UW 500 - Allow YAML in all conversions

docs/sections/user_guide/cli/mode_config.rst

@@ -434,16 +434,16 @@ and an additional supplemental YAML file ``values2.yaml`` with the following con
 .. note:: In recognition of the different sets of value types representable in each config format, ``uw`` supports two format-combination schemes:
 
    1. **Output matches input:** The format of the output config matches that of the input config.
-   2. **Input is YAML:** If the input config is YAML, any output format may be requested. In the worst case, values always have a string representation, but note that, for example, the string representation of a YAML sequence (Python ``list``) in an INI output config may not be useful.
+   2. **Output or input is YAML:** If either config is YAML, any other format may be requested. In the worst case, output values always have a string representation, but note that, for example, the string representation of a YAML sequence (Python ``list``) in an INI output config may not be useful.
 
    In all cases, any supplemental configs must be in the same format as the input config and must have recognized extensions.
 
    ``uw`` considers invalid combination requests errors:
 
    .. code-block:: text
 
-      $ uw config realize --input-file b.nml --output-file a.yaml
-      Output format yaml must match input format nml
+      $ uw config realize --input-file b.nml --output-file a.ini
+      Output format ini must match input format nml unless either is YAML
 
    .. code-block:: text
 

src/uwtools/config/tools.py

@@ -251,8 +251,11 @@ def _validate_format_output(input_fmt: str, output_fmt: str) -> None:
     :param output_fmt: Output format.
     :raises: UWError if output format is incompatible.
     """
-    if not input_fmt in (FORMAT.yaml, output_fmt):
-        raise UWError("Output format %s must match input format %s" % (output_fmt, input_fmt))
+    if FORMAT.yaml not in (input_fmt, output_fmt) and input_fmt != output_fmt:
+        raise UWError(
+            "Output format %s must match input format %s unless either is YAML"
+            % (output_fmt, input_fmt)
+        )
 
 
 def _validate_format_supplemental(

src/uwtools/tests/config/test_tools.py

@@ -645,12 +645,15 @@ def test__realize_config_values_needed_negative_results(caplog, tmp_path):
 @pytest.mark.parametrize("output_fmt", FORMAT.extensions())
 def test__validate_format_output(input_fmt, output_fmt):
     call = lambda: tools._validate_format_output(input_fmt=input_fmt, output_fmt=output_fmt)
-    if input_fmt in (FORMAT.yaml, output_fmt):
+    if FORMAT.yaml in (input_fmt, output_fmt) or input_fmt == output_fmt:
         call()  # no exception raised
     else:
         with raises(UWError) as e:
             call()
-        assert str(e.value) == f"Output format {output_fmt} must match input format {input_fmt}"
+        assert (
+            str(e.value) == f"Output format {output_fmt} must match input format {input_fmt}"
+            " unless either is YAML"
+        )
 
 
 def test__validate_format_supplemental_fail_obj():