rely on is_empty property instead of __len__ (#12347)

spacy/errors.py

@@ -444,8 +444,7 @@ class Errors(metaclass=ErrorsWithCodes):
     E133 = ("The sum of prior probabilities for alias '{alias}' should not "
             "exceed 1, but found {sum}.")
     E134 = ("Entity '{entity}' is not defined in the Knowledge Base.")
-    E139 = ("Knowledge base for component '{name}' is empty. Use the methods "
-            "`kb.add_entity` and `kb.add_alias` to add entries.")
+    E139 = ("Knowledge base for component '{name}' is empty.")
     E140 = ("The list of entities, prior probabilities and entity vectors "
             "should be of equal length.")
     E141 = ("Entity vectors should be of length {required} instead of the "

spacy/kb/kb_in_memory.pyx

@@ -46,6 +46,9 @@ cdef class InMemoryLookupKB(KnowledgeBase):
         self._alias_index = PreshMap(nr_aliases + 1)
         self._aliases_table = alias_vec(nr_aliases + 1)
 
+    def is_empty(self):
+        return len(self) == 0
+
     def __len__(self):
         return self.get_size_entities()
 

spacy/pipeline/entity_linker.py

@@ -250,7 +250,7 @@ class EntityLinker(TrainablePipe):
         # Raise an error if the knowledge base is not initialized.
         if self.kb is None:
             raise ValueError(Errors.E1018.format(name=self.name))
-        if len(self.kb) == 0:
+        if hasattr(self.kb, "is_empty") and self.kb.is_empty():
             raise ValueError(Errors.E139.format(name=self.name))
 
     def initialize(

spacy/tests/pipeline/test_entity_linker.py

@@ -353,6 +353,9 @@ def test_kb_default(nlp):
     """Test that the default (empty) KB is loaded upon construction"""
     entity_linker = nlp.add_pipe("entity_linker", config={})
     assert len(entity_linker.kb) == 0
+    with pytest.raises(ValueError, match="E139"):
+        # this raises an error because the KB is empty
+        entity_linker.validate_kb()
     assert entity_linker.kb.get_size_entities() == 0
     assert entity_linker.kb.get_size_aliases() == 0
     # 64 is the default value from pipeline.entity_linker