Compare sets in a match expression

Question

I want to compare sets in a match expression in Python. So imagine you want to compare multiple variables but they do not have any order. They only have to include the same values as a whole. In this case you should use match if you have multiple combinations you want to test for the sake of readiness. That could look like the following:

match set((string1, string2, string3)):
    case set((' Hello', ' world', '!')):
        ...
    case set(('beatufiful', 'fair', 'nice')):
        ...
    case set(('extremly', 'hyper', 'strongly')):
        ...

However, this approach does not work and now I fear I it may not be possible to compare sets this way. Comparing through '{'...',...}' expressions gives a syntax error. However comparing the following way works:

match 1:
    case 1 if set((' Hello', ' world', '!')) == set((string1, string2, string3)):
        ...
    case 1 if set(('beautiful', 'fair', 'nice')) == set((string1, string2, string3)):
        ...
    case 1 if set(('extremely', 'hyper', 'strongly')) == set((string1, string2, string3)):
        ...

(I tested it accurately, for every (special) occasion with unittest.)

But this is not the best approach for the unnecessary code.

So how do I get to a cleaner variant?

@Barmar because peps.python.org/pep-0622/#rationale-and-goals. it is more concise. Also stackoverflow.com/questions/67961895/… — Ichwerdennauchsonst
– Ichwerdennauchsonst, Commented Oct 27, 2024 at 18:25
You are not trying to match the structure of the data. That's what that rationale would be about. — mkrieger1
– mkrieger1, Commented Oct 27, 2024 at 18:27
Sets aren't really structured in the way match statements aim to exploit. — chepner
– chepner, Commented Oct 27, 2024 at 18:27

wim · Accepted Answer · 2024-10-27 18:41:22Z

2

Why do you want to use structural pattern matching for this? There is no "structure" to match, so an if/elif would be more usual.

if s == {" Hello", " world", "!"}:
    ...
elif s == {"beatufiful", "fair", "nice"}:
    ...
elif s == {"extremly", "hyper", "strongly"}:
    ...

If you want to associate a bunch of sets with values, you can use a dict.

lookup = {
    frozenset((" Hello", " world", "!")): "case 1",
    frozenset(("beatufiful", "fair", "nice")): "case 2",
    frozenset(("extremly", "hyper", "strongly")): "case 3",
}

It will require using frozenset keys, because normal sets are unhashable.

answered Oct 27, 2024 at 18:41

wim

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blhsing · Accepted Answer · 2024-10-28 01:21:07Z

2

An expression after a case statement is not a normal expression, but a pattern. A set can't be used as a pattern for structural matching since it is unordered in Python and can't be looked up by keys, offering no way of mapping a value within the container to a capture variable.

If you want to match a set by a value you can instead define the value as a class variable so the value can be specified as a dotted name to prevent it from being treated as a pattern:

class Phrase:
    GREETING = {'Hello', 'world', '!'}
    PRAISE = {'beautiful', 'fair', 'nice'}
    EMPHASIS = {'extremely', 'hyper', 'strongly'}

match {'Hello', '!', 'world'}:
    case Phrase.GREETING:
        print('Greeting')
    case Phrase.PRAISE:
        print('Praise')
    case Phrase.EMPHASIS:
        print('Emphasis')

This outputs:

Greeting

Demo here

edited Oct 28, 2024 at 1:21

answered Oct 28, 2024 at 1:00

blhsing

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2 Comments

no comment Over a year ago

Ah, so that's why mapping patterns do exist? Because dicts are ordered?

blhsing Over a year ago

Dicts offer keys for mapping. Tuples and lists offer indices for mapping. I've updated the answer with clarification then.

mkrieger1 · Accepted Answer · 2024-10-27 18:36:59Z

1

It would be possible to do this by converting the input set into a canonical tuple form so that you could use fixed values to compare against.

One possible canonical form would be sorted alphabetically:

match sorted(set((string1, string2, string3))):
    case (' Hello',' world', '!'):
        ...
    case ('beatufiful', 'fair', 'nice'):
        ...
    case ('extremly', 'hyper', 'strongly'):
        ...

(Your example comparison tuples are already sorted, but if not you would have to sort them as well.)

answered Oct 27, 2024 at 18:36

mkrieger1

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