Add Token.is_sent_start property, so can deprecate Token.sent_start

spacy/tests/doc/test_token_api.py

@@ -155,13 +155,13 @@ def test_doc_token_api_head_setter(en_tokenizer):
     assert doc[2].left_edge.i == 0
 
 
-def test_sent_start(en_tokenizer):
+def test_is_sent_start(en_tokenizer):
     doc = en_tokenizer(u'This is a sentence. This is another.')
-    assert not doc[0].sent_start
-    assert not doc[5].sent_start
-    doc[5].sent_start = True
-    assert doc[5].sent_start
-    assert not doc[0].sent_start
+    assert doc[5].is_sent_start is None
+    doc[5].is_sent_start = True
+    assert doc[5].is_sent_start is True
+    # Backwards compatibility
+    assert doc[0].sent_start is False
     doc.is_parsed = True
     assert len(list(doc.sents)) == 2
 

spacy/tokens/token.pyx

@@ -330,9 +330,29 @@ cdef class Token:
             return self.c.r_kids
 
     property sent_start:
-        # TODO: fix and document
+        # TODO deprecation warning
         def __get__(self):
-            return self.c.sent_start
+            # Handle broken backwards compatibility case: doc[0].sent_start
+            # was False.
+            if self.i == 0:
+                return False
+            else:
+                return self.sent_start
+
+        def __set__(self, value):
+            self.is_sent_start = value
+
+    property is_sent_start:
+        """RETURNS (bool / None): Whether the token starts a sentence.
+            None if unknown.
+        """
+        def __get__(self):
+            if self.c.sent_start == 0:
+                return None
+            elif self.c.sent_start < 0:
+                return False
+            else:
+                return True
 
         def __set__(self, value):
             if self.doc.is_parsed: