Hi Caleb,
On Tue, Feb 23, 2021 at 11:05 PM Caleb Donovick <donov...@cs.stanford.edu>
wrote:
> What is the motivation for returning `None` on empty splits? I feel like
> this creates an unnecessary asymmetry. I don't personally have a use
> case for the this feature so I may be missing something but it seems like
> it would force an annoying pattern:
>
Split is used by the interpreter to implement except*. It needs to check
whether the first value of the split is empty or not (this determines
whether the except* clause executes).
This is more efficiently done with None. And then there is creating an
object that you don't need.
The intention is that you use except* rather than do this:
> ```
> try:
> foo()
> except ExceptionGroup as eg:
> g1, g2 = eg.split(predicate)
> for e in g1:
> handle_true(e)
> for e in g2:
> handle_false(e)
> ```
>
(as an aside - exception groups ended up not being iterable - that is one
of the rejected ideas, see explanation there).
> Also this creates an subtle difference with subgroup:
>
> ```
> g1, g2 = eg.split(predicate)
> h1, h2 = eg.subgroup(predicate), eg.subgroup(lambda e: not predicate(e))
> assert g1 == h1 and g2 == h2 # only true if `None not in {g1, g2}`
> ```
>
An empty subgroup should also return None. I see this is not mentioned in
the PEP, so I will add it. Thanks.
Irit
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