Also raise error in Span.__reduce__

spacy/errors.py

@@ -308,6 +308,12 @@ class Errors(object):
             "would always have to include its Doc and Vocab, which has "
             "practically no advantage over pickling the parent Doc directly. "
             "So instead of pickling the token, pickle the Doc it belongs to.")
+    E112 = ("Pickling a span is not supported, because spans are only views "
+            "of the parent Doc and can't exist on their own. A pickled span "
+            "would always have to include its Doc and Vocab, which has "
+            "practically no advantage over pickling the parent Doc directly. "
+            "So instead of pickling the span, pickle the Doc it belongs to or "
+            "use Span.as_doc to convert the span to a standalone Doc object.")
 
 @add_codes
 class TempErrors(object):

spacy/tests/regression/test_issue2833.py

@@ -7,7 +7,9 @@ from spacy.compat import pickle
 
 
 def test_issue2833(en_vocab):
-    """Test that a custom error is raised if a token is pickled."""
+    """Test that a custom error is raised if a token or span is pickled."""
     doc = Doc(en_vocab, words=["Hello", "world"])
     with pytest.raises(NotImplementedError):
         pickle.dumps(doc[0])
+    with pytest.raises(NotImplementedError):
+        pickle.dumps(doc[0:2])

spacy/tokens/span.pyx

@@ -141,6 +141,9 @@ cdef class Span:
         for i in range(self.start, self.end):
             yield self.doc[i]
 
+    def __reduce__(self):
+        raise NotImplementedError(Errors.E112)
+
     @property
     def _(self):
         """User space for adding custom attribute extensions."""