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Author steven.daprano
Recipients Saimadhav.Heblikar, cool-RR, eric.smith, ethan.furman, josh.r, mark.dickinson, pitrou, r.david.murray, rhettinger, scoder, serhiy.storchaka, steven.daprano
Date 2014-10-04.15:24:22
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On Sat, Oct 04, 2014 at 10:43:02AM +0000, Antoine Pitrou wrote:

> Just because something is discussed doesn't mean it needs a PEP.

I know that in general discussions do not need a PEP, but we seem to be 
rushing awfully fast into writing code before we even know what the code 
is supposed to do. Perhaps "need a PEP" was too strong, but we surely 
need to do something to decide on constistent and well-defined 
behaviour. As you pointed out earlier, Counters aren't precisely 
multisets, they're more like a hybrid mapping/multiset. It's not clear 
what behaviour subset and superset should have for such a hybrid. By 
rushing into code before deciding on what the code is supposed to do, we 
end up with inconsistent behaviour.

Here are a pair of Counters which compare "is subset", but not "is 
subset or equal" using Ram's patch:

py> Counter(a=1, b=0) < Counter(a=2)
py> Counter(a=1, b=0) <= Counter(a=2)

On the other hand, here's a pair which compares "is subset or equal", 
but not ("is subset" or "equal"):

py> Counter(a=0) <= Counter(b=0)
py> (Counter(a=0) < Counter(b=0)) or (Counter(a=0) == Counter(b=0))

Adding elements with count=0 changes whether a set is a subset or not:

py> Counter(a=0) < Counter(b=0)
py> Counter(a=0) < Counter(b=0, c=0)
py> Counter(a=0, b=0) < Counter(b=0, c=0)

I'm not convinced that mixed Counter and regular dict comparisons should 
be supported, but Ram's patch supports any Mapping type. Is that a good 
idea? There's been no discussion on that question.

Can somebody explain to me what this means? How is this meaningfully a 
subset operation?

py> Counter(a="eggs") < Counter(a="spam")

I supported adding these operations to Counter, but the last thing I 
want is to enshrine a set of ad-hoc and inconsistent semantics that 
don't match standard multiset behaviour and are defined by the 
implementation, instead of having the implementation defined by the 
desired semantics.
Date User Action Args
2014-10-04 15:24:23steven.dapranosetrecipients: + steven.daprano, rhettinger, mark.dickinson, pitrou, scoder, eric.smith, r.david.murray, cool-RR, ethan.furman, serhiy.storchaka, Saimadhav.Heblikar, josh.r
2014-10-04 15:24:23steven.dapranolinkissue22515 messages
2014-10-04 15:24:22steven.dapranocreate