Set kb_id to "" (empty string; id 0) instead of NIL for no prediction in entity_linker

[kinghuang]

When there is no prediction or an entity is discarded, the EntityLinker sets the kb_id to NIL, instead of an empty string. This means that there are two values for no kb_id: an empty string (id 0) or NIL.

spaCy/spacy/pipeline/entity_linker.py

Line 105 in 5279c7c

NIL = "NIL" # string used to refer to a non-existing link

spaCy/spacy/pipeline/entity_linker.py

Lines 331 to 332 in 5279c7c

	# no prediction possible for this entity - setting to NIL
	final_kb_ids.append(self.NIL)

I don't think the entity_linker should be using a special string, NIL, to represent no kb_id. This complicates code, which can't depend on Span.kb_id or Span.kb_id_ evaluating to a falsy value when there is no kb_id. And, it differs from entity labels, where it is consistently an empty string for no label.

[svlandeg]

Hi,

I think our original reasoning was that there's a difference between "not set" and "no link possible". For instance, I myself am not represented in Wikidata, so any text including a reference to my name, should get a gold annotation NIL to specifically state there is no suitable link.

[kinghuang]

Ah, I see. However, I'd argue this is no different than the EntityRecognizer not having a label to apply to a range of tokens. There's no distinction between "not set" and "no entity" on Span.label_. However, there is a different on the corresponding Token.ent_iob_ attribute (empty vs O).

Can the NIL be kept on the Token.ent_kb_id_ attribute, but an empty value on the Span.kb_id_ attribute, like how entity labels are handled?

[svlandeg]

I think right now we don't want to change the behaviour, but I'll put this in the backlog to (re)consider in the future!

[kinghuang]

Ok. FYI, I have run into a complication with the NIL kb_id when using multiple entity_linker components with overwrite=False. The entity linker will only set kb_id when it is empty/0, not when it's NIL.

spaCy/spacy/pipeline/entity_linker.py

Line 416 in 86fa37e

if token.ent_kb_id == 0 or overwrite:

[svlandeg]

Thanks, that's useful to add to the consideration! (I'm sure you've figured that out, but a quick workaround is a custom component inbetween the two EL's that sets NIL's back to empty...)