Emit empty document for empty multi streams and add regression tests

Author: t-kalinowski

src/lib.rs

@@ -1007,7 +1007,7 @@ fn format_yaml_impl(value: &Yaml<'static>, multi: bool) -> Result<String> {
             }
         };
         if docs.is_empty() {
-            return Ok(String::new());
+            return Ok(String::from("---\n"));
         }
         emit_yaml_documents(&docs, true)
     } else {

tests_py/test_format_parse.py

@@ -104,6 +104,13 @@ def test_format_yaml_round_trips_multi_document_streams():
     assert yaml12.parse_yaml(encoded, multi=True) == docs
 
 
+def test_format_yaml_multi_empty_sequence_emits_empty_document():
+    encoded = yaml12.format_yaml([], multi=True)
+
+    assert encoded == "---\n...\n"
+    assert yaml12.parse_yaml(encoded, multi=True) == [None]
+
+
 def test_format_yaml_rejects_non_sequence_for_multi():
     with pytest.raises(TypeError, match="`value` must be a sequence when `multi=True`"):
         yaml12.format_yaml({"foo": 1}, multi=True)

tests_py/test_read_write.py

@@ -54,6 +54,15 @@ def test_write_and_read_multi_document_streams(tmp_path: Path):
     assert yaml12.read_yaml(str(path), multi=True) == docs
 
 
+def test_write_yaml_multi_empty_sequence_emits_empty_document(tmp_path: Path):
+    path = tmp_path / "yaml12-empty-multi.yaml"
+
+    yaml12.write_yaml([], str(path), multi=True)
+
+    assert path.read_text(encoding="utf-8") == "---\n...\n"
+    assert yaml12.read_yaml(str(path), multi=True) == [None]
+
+
 def test_write_yaml_flushes_final_newline_for_files(tmp_path: Path):
     path = tmp_path / "yaml12-flush.yaml"
     value = {"foo": 1}