error msg and unit tests for setting kb_id on span

spacy/errors.py

@@ -371,6 +371,11 @@ class Errors(object):
             "with spacy >= 2.1.0. To fix this, reinstall Python and use a wide "
             "unicode build instead. You can also rebuild Python and set the "
             "--enable-unicode=ucs4 flag.")
+    E131 = ("Cannot write the kb_id of an existing Span object because a Span "
+            "is a read-only view of the underlying Token objects stored in the Doc. "
+            "Instead, create a new Span object and specify the `kb_id` keyword argument, "
+            "for example:\nfrom spacy.tokens import Span\n"
+            "span = Span(doc, start={start}, end={end}, label='{label}', kb_id='{kb_id}')")
 
 
 @add_codes

spacy/tests/doc/test_span.py

@@ -172,10 +172,12 @@ def test_span_as_doc(doc):
     assert span_doc[0].idx == 0
 
 
-def test_span_string_label(doc):
-    span = Span(doc, 0, 1, label="hello")
+def test_span_string_label_kb_id(doc):
+    span = Span(doc, 0, 1, label="hello", kb_id="Q342")
     assert span.label_ == "hello"
     assert span.label == doc.vocab.strings["hello"]
+    assert span.kb_id_ == "Q342"
+    assert span.kb_id == doc.vocab.strings["Q342"]
 
 
 def test_span_label_readonly(doc):
@@ -184,6 +186,12 @@ def test_span_label_readonly(doc):
         span.label_ = "hello"
 
 
+def test_span_kb_id_readonly(doc):
+    span = Span(doc, 0, 1)
+    with pytest.raises(NotImplementedError):
+        span.kb_id_ = "Q342"
+
+
 def test_span_ents_property(doc):
     """Test span.ents for the """
     doc.ents = [

spacy/tokens/span.pyx

@@ -111,6 +111,8 @@ cdef class Span:
             self.end_char = 0
         if isinstance(label, basestring_):
             label = doc.vocab.strings.add(label)
+        if isinstance(kb_id, basestring_):
+            kb_id = doc.vocab.strings.add(kb_id)
         if label not in doc.vocab.strings:
             raise ValueError(Errors.E084.format(label=label))
         self.label = label
@@ -662,9 +664,14 @@ cdef class Span:
         def __get__(self):
             return self.doc.vocab.strings[self.kb_id]
 
-        # TODO: custom error msg like for label_
         def __set__(self, unicode kb_id_):
-            raise NotImplementedError(TempErrors.T007.format(attr='kb_id_'))
+            if not kb_id_:
+                kb_id_ = ''
+            current_label = self.label_
+            if not current_label:
+                current_label = ''
+            raise NotImplementedError(Errors.E131.format(start=self.start, end=self.end,
+                                                         label=current_label, kb_id=kb_id_))
 
 
 cdef int _count_words_to_root(const TokenC* token, int sent_length) except -1: