Response to review

- Ensure --json help message is useful.
- Make the null value reporting only happen to actual null values.

cylc/flow/parsec/OrderedDict.py

@@ -100,6 +100,19 @@ def prepend(self, key, value):
         self[key] = value
         self.move_to_end(key, last=False)
 
+    @staticmethod
+    def repl_val(target, replace, replacement):
+        """Replace dictionary values with a string.
+
+        Designed to be used recursively.
+        """
+        for key, val in target.items():
+            if isinstance(val, dict):
+                OrderedDictWithDefaults.repl_val(
+                    val, replace, replacement)
+            elif val == replace:
+                target[key] = replacement
+
 
 class DictTree:
     """An object providing a single point of access to a tree of dicts.

cylc/flow/parsec/config.py

@@ -208,11 +208,9 @@ def jdump(
             if not keys:
                 keys = []
             cfg = self.get(keys, sparse)
+            cfg.repl_val(cfg, None, none_str)
             data = json.dumps(cfg, indent=indent)
 
-            # We can replace null values with our none_str:
-            data = data.replace('null', f'"{none_str}"')
-
         print(data, file=handle or sys.stdout)
 
     def mdump(self, mkeys=None, sparse=False, prefix='',

cylc/flow/scripts/config.py

@@ -113,8 +113,7 @@ def get_option_parser() -> COP:
     parser.add_option(
         '--json',
         help=(
-            'Print metadata from a Cylc configuration.'
-        ),
+            'Returns config as JSON rather than Cylc Config format.'),
         default=False,
         action='store_true',
         dest='json'