fix(yaml): emit null, not empty scalars

src/remarshal/main.py

@@ -646,6 +646,10 @@ def stringify_null(x: Any) -> Any:
         raise ValueError(msg)
 
 
+def _yaml_represent_none(self, data):
+    return self.represent_scalar("tag:yaml.org,2002:null", "null")
+
+
 def _encode_yaml(data: Document, *, yaml_options: YAMLOptions) -> str:
     yaml = ruamel.yaml.YAML()
     yaml.default_flow_style = False
@@ -654,6 +658,8 @@ def _encode_yaml(data: Document, *, yaml_options: YAMLOptions) -> str:
     yaml.indent = yaml_options.indent
     yaml.width = yaml_options.width
 
+    yaml.representer.add_representer(type(None), _yaml_represent_none)
+
     try:
         out = StringIO()
         yaml.dump(

tests/bool-null-key.yaml

@@ -1,4 +1,4 @@
 True: foo
 false: oof
 another: bar
-NULL: "nothin'"
+NULL: nothin'

tests/null.json

@@ -0,0 +1,3 @@
+{
+    "foo": null
+}

tests/null.yaml

@@ -0,0 +1 @@
+foo: null

tests/test_remarshal.py

@@ -1,6 +1,6 @@
 #! /usr/bin/env python
 # Remarshal, a utility to convert between serialization formats.
-# Copyright (c) 2014-2020, 2023 D. Bohdan
+# Copyright (c) 2014-2020, 2023-2024 D. Bohdan
 # License: MIT
 
 from __future__ import annotations
@@ -364,6 +364,11 @@ def test_binary_to_yaml(self, convert_and_read) -> None:
     def test_binary_to_cbor(self, convert_and_read) -> None:
         convert_and_read("bin.msgpack", "msgpack", "cbor")
 
+    def test_yaml_null(self, convert_and_read) -> None:
+        output = convert_and_read("null.json", "json", "yaml")
+        reference = read_file("null.yaml")
+        assert output == reference
+
     def test_yaml_style_default(self, convert_and_read) -> None:
         output = convert_and_read("long-line.json", "json", "yaml")
         reference = read_file("long-line-default.yaml")
@@ -544,6 +549,16 @@ def test_yaml2toml_bool_null_key(self, convert_and_read) -> None:
         reference = read_file("bool-null-key.toml")
         assert output == reference
 
+    def test_yaml2yaml_bool_null_key(self, convert_and_read) -> None:
+        output = convert_and_read(
+            "bool-null-key.yaml",
+            "yaml",
+            "yaml",
+        )
+        reference = read_file("bool-null-key.yaml")
+
+        assert output == reference.lower()
+
     def test_yaml2toml_timestamp_key(self, convert_and_read) -> None:
         output = convert_and_read(
             "timestamp-key.yaml",