parameterization: excluded values

[oliver-sanders]

Describe exactly what you would like to see in an upcoming release
A mechanism for excluding parametrised values e.g:

    foo => bar<pub!=prancing_pony> => baz

I think the use cases for this type of functionality are probably going to hit the limitation of parameters in other ways so #1962 may be the best way to solve this. Some examples:

• Exclude a single value <pub!=prancing_pony>
• Exclude a set of values <pub!=[prancing_pony, golden_dragon]>
• Exclude items from another parametrization <pub!=<old_pub>>

Alternative workaround might be to create a new parameter defined based on another one:

[cylc]
    [[parameters]]
        foo = 1..10
        bar = [x for x in foo if x != 5]

Additional context

Somewhat related to:

• parameterization: allow access to indexed entries #2076
• parameterization: allow missing value exclusion of entries #2290

Pull requests welcome!
This is an Open Source project - please consider contributing code yourself
(please read CONTRIBUTING.md before starting any work though).

[oliver-sanders]

Likely the best solution is to supersede this with #1962

[hjoliver]

I'm happy to say if users need this sort of "advanced" parameterization they can a) roll their own with Jinja2 loops; or b) wait for #1962. It would be nice to have, but may be difficult to implement in the current graph string parser (?) and it's not a blocker because of a).