Pattern matching multiple variables in Python

Question

In OCaml or similar language I would do something like this:

let hello a b c =
  match (a, b, c) with
    | (1, 1, 1) -> 1
    | (2, 2, 2) -> 2
    | (_, _, 3) -> 3
    | (_, _, _) -> 4

Is there a way in Python to have this? Right now I am using

if a==1 and b==1 and c==1:
  return 1
elif a==2 and b==2 and c==2:
  return 2
elif c==3:
  return 3
else:
  return 4

Is this the most idiomatic Python to achieve that?

For the triple case you can use, well, a triple! The if a == 1 and b == 1 and c == 1 becomes if (a, b, c) == (1, 1, 1) for example. — Silver
– Silver, Commented Jan 27, 2018 at 10:58

kabanus · Accepted Answer · 2018-01-27 11:17:07Z

1

As @SilverSlash mentions you can group your variables to a tuple on the fly. A way to save re-writing (a,b,c) is to use a loop, and use the variables themselves for the wildcards:

def hello (a,b,c):
    for (match,result) in (
        ((1,1,1),1),
        ((2,2,2),2),
        ...,
        ((a,b,3),3),
        ...):
        if (a,b,c) == match:
            return result
    return "elseValue"

This looks kind of like the code you're used to. I couldn't say if it's idiomatic or not, but I think it's equally OK as a standard if-else list. It's a bit of a waste of run-time doing the full list check if you just want to look at the last element, but it may not matter for many applications.

Even if you go for an if-else though, I would definitely group to tuples rather than use and between each element. That (I think - maybe a bit of an opinion based question?) is more idiomatic.

edited Jan 27, 2018 at 11:17

answered Jan 27, 2018 at 11:03

kabanus

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