Python: Acceptance of Pattern Matching – PEP 634

Python: Acceptance of Pattern Matching – PEP 634(mail.python.org)

48 points by potomak 5 years ago | 12 comments

JNRowe 5 years ago |

Direct links to the PEPs themselves, to perhaps save others a little time.

Accepted:

Specification https://www.python.org/dev/peps/pep-0634/

Motivation and Rationale https://www.python.org/dev/peps/pep-0635/

Tutorial https://www.python.org/dev/peps/pep-0636/

Rejected:

Unused variable syntax https://www.python.org/dev/peps/pep-0640/

Explicit Pattern Syntax for Structural Pattern Matching https://www.python.org/dev/peps/pep-0642/

pansa2 5 years ago |

It's interesting that pattern matching has been accepted despite a poll showing a majority (65%) of core developers opposed to this particular proposal, and a plurality (44%) opposed to pattern matching altogether.

https://discuss.python.org/t/gauging-sentiment-on-pattern-ma...

rcxdude 5 years ago | |

I'm not sure if you can read the 'accept 634 + another' options as opposed to 'accept 634' (that said you can't necessarily read it as in support of it either. This poll is a bit of a mess in that regard). But it's certainly contentious (all options including accepting 634 still only comes to 50%).

pansa2 5 years ago | | |

> I'm not sure if you can read the 'accept 634 + another' options as opposed to 'accept 634'

However, I’d say that “accept 634 + another” is clearly opposed to “accept 634 & reject both others”, which is the decision that has been made.

pfalcon 5 years ago | |

Don't forget that the poll is dated 2020-11-20 (and majority of votes were cast soon). There was a lot of discussion (and thus maybe elaboration of opinions) since that.

pansa2 5 years ago |

Is it still the case that these two pieces of code have different behaviour?

    match status:
        case 404:
            return "Not found"

    not_found = 404
    match status:
        case not_found:
            return "Not found"

IIRC in an earlier proposal, the former would check for equality (`status == 404`) and the latter would perform an assignment (`not_found = status`).

bilboa 5 years ago | |

Yes, the variable names in the case patterns are treated like variables on the left hand side of an assignment statement. Whatever value they may have had before is irrelevant. A pattern which consists solely of a variable name is a wildcard pattern that matches anything, just as it would be on the left side of an assignment.

Edit: It is possible to use constants or variables in a case pattern, but they have to be dotted names to not be treated like a capture pattern. So this would work for your example:

  class HttpStatus(Enum):
      NOT_FOUND = 404

  match status:
     case HttpStatus.NOT_FOUND:
         return "Not found"

wodenokoto 5 years ago | | |

Do you know what would happen with:

    def _id(x): return x

    not_found = 404
    match status:
        case _id(not_found):
            return "Not found"

airstrike 5 years ago | |

Yes, and this is incredibly disappointing.

Couldn't we achieve the same functionality with a little less ambiguity using the following syntax?:

    not_found = 404
    match status:
        case not_found:
            return "Not found"
            
        case _ as foo:
            do_something(foo)
            return "Match anything"

it even works for the example in PEP 365

    match json_pet:
        case {"type": "cat", "name": _ as name, "pattern": _ as pattern}:
            return Cat(name, pattern)
        case {"type": "dog", "name": _ as name, "breed": _ as breed}:
            return Dog(name, breed)
        case _:
            raise ValueError("Not a suitable pet")

pfalcon 5 years ago | | |

I'm with everyone on "incredibly disappointing" valuation. But being pragmatic, I had to accept the idea "it can be addressed later" vs continuing this mess. And there was truly mess, e.g. PEP642, which initially tried to addressed just this issue, turned in a hairy monster trying to random-patch pattern matching considerably in rather weird ways.

reagle 5 years ago | |

I don’t understand this. How are we supposed to read the case word? Is there an online interpreter for this so we can play with it?

match json_pet: case {"type": "cat", "name": _ as name, "pattern": _ as pattern}: return Cat(name, pattern) case {"type": "dog", "name": _ as name, "breed": _ as breed}: return Dog(name, breed) case _: raise ValueError("Not a suitable pet")