Use underscore (_) as a "default" key for Dictionary

[shraiwi]

Describe the project you are working on

I am working on a VR application with low-latency streaming

Describe the problem or limitation you are having in your project

I am mapping WebXR strings to enum values using a dictionary, like so:

# ClientMode is an enum
const WEBXR_FEATURE_MAPPING : Dictionary = {
	"local-floor": ClientMode.CLIENT_VR_3DOF,
	"bounded-floor": ClientMode.CLIENT_VR_6DOF,
};

But I'd like to avoid the overhead of using the .get() function every time to handle null coalescence.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

I noticed that the match...case branch uses the _ token as a wildcard. I believe that it would be intuitive for GDScript dictionaries to have this property as well. Instead of defining a default value during the .get() function call, I think it would be easier for programmers to define a default key-value pair upon declaration.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Accessing a dictionary this way is redundant and unintuitive:

const WEBXR_FEATURE_MAPPING : Dictionary = {
	"local-floor": ClientMode.CLIENT_VR_3DOF,
	"bounded-floor": ClientMode.CLIENT_VR_6DOF,
};

# for multiple calls, this gets unwieldy
WEBXR_FEATURE_MAPPING.get("local", ClientMode.CLIENT_FLAT); # returns ClientMode.CLIENT_FLAT
WEBXR_FEATURE_MAPPING.get("local-floor", ClientMode.CLIENT_FLAT); # returns ClientMode.CLIENT_VR_3DOF

But with the underscore wildcard, default values are be a breeze:

const WEBXR_FEATURE_MAPPING : Dictionary = {
	"local-floor": ClientMode.CLIENT_VR_3DOF,
	"bounded-floor": ClientMode.CLIENT_VR_6DOF,
	_: ClientMode.CLIENT_FLAT, # '_' denotes a default value
};

# this stays clear and concise
WEBXR_FEATURE_MAPPING["local"]; # returns ClientMode.CLIENT_FLAT
WEBXR_FEATURE_MAPPING["local-floor"]; # returns ClientMode.CLIENT_VR_3DOF

# of course, we can always override the default in the function call
WEBXR_FEATURE_MAPPING.get("local", ClientMode.CLIENT_LOCAL); # returns ClientMode.CLIENT_LOCAL

If this enhancement will not be used often, can it be worked around with a few lines of script?

While it's possible to have default keys declared in the function call, it is easier to have them declared upon creation.

Is there a reason why this should be core and not an add-on in the asset library?

This is a change to a GDScript core object, so there's no way to implement this otherwise.

[YuriSizov]

I don't think this is a common use-case for dictionaries and you can always make a method that would know to fallback on a special key or a predefined value.

func get_webxr_feature(key : String):
  if (WEBXR_FEATURE_MAPPING.has(key)):
    return WEBXR_FEATURE_MAPPING[key]

  return WEBXR_FEATURE_MAPPING["_"] # or just ClientMode.CLIENT_LOCAL

[Calinou]

See also #1321.

[KoBeWi]

But I'd like to avoid the overhead of using the .get() function every time to handle null coalescence.

I don't think there is much more overhead than with the [] operator or with the proposed changes.

[zinnschlag]

Also, what happens if you do want to put a _ as regular key into a dictionary? I don't think having such kludges is a good idea. They are likely to trip people up sooner or later. Sounds like a violation of the Principle of least astonishment.

If we really want to have this feature, it might be better to consider something like Python's defaultdict. But I am not convinced that it is worth it the additional type. As @pycbouh has pointed out, the issue can be worked around easily.

[shraiwi]

Sounds like a violation of the Principle of least astonishment.

My original reasoning was that since the match..case pattern uses the underscore as a wildcard, the dictionary should too, but your reasoning makes sense as well. The group that it benefits from a change like this is so small that it really isn't worth implementing it. Thanks for the feedback!