Add Span index boundary checks (#5861)

* Add Span index boundary checks

* Return Span-specific IndexError in all cases

* Simplify and fix if/else

spacy/errors.py

@@ -588,6 +588,7 @@ class Errors(object):
     E199 = ("Unable to merge 0-length span at doc[{start}:{end}].")
     E200 = ("Specifying a base model with a pretrained component '{component}' "
             "can not be combined with adding a pretrained Tok2Vec layer.")
+    E201 = ("Span index out of range.")
 
 
 @add_codes

spacy/tests/doc/test_span.py

@@ -287,3 +287,15 @@ def test_span_eq_hash(doc, doc_not_parsed):
     assert hash(doc[0:2]) == hash(doc[0:2])
     assert hash(doc[0:2]) != hash(doc[1:3])
     assert hash(doc[0:2]) != hash(doc_not_parsed[0:2])
+
+
+def test_span_boundaries(doc):
+    start = 1
+    end = 5
+    span = doc[start:end]
+    for i in range(start, end):
+        assert span[i - start] == doc[i]
+    with pytest.raises(IndexError):
+        _ = span[-5]
+    with pytest.raises(IndexError):
+        _ = span[5]

spacy/tokens/span.pyx

@@ -181,9 +181,13 @@ cdef class Span:
             return Span(self.doc, start + self.start, end + self.start)
         else:
             if i < 0:
-                return self.doc[self.end + i]
+                token_i = self.end + i
             else:
-                return self.doc[self.start + i]
+                token_i = self.start + i
+            if self.start <= token_i < self.end:
+                return self.doc[token_i]
+            else:
+                raise IndexError(Errors.E201)
 
     def __iter__(self):
         """Iterate over `Token` objects.