On Apr 8, 2015, at 05:34, Anthony Towns <firstname.lastname@example.org
Ah, I see. I was thrown by := looking like assignment in Pascal or some of the mutable ML variants, where it obviously can't be part of a subexpression, but once you get past that, yeah, it's obvious. Sorry.
But argspec is more powerful and simpler than that--well, not argspec, but fullargspec and especially Signature. Just use inspect.signature and its bind method. That gives you a BoundArguments, and then you can just do boundargs.args to match by position or boundargs.arguments['spam'] to match by keyword; for an argument that could have been passed either way, the value will show up both ways. (And for args that were passed into *args, however they got there, that works too--for example, if you def __init__(self, x, *args) and then do signature(cls.__init__).bind(1, 2, 3), you can see the 3 as args or as arguments['args'].)
Meanwhile, I'm a bit wary about type implicitly assuming that attributes and __init__ parameters match one-to-one like this. Sure, that's often right, and it will often raise an AttributeError or a TypeError (from Signature.bind) when it's not right--but those cases where it does the wrong thing will be pretty surprising, and hard for a novice to work around. Making classes explicitly opt in to matching seems safer.
On the other hand, allowing them to opt in with a dead-simple decorator like @simplematch (or a base class or metaclass or register function or whatever) that just adds a __deconstruct__ method like the one you defined does seem a lot easier than any of the ideas I originally suggested.
One last thing here: I like your NotDeconstructable; if we make that a subclass of ValueError, we can change normal tuple unpacking to raise that as well, without breaking any code.
Sure, but that's a pretty bad way to process Python lists (you end up making N copies of average length N/2, plus N stack frames out of a finite limit, and the code ends up more verbose and less readable), so anything that makes that Scheme/ML style look more inviting than iteration is probably not an advantage...
You're starting to turn me around on my dislike of using pattern matching for regexps; that's more readable than existing re code...
I originally thought the extra indirection of letting something pose as the match value's type while actually not being that type was unnecessary, but this case, an re object's type posing as the string's type, seems like a great counterexample.
And it doesn't add much complexity, so... I think it's worth it.
I don't know about that. It wouldn't be hard to remember the rule once you learn it, but it may not be that easy to learn, because at first glance it doesn't make sense. It's weird to have "foo" mean "assign to foo" but "foo as bar" in the exact same context mean "use the value of foo, then assign to bar". It's unprecedented in Python (with doesn't assign to the context manager name instead of using it as an expression if you leave off the "as") and I can't think of any other language that does something equivalent. (And I'm pretty sure it would inspire rants about "warts" by a certain person on this list. :)
And again, unless use cases for pattern matching in Python turn out to be very different than in ML and friends (which is certainly _possible_, but not something I'd want to just assume--if we don't expect somewhat similar uses, why are we even looking to them for inspiration?), I think the need for matching a variable's value won't be that common, so we shouldn't waste the nicest syntactic form on it... (Although I'm definitely not sold on my throwaway suggestion of "@foo", which has misleading parallels both in Python and in some of the languages the syntax is inspired by...)
Yeah, I tried to come up with a rule that disallowed literal values in an assignment (but not case) target list unless there's some kind of "real pattern" somewhere within (possibly recursively) the target list, but all of the possibilities seem horrible. So I decided the best thing to do was just not mention that "0 = n" is no longer a syntax error, but a legal runtime assertion that hopefully people don't use.
Also, while I don't think people should explicitly write "0 = n", I can imagine a code generator or MacroPy macro or whatever that takes generates a pattern based on its arguments that might want to generate something like "0 = n" for the no-args case, and why not allow it?
Anyway, I think your additions and changes make a much better proposal than what I originally had, and we're now probably on track to the best Pythonic design for ML-style pattern matching--but I still don't think it's worth adding to Python, or even writing as a PEP for Guido to reject. Have you ever written code where you really missed the feature (except in the first hour back on Python after a week of Haskell hacking)? Or seen any real code that would be substantially improved? If not, this still just seems like a feature for its own sake, or more than one way to do it for no reason.