bug(skore): not possible to choose default backend in a nested context

[glemaitre]

What would you like to say?

In #1924, we chose to not use "default" thinking that it was the same semantic than None.

However, those two options have different semantics:

• None means that we do not change the backend
• "default" means that we use the default backend set by the Display

It starts to matter in nested context. Right now, we are unable to reset a context to the default backend that would be equivalent to:

with config_context(plot_backend="default"):  
    with config_context(plot_backend="plotly"):
        print(get_config()["plot_backend"])

We are only able to write:

with config_context(plot_backend=None):  
    with config_context(plot_backend="plotly"):
        print(get_config()["plot_backend"])

that will let the "plotly" backend on. The problem is that one might not be the code owner of the internal config_context and thus cannot really modify the inner call.

We used this pattern in scikit-learn for the transform_output parameter or in skrub for the enable_subsampling parameter and faced similar issue.

[thomass-dev]

Sorry, i don't get it.

Currently, there is no default at Display level. There is only a default behavior at the global scope, that is matplotlib:

_global_config: dict[str, Any] = {
    "show_progress": True,
    "plot_backend": "matplotlib",
}

So in state, these 3 should be strictly equivalent:

with config_context(plot_backend="plotly"):
    print(get_config()["plot_backend"])

with config_context(plot_backend="default"):  
    with config_context(plot_backend="plotly"):
        print(get_config()["plot_backend"])

with config_context(plot_backend=None):  
    with config_context(plot_backend="plotly"):
        print(get_config()["plot_backend"])

If you want to add a default at each display level, we should:

• introduce default-global for the default defined at the global scope,
• introduce default-display for the default defined at the display scope,
• replace None by a sentinel; we shouldn't be able to do with config_context(plot_backend=None): which IMO has more naturally the sense "disable the backend" more than "do nothing".