On Thu, Mar 21, 2019 at 9:17 AM Serhiy Storchaka <storchaka@gmail.com> wrote:
21.03.19 14:51, Chris Angelico пише:
> ... then, in the interests of productive discussion, could you please
> explain? What is it about dict addition that makes it harder to
> understand than other addition?

Currently the + operator has 2 meanings for builtin types (both are
widely used), after adding it for dicts it will have 3 meanings.

3 > 2, is not?

It depends how abstractly you define the "meanings".

If you define + as "arithmetic addition" and "sequence concatenation", then yes, there are 2. But novices have to learn that the same concatenation operator applies to strings as well as lists/tuples. And when reading x + y, it is probably relevant whether x and y are numbers, strings, or sequence containers like lists.

The proposal would generalize "sequence concatenation" to something like "asymmetric sequence/collection combination". (Asymmetric because d1 + d2 may not equal d2 + d1.) It seems a natural extension to me, though the | alternative is also reasonable (interpreted as taking the OR of keys in the two dicts; but unlike unioning two sets, the dict-merge operator would be asymmetric). The third proposed alternative, <<, has no "baggage" from an existing use as a combination operator, but at the same time it is a more obscure choice.