated() built-in, Let
> the bike-shedding begin: should such a thing be spelled ?
>
> new_dict = a + b + c + d
>
> Pros: + is short to type; subclasses can control the type of new_dict.
> Cons: dict addition isn't obvious.

I think it's more obvious.  It only needs __add__ and __iadd__ methods to 
make it consistent with the list type.

The cons is that somewhere someone could be catching TypeError to 
differentiate dict from other types while adding.  But it's just as likely 
they are doing so in order to add them after a TypeError occurs.

I think this added consistency between lists and dicts would be useful.

But, Putting __add__ and __iadd__ methods on dicts seems like something 
that was probably discussed in length before, and I wonder what reasons 
where given for not doing it then.

Cheers,
   Ron