On Wed, Jun 24, 2020 at 11:05 AM Brandt Bucher <brandtbuc...@gmail.com> wrote:
> Guido van Rossum wrote: > > IIUC the pattern matching uses either .get(key, <sentinel>) or > .__contains__(key) followed by .__getitem__(key). Neither of those will > auto-add the item to a defaultdict (and the Mapping protocol supports > both). @Brandt: what does your implementation currently do? Do you think we > need to specify this in the PEP? > > I still prefer under-specifying rather than over-specifying, in order to > allow the implementation some flexibility. Basically, "if you've got a > funny mapping, it may behave funny". > > For example, we currently perform a length check before even getting to > this point, which is allowed because the spec isn't very strict. This is a > good thing; we already have a section in the PEP that explicitly allows > compiler to perform transformations such as C(x) | C(y) -> C(x | y). > > If we have to specify it, __contains__ followed by __getitem__ is the way > to do it. The current behavior is subtly different (and probably wrong), > but I was going to change it today anyways... I had an outstanding TODO for > it. > So why not .get(key, <sentinel>)? You can reuse the sentinel, and this way it's a single call instead of two -- e.g. the code in Mapping implements both __contains__() and get() by calling __getitem__() and catching KeyError. -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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