Ignore "unset" values from ValueFrom?

[pawamoy]

I have this option:

    sources_directory: An[
        Path,
        cappa.Arg(
            short="-s",
            long=True,
            default=FromConfig("index_sources_directory", "index.sources-directory"),
            show_default="`index.sources-directory` config-value or `{default}`",
            group=_GROUP_OPTIONS,
        ),
        Doc("Directory where the sources are stored."),
    ] = defaults.DEFAULT_REPO_DIR

I noticed that if I don't specify a value on the command line, Cappa will use the value obtained from my FromConfig default value (which is a subclass of ValueFrom, just for convenience). But in this case, the value is an unset object. How would I tell Cappa to consider this value as "missing", so that it continues and actually uses the class attribute default, defaults.DEFAULT_REPO_DIR?

class Unset:
    """A sentinel value for unset configuration options."""

    def __bool__(self) -> bool:
        return False

    def __repr__(self) -> str:
        return "<unset>"


unset = Unset()

[pawamoy]

I realize I could enhance FromConfig again to pass it yet another default value, to use if unset is found in the config:

FromConfig("index_sources_directory", "index.sources-directory", defaults.DEFAULT_REPO_DIR),

...but I was wondering if there's a way to avoid repetition 🙂

[pawamoy]

Ah, that doesn't solve it, Cappa then gives me None. The solution is to return cappa.type_view.Empty:

return cappa.type_view.Empty if value is Config.unset else value

[DanCardin]

Right, you rightly pointed out during impl that None might be a valid value in the context of this feature. So Empty would be what you want. Empty should probably be being reexported at the top level, type_view is an irrelevant concept to users.

you tell me if it's worth considering a convenience api for this ^ pattern? I could imagine some Empty.map_empty_sentinel(Config.unset)(value) or somesuch that makes it...maybe moderately less work. but it's still fairly specific to your empty sentinal usecase so 🤷

[pawamoy]

Just exposing Empty at the top-level (and documenting it) would be perfect 🙂

[DanCardin]

#196 for linking's sake.