MetaCode brainstorming

[phackstock]

Following our discussion earlier, I'm sketching out some ideas for a MetaCodeList.
I think it's easiest to start at what the yaml file is supposed to look like:

- Meta category with boolean values:
  allowed_values: [true, false]
  default: false
- Meta category for tracking versions:
  data_type: int
  required: true
- Meta category only required for Downscaling:
  apply_to: "Downscaling*"
  required: True

the three above examples illustrate some of the use cases that I can see, listing them in order:

1. Meta category with boolean values has allowed values true and false, defaulting to false
2. Meta category for tracking versions is required and requires an integer. Allowed values not really make sense here since we expect increasing integer numbers.
3. Meta category only required for Downscaling should only be applied to models matching the pattern Downscaling*, in which case, however, it is required.

Looking at this again @danielhuppmann I'll retract my previous suggestion of making the entries in allowed_values MetaCodes as that seems too complicated and not every entry in the sample MetaCodeList even features allowed_values.

It's probably better to translate the above structure into something like:

class MetaCode(Code):
    allowed_values: Optional(list[any])
    default: Optional(any)
    data_type: Optional(type)
    required: Optional(bool)

then we'd have a MetaCodeList to hold a list of MetaCodes.

[danielhuppmann]

I think that this is a good model to follow, but I suggest to implement it step by step.

The first PR should be the MetaCode and MetaCodeList classes read from all yaml files in a meta folder, with only the attribute allowed_values.

[danielhuppmann]

The second PR should be a MetaValidator (with an apply function), which can be passed to the the processor-list in the process-function...

[phackstock]

@danielhuppmann sounds like a sensible approach to me

[danielhuppmann]

@GretchenSchowalter, can you take a first stab at this? Please look at this tutorial to better understand the "meta" indicators: https://pyam-iamc.readthedocs.io/en/stable/tutorials/pyam_first_steps.html

[phackstock]

@danielhuppmann, I'd say we can close this, right?

[danielhuppmann]

Closing, with a follow-up at #270