On Sun, Oct 20, 2019 at 11:48:10PM -0000, Dominik Vilsmeier wrote:
> Regarding "|" operator, I think a drawback is the resemblance with
> "or" (after all it's associated with "__or__") so people might assume
> behavior similar to `x or y` where `x` takes precedence (for truthy
> values of `x`). So when reading `d1 | d2` one could falsely assume
> that values in `d1` take precedence over the ones in `d2` for
> conflicting keys. And this is also the existing `set` behavior (though
> it's not really relevant in this case):
There's a much easier way to demonstrate what you did:
>>> {1} | {1.0}
{1}
In any case, dict.update already has this behaviour:
>>> d = {1: 'a'}
>>> d.update({1.0: 'A'})
>>> d
{1: 'A'}
The existing key is kept, only the value is changed. The PEP gives a
proposed implementation, which if I remember correctly is:
# d1 | d2
d = d1.copy()
d.update(d2)
so it will keep the current dict behaviour:
- keys are stable (first key seen wins)
- values are updated (last value seen wins)
I think that, strictly speaking, this "keys are stable" behaviour is not
guaranteed by the language reference. But it's probably so deeply built
into the implementation of dicts that it is unlike to ever change. (I
think Guido mentioned something about it being a side-effect of the way
dict `__setitem__` works?)
--
Steven
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