[Python-Dev] PEP 622 and Walrus operator

27 Aug 2020

      I'm intrigued by PEP 622. It sounds like a great addition, as the current PEP mentions, the lack of being able to match against a local variable seems like a great limitation. 

I'll admit to being over my head here; last time I did language design was undergrad CS 15 years ago. However, I was wondering if the following was considered (if it was, it isn't mentioned in rejected ideas): Force all assignment to happen through the walrus operator. We have assignment expressions, let's use them! This then frees up variable names to act and look like normal matches. The code first mentioned in the overview would then look like this:

def make_point_3d(pt):
    match pt:
        case (x := _, y := _):
            return Point3d(x, y, 0)
        case (x := _, y := _, z := _):
            return Point3d(x, y, z)
        case Point2d(x := _, y := _):
            return Point3d(x, y, 0)
        case Point3d(_, _, _):
            return pt
        case _:
            raise TypeError("not a point we support")

Sequence pattern could look like this:

From sequence patterns:

match collection:
    case 1, [x, others := *_]: # or others := *rest
        print("Got 1 and a nested sequence")
    case (1, x := _):
        print(f"Got 1 and {x}")

I'll admit to it not being as elegant, but it's also not as confusing. This would eliminate the discrepancy between "case name" and "case obj.name". 

x = 1
obj.x = 2
match something:
   case obj.x:
     print('something is 2')
   case x:
    print('something should be 1....oops')

The problem with just leaving it to a SyntaxWarning is that 1) those get ignored, leading to bugs. 2) Even if followed, you'll end up with a lot of code looking something like this:
# x, y previously declared
dummy = SimpleNamespace()
dummy.x = x
dummy.y = y
match something:
   case dummy.x:
     print('something is x')
   case dummy.y:
    print('something is y')
   ...

[Python-Dev] PEP 622 and Walrus operator

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