[Python-ideas] PEP: Dict addition and subtraction

Chris Angelico rosuav at gmail.com
Wed Mar 6 07:53:47 EST 2019

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",

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.


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