Add optional default value to get_meta()

[KoBeWi]

Closes godotengine/godot-proposals#2383

[Calinou]

I'd prefer to add an optional third argument to get_meta(), which would be more similar to Dictionary.get().

Having a separate method is warranted for having get_meta_or_null() in the future (once we make get_meta() return an error when the value doesn't exist and no default is specified). But for returning a default value, I wouldn't use a separate method entirely.

[KoBeWi]

get_meta_or_null()

I don't think having such method makes sense. We have get_node_or_null() because you can literally only return either node or null. In case of Dictionary or meta, the default value can be any Variant. And returning null in case of missing value means that you have to add more code if you want a different default.

Having a separate method is warranted for having get_meta_or_null() in the future (once we make get_meta() return an error when the value doesn't exist and no default is specified).

This basically means that we'd add get_meta_or_null() just to have a version of get_meta() with null default value that doesn't throw error. Instead of having two methods with default for such a weird reason, it's better to have one.

[akien-mga]

We discussed in PR review meeting that this makes sense, but could actually be done by adding the default value to get_meta itself, defaulting to null. If it's null (no custom default set by the user), it would throw an error when the meta isn't found. Otherwise, it returns the non-null default.

As mentioned in the PR review, that doesn't cover the use case that get_meta_or_default(meta, null) would, but we thought that this specific use case doesn't necessarily need to be supported (and can be handled with has_meta).

[KoBeWi]

I checked my code for use of get("sth", null) and found this:

var icon = icon_cache.get(tab_scenes[i], null)
if icon:
	slot.set_texture(icon)
else:
	generate_icon(instance as Node2D, slot)

The get_meta() equivalent would be:

if node.has_meta("meta"):
    value = node.get_meta("meta")
else:
    value = something_else

In such cases I need to access the "meta" twice. But I guess it's rare enough to live with that. I'll make the discussed changes.

[EricEzaM]

As mentioned in the PR review, that doesn't cover the use case that get_meta_or_default(meta, null) would, but we thought that this specific use case doesn't necessarily need to be supported (and can be handled with has_meta).

Tbh I think the _or_default method is useful and covers a lot of cases. Searching for has_meta( in the code base returns a number of results where this new method could be used to simplify the code. I think adding a param to get_meta (as described in quoted text above) but not having it work (without error) 100% of the time is a bit of a half measure.

If it were my choice I would either have the _or_default version, or veto this overall.

[KoBeWi]

adding the default value to get_meta itself

Done.

[akien-mga]

Does this need a rebase after #59452 or is it fine as is?

[akien-mga]

Approved based on what we discussed last time.

[KoBeWi]

Tested, still works.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.5.