xtriggers: cannot use python collections in xtrigger signatures

[ColemanTom]

Xtriggers: Docs and code don't align or am I misinterpreting?

Looking at the docs for custom xtriggers I feel like it is saying the arguments can be any type, for example, floats, strings, lists. But, at least when I've tried to do something like my_xtrig = my_xtrigger(foo=[1,2]) cylc does not like this and fails (I think it splits on the comma?). Similarly ["A", "B"] fails as it tries to unqoute strings and fails.

So either I'm doing something wrong, or xtriggers do not handle all data types

[oliver-sanders]

I'm afraid it's the latter.

Confirmed with this doctest:

diff --git a/cylc/flow/parsec/validate.py b/cylc/flow/parsec/validate.py
index de74c8fcc..34226d963 100644
--- a/cylc/flow/parsec/validate.py
+++ b/cylc/flow/parsec/validate.py
@@ -1151,6 +1151,10 @@ class CylcConfigValidator(ParsecValidator):
             >>> x.func_kwargs
             {'x': True, 'y': 1.1}
 
+            >>> x = xtrig('a([1,2])', [None])
+            >>> x.func_args
+            [[1, 2]]
+
         """
         label = keys[-1]
         value = cls.strip_and_unquote(keys, value)

As it stands, the xtrigger signature parsing is a bit simplistic and doesn't support all valid Python syntax.

Ideally we would use Python AST to parse these expressions (#5588), however, we made the mistake of allowing "implicit strings", e.g, [xtriggers]foo = foo(abc) (that should be "abc" not abc).

So we would need to deprecate and remove support for this to resolve properly.