How to spell anonymous default case?

[gvanrossum]

We could either use case _: ... or else: ....

In favor of else, this is what you use for if-elif-elif.

Against else:

• We already have too many different uses of else (for-else/while-else, if-else, try-except-else).
• You don't need it, since you can write case _: ...
• The special meaning of _ as a pattern is useful inside nested patterns too, e.g.

match x:
    case [1, 2, _]: print("Starts with 1, 2")
    ...

[Tobias-Kohn]

I feel this discussion is closely connected to issue #5 (as I mentioned there) and depends on the semantics that we want to convey. If matches have to be exhaustive, I would argue strongly against the use of else, otherwise I would just favour case _ ;-).

My issue with else is that it is IMHO not necessarily clear what it connects to. This is different with if-elif-else chains, as the else there is an intrinsic part of the entire design (as an extension of if-else). Notwithstanding, having taught Python for quite some time now, I often found my students struggling with the semantics of if-if-else vs. if-elif-else, making me quite aware of the difficulty of such constructs. Moreover, a couple of students replaced all ifs with elifs to have a consistent picture of elif-elif-elif. Consistency in the syntax (i.e. using case everywhere) might therefore help learners.

Anyway, using the case _: variant, we would have something like:

match x:
    case [1, 2, _]: print("Starts with 1, 2")
    case _: print("Anything else")

However, if we went for an else, I would place an else one level higher like so:

match x:
    case [1, 2, _]: print("Starts with 1, 2")
else:
    print("No pattern matched the value")

The reason for this being that the else belongs logically to the match-statement, expressing that no pattern actually matched the value. An else on the same level as the cases would read much more as if belonging to that last case in the list. While we probably all picture the cases to replace if-elif-else-chains, this is not necessarily obvious to others. In principle, we could even have something like (well, now, this looks very ugly to me, to be honest):

match x:
    case [1, 2, _]:
        print("Starts with 1, 2")
    else:
        print("Starts with something else...")
    case [_, 9]:
        print("and ends in nine!")

[gvanrossum]

Okay, there's a principle of using the fewest ingredients necessary somewhere. Let's use 'case _:' instead of 'else:'.