Require HEAD for is_parsed in Doc.from_array() (#5011)

Modify flag settings so that `DEP` is not sufficient to set `is_parsed`
and only run `set_children_from_heads()` if `HEAD` is provided.

Then the combination `[SENT_START, DEP]` will set deps and not clobber
sent starts with a lot of one-word sentences.

spacy/tests/doc/test_doc_api.py

@@ -7,7 +7,7 @@ import numpy
 from spacy.tokens import Doc, Span
 from spacy.vocab import Vocab
 from spacy.errors import ModelsWarning
-from spacy.attrs import ENT_TYPE, ENT_IOB
+from spacy.attrs import ENT_TYPE, ENT_IOB, SENT_START, HEAD, DEP
 
 from ..util import get_doc
 
@@ -274,6 +274,39 @@ def test_doc_is_nered(en_vocab):
     assert new_doc.is_nered
 
 
+def test_doc_from_array_sent_starts(en_vocab):
+    words = ["I", "live", "in", "New", "York", ".", "I", "like", "cats", "."]
+    heads = [0, 0, 0, 0, 0, 0, 6, 6, 6, 6]
+    deps = ["ROOT", "dep", "dep", "dep", "dep", "dep", "ROOT", "dep", "dep", "dep", "dep"]
+    doc = Doc(en_vocab, words=words)
+    for i, (dep, head) in enumerate(zip(deps, heads)):
+        doc[i].dep_ = dep
+        doc[i].head = doc[head]
+        if head == i:
+            doc[i].is_sent_start = True
+    doc.is_parsed
+
+    attrs = [SENT_START, HEAD]
+    arr = doc.to_array(attrs)
+    new_doc = Doc(en_vocab, words=words)
+    with pytest.raises(ValueError):
+        new_doc.from_array(attrs, arr)
+
+    attrs = [SENT_START, DEP]
+    arr = doc.to_array(attrs)
+    new_doc = Doc(en_vocab, words=words)
+    new_doc.from_array(attrs, arr)
+    assert [t.is_sent_start for t in doc] == [t.is_sent_start for t in new_doc]
+    assert not new_doc.is_parsed
+
+    attrs = [HEAD, DEP]
+    arr = doc.to_array(attrs)
+    new_doc = Doc(en_vocab, words=words)
+    new_doc.from_array(attrs, arr)
+    assert [t.is_sent_start for t in doc] == [t.is_sent_start for t in new_doc]
+    assert new_doc.is_parsed
+
+
 def test_doc_lang(en_vocab):
     doc = Doc(en_vocab, words=["Hello", "world"])
     assert doc.lang_ == "en"

spacy/tokens/doc.pyx

@@ -813,7 +813,7 @@ cdef class Doc:
                 if attr_ids[j] != TAG:
                     Token.set_struct_attr(token, attr_ids[j], array[i, j])
         # Set flags
-        self.is_parsed = bool(self.is_parsed or HEAD in attrs or DEP in attrs)
+        self.is_parsed = bool(self.is_parsed or HEAD in attrs)
         self.is_tagged = bool(self.is_tagged or TAG in attrs or POS in attrs)
         # If document is parsed, set children
         if self.is_parsed: