On Tue, Oct 22, 2019 at 12:05 AM Jan Greis <jan.r.greis@gmail.com> wrote:
On 21/10/2019 21:14, Dominik Vilsmeier wrote:
> Exactly, so the dict "+" behavior would match the set "|" behavior, preserving the keys. But how many users will be concerned about whether the keys are going to be preserved? I guess almost everybody will want to know what happens with the values, and that question remains unanswered by just looking at the "+" or "|" syntax. It's reasonable to assume that values are preserved as well, i.e. `d1 + d2` adds the missing keys from `d2` to `d1`. Of course, once you know that "+" is actually similar to "update" you can infer that the last value wins.

There's one reason for + which I feel is being missed (though I think
someone may have briefly mentioned it last time this topic was brought
up): If we look at the behaviour of dict literals, adding two dicts
actually behaves like concatenation in the sense that

{"key1": "val1", "key2": "val2", "key1": "val3"} == {"key1": "val3",
"key2": "val2"}

which is exactly what we would get by adding {"key1": "val1", "key2":
"val2"} and {"key1": "val3"}

It is not a "concatenation" though, because you lost {"key1": "val1"} in the process. The concatenation is not _just_ "writing something after something", you can do it with anything, but the actual operation, producing the result.


so using + we would actually have

{"key1": "val1", "key2": "val2"} + {"key1": "val3"} ==  {"key1": "val1",
"key2": "val2", "key1": "val3"}
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