Fix sentence indexing bug in Span.sents (#12405)

* Add test for partial sentences in ent.sents.

* Removed unneeded import.

* Format. Simplify code.

spacy/tests/doc/test_span.py

@@ -700,3 +700,19 @@ def test_span_group_copy(doc):
     assert len(doc.spans["test"]) == 3
     # check that the copy spans were not modified and this is an isolated doc
     assert len(doc_copy.spans["test"]) == 2
+
+
+def test_for_partial_ent_sents():
+    """Spans may be associated with multiple sentences. These .sents should always be complete, not partial, sentences,
+    which this tests for.
+    """
+    doc = Doc(
+        English().vocab,
+        words=["Mahler's", "Symphony", "No.", "8", "was", "beautiful."],
+        sent_starts=[1, 0, 0, 1, 0, 0],
+    )
+    doc.set_ents([Span(doc, 1, 4, "WORK")])
+    # The specified entity is associated with both sentences in this doc, so we expect all sentences in the doc to be
+    # equal to the sentences referenced in ent.sents.
+    for doc_sent, ent_sent in zip(doc.sents, doc.ents[0].sents):
+        assert doc_sent == ent_sent

spacy/tokens/span.pyx

@@ -460,9 +460,8 @@ cdef class Span:
                     start = i
                     if start >= self.end:
                         break
-            if start < self.end:
-                yield Span(self.doc, start, self.end)
-
+                elif i == self.doc.length - 1:
+                    yield Span(self.doc, start, self.doc.length)
 
     @property
     def ents(self):