using "constant" variables in python match statement

Question

I am writing a program and python and want to use the match-case statement allowed in python 3.10 to be able to switch on 'enum' values i.e:

match token.type:
  case TOK_INT: #0
    # do stuff
  case TOK_STRING: #1
    # do stuff
  etc...

however trying to do this causes python to throw a SyntaxError: name capture 'TOK_INT' makes remaining patterns unreachable error. I know I can fix it by manually typing the number associated with each 'enum' variable but I am wondering if there is a better way of going about this?

Your inappropriate tag `switch-statement` shows that you have not understood, that python's [structural pattern matching](https://www.python.org/dev/peps/pep-0636/) is [not a switch statement](https://www.youtube.com/watch?v=ASRqxDGutpA). — Richard Neumann, Feb 17 '22 at 00:52
@RichardNeumann i am well aware, in my use case however that's all I'm really trying to do but thanks for the heads up anyways. — james ray, Feb 17 '22 at 00:55
Your problem is still that you have not understood what structural pattern matching does. It maches structures. And a single name (aka variable) has the same structure as any other single name. So the first *structural* match shadows all subsequent *structural* matches. I.e. there is no value matching. Read the docs and / or watch the video. — Richard Neumann, Feb 17 '22 at 01:01
@RichardNeumann once again, I am aware. the more proper thing here is to just use a if-else chain and be done with it. I ask this question because I am not making production level code and I want something that looks nice, not because I'm looking for the most proper / correct this to do. I only wanted something that can be used LIKE a switch, not a literally switch. — james ray, Feb 17 '22 at 01:11

score 3 · Accepted Answer · answered Feb 17 '22 at 01:02

3

This will work with any dotted name (like math.pi). However an unqualified name (i.e. a bare name with no dots) will be always interpreted as a capture pattern, so avoid that ambiguity by always using qualified constants in patterns.

you can refer here

answered Feb 17 '22 at 01:02

jinger7281

71
2

...or don't abuse structual pattern matching as an if-else replacement at all. – Richard Neumann Feb 17 '22 at 01:03
@jasonharper give the exact answer before me – jinger7281 Feb 17 '22 at 01:05
1

@RichardNeumann why is that abuse to use match as switch? (Structural) matching on values of an Enum type (i.e using a match as a switch) is a better idiom than if-else tree, is it not? – alexei Dec 08 '22 at 05:31

using "constant" variables in python match statement

1 Answers1