Comment by pansa2

2 days ago

The real crime is the design of Python's pattern matching in the first place:

    match status:
        case 404:
            return "Not found"

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

Everywhere else in the language, you can give a constant a name without changing the code's behaviour. But in this case, the two snippets are very different: the first checks for equality (`status == 404`) and the second performs an assignment (`not_found = status`).

https://x.com/brandon_rhodes/status/1360226108399099909

22 comments

pansa2

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user453  2 days ago

That's destructuring, a feature not a bug. Same as it works in any functional language - and tremendously useful once you get the hang of it

  • pansa2  2 days ago

    At least functional languages tend to have block scope, so the latter snippet introduces a new variable that shadows `not_found` instead of mutating it.

  • b3orn  2 days ago

    No, at least in Erlang a variable is assigned once, you can then match against that variable as it can't be reassigned:

        NotFound = 404,
    case Status of
        NotFound -> "Not Found";
        _ -> "Other Status"
    end.

    That snippet will return "Other Status" for Status = 400. The Python equivalent of that snippet is a SyntaxError as the first case is a catch all and the rest is unreachable.

  • quotemstr  2 days ago

    Destructuring is a feature. Making it easy to confuse value capture and value reference was an error. Add single-namespace locals and you have a calamity.

  • mcdeltat  2 days ago

    Destructuring yes but you can still argue it's poorly designed. Particularly unintuitive because matching on a nested name e.g. module.CONSTANT works for matching and doesn't destructure. It's just the use of an unnested name which does destructuring.

    What Python needs is what Elixir has. A "pin" operator that forces the variable to be used as its value for matching, rather than destructuring.

  • xdennis  2 days ago

    It wouldn't be a problem is Python had block level variable scope. Having that destructuring be limited to the 'case' would be fine, but obliterating the variable outside of the match is very unexpected.

quotemstr  2 days ago

And there was a much better proposal that got rejected in favor of what we got: https://peps.python.org/pep-0642/

  • danudey  2 days ago

    The very first example there shows a match/case block where almost every single case just runs "pass" and yet every single one has a side effect. It's very difficult to read at first, difficult to understand if you're new to the syntax, and is built entirely around side effects. This might be one of the worst PEPs I've ever seen just based on that example alone.

    Fun fact: you can do the same thing with the current match/case, except that you have to put your logic in the body of the case so that it's obvious what's happening.

almostgotcaught  2 days ago

because it's not a `switch` it's a `match` ie pattern matching...