[Python-ideas] PEP: Dict addition and subtraction

Brice Parent contact at brice.xyz
Wed Mar 6 08:40:00 EST 2019

Le 06/03/2019 à 13:53, Chris Angelico a écrit :
> On Wed, Mar 6, 2019 at 11:18 PM Brice Parent <contact at brice.xyz> wrote:
>> The major implication to such a
>> modification of the Dict.update method, is that when you're using it
>> with keyword arguments (by opposition to passing another dict/iterable
>> as positional), you're making a small non-backward compatible change in
>> that if in some code, someone was already using the keyword that would
>> be chosing (here "on_collision"), their code would be broken by the new
>> feature.
>> Anyway, if
>> the keyword is slected wisely, the collision case will almost never
>> happen, and be quite easy to correct if it ever happened.
> You can make it unlikely, yes, but I'd dispute "easy to correct".
> Let's suppose that someone had indeed used the chosen keyword (and
> remember, the more descriptive the argument name, the more likely that
> it'll be useful elsewhere and therefore have a collision). How would
> they discover this? If they're really lucky, there MIGHT be an
> exception (if on_collision accepts only a handful of keywords, and the
> collision isn't one of them), but if your new feature is sufficiently
> flexible, that might not happen. There'll just be incorrect behaviour.
> As APIs go, using specific keyword args at the same time as **kw is a
> bit odd. Consider:
> button_options.update(button_info, on_click=frobnicate, style="KDE",
> on_collision="replace")
> It's definitely not obvious which of those will end up in the
> dictionary and which won't. Big -1 from me on that change.
That's indeed a good point. Even if the correction is quite easy to make 
in most cases. With keyword only changes:

button_options.update(dict(on_click=frobnicate, style="KDE", on_collision="replace"))  # or button_options.update(dict(on_collision="replace"), on_click=frobnicate, style="KDE")

In the exact case you proposed, it could become a 2-liners:

button_options.update(dict(on_click=frobnicate, style="KDE", on_collision="replace"))

In my code, I would probably make it into 2 lines, to make clear that we 
have 2 levels of data merging, one that is general (the first), and one 
that is specific to this use-case (as it's hard written in the code), 
but not everyone doesn't care about the number of lines.

But for the other part of your message, I 100% agree with you. The main 
problem with such a change is not (to me) that it can break some edge 
cases, but that it would potentially break them silently. And that, I 
agree, is worth a big -1 I guess.

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