feat: allow eval on non existing namespace

[nichmor]

This PR aims to add allow_missing_selector argument for RecipeLoader which will be used by conda-smithy linter to suggest some lints for osx platform. To do this, we need to ignore other selectors ( treat them as true ) so the osx one will be evaluated.
Also, I'm adding flatten_lists that will make just a list of values for this case:

python:
if: osx
  - 3.8
 
python:
if: unix
 - 3.9

So instead of having python: [[3.8], [3.9]] we will have python: [3.8, 3.9]

[baszalmstra]

Are these methods considered to be used outside of this class? E.g. are they public? If not maybe we should prepend them with a _. Otherwise, disregard this comment.

[nichmor]

I've moved flatten_lists and remove_empty_keys in utils.py so it could be reused in future.
wdyt?

[baszalmstra]

utils.py is still a "public" module though. Personally I would either make utils "non-public" by prepending a _ or prepending the functions.

The thing is that if we don't mark them as non-public they are public and people might start depending on these symbols.

[nichmor]

sorry but why it's a bad idea to expose this utills functions for the public?
flattening a lists and remove empty keys seems like a generic problem that isn't a rattler/conda specific

[baszalmstra]

I think it's a preference but to me, the scope of this library is not providing utility functions to work with lists and dictionaries but rather to work with recipe.yamls. In my eyes, we should be really careful to increase the API of the library. People will start depending on anything we expose publicly which makes it harder for us to refactor things in the library later on. For instance, just moving a function from one module to another will then already be a breaking change.

WDYT @Hofer-Julian @tdejager

[nichmor]

got it, let me make them private

[Hofer-Julian]

I agree with @baszalmstra.

Regarding making all functions private vs making the module private, I think making the functions private is the way to go. It is more common in my experience.