label in span not writable anymore (#3408)

* label in span not writable anymore

* more explicit unit test and error message for readonly label

* bit more explanation (view)

* error msg tailored to specific case

* fix None case

spacy/errors.py

@@ -358,6 +358,11 @@ class Errors(object):
             "arguments to exclude fields from being serialized or deserialized "
             "is now deprecated. Please use the `exclude` argument instead. "
             "For example: exclude=['{arg}'].")
+    E129 = ("Cannot write the label 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 `label` keyword argument, "
+            "for example:\nfrom spacy.tokens import Span\n"
+            "span = Span(doc, start={start}, end={end}, label='{label}')")
 
 
 @add_codes

spacy/tests/doc/test_span.py

@@ -178,11 +178,10 @@ def test_span_string_label(doc):
     assert span.label == doc.vocab.strings["hello"]
 
 
-def test_span_string_set_label(doc):
+def test_span_label_readonly(doc):
     span = Span(doc, 0, 1)
+    with pytest.raises(NotImplementedError):
         span.label_ = "hello"
-    assert span.label_ == "hello"
-    assert span.label == doc.vocab.strings["hello"]
 
 
 def test_span_ents_property(doc):

spacy/tokens/span.pyx

@@ -653,7 +653,9 @@ cdef class Span:
             return self.doc.vocab.strings[self.label]
 
         def __set__(self, unicode label_):
-            self.label = self.doc.vocab.strings.add(label_)
+            if not label_:
+                label_ = ''
+            raise NotImplementedError(Errors.E129.format(start=self.start, end=self.end, label=label_))
 
 
 cdef int _count_words_to_root(const TokenC* token, int sent_length) except -1: