[Python-ideas] Re: PEP 584: Add + and += operators to the built-in dict class.
On 22/10/2019 06:43, Richard Musil wrote:
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.
My point is that if I saw {"key1": "val1", "key2": "val2"} + {"key1":
"val3"}, I would expect that it would be equivalent to {"key1": "val1",
"key2": "val2", "key1": "val3"}.
Similarly, I would expect that
deque([1, 2, 3], maxlen=4) + deque([4, 5]) == deque([1, 2, 3, 4, 5],
maxlen=4) == deque([2, 3, 4, 5], maxlen=4)
which indeed is true.
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[Python-ideas] Re: PEP 584: Add + and += operators to the built-in dict class.
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"}
so using + we would actually have
{"key1": "val1", "key2": "val2"} + {"key1": "val3"} == {"key1": "val1",
"key2": "val2", "key1": "val3"}
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