UW-269 Extend set_config.py tool to "export" variables (#280)

* added export_variables function

* updated section_path type

* config_dict -> config

* updated doc string

* ordered type values

* updated function name

* added dict type checking

* added list handling

* added helper functions

* adding Paul's reorg

* re-order functions

* decluttering

* convert to sorted set

* output_lines as a list

* removed list functionality

* removed superfluous test

* fixed list handling error

* reorg

* disable protected access

* name change

src/uwtools/config.py

@@ -21,6 +21,7 @@
 import yaml
 
 from uwtools import exceptions, logger
+from uwtools.exceptions import UWConfigError
 from uwtools.j2template import J2Template
 from uwtools.logger import Logger
 from uwtools.utils import cli_helpers
@@ -653,6 +654,20 @@ def dump_file_from_dict(path: str, cfg: dict, opts: Optional[ns] = None) -> None
             file_name.write("\n".join(lines))
 
 
+# Private functions
+
+
+def _log_and_error(msg: str, log: Logger) -> None:
+    """
+    Will log a user-provided error message and raise a UWConfigError with the same message.
+    """
+    log.error(msg)
+    raise UWConfigError(msg)
+
+
+# Public functions
+
+
 def create_config_obj(
     input_base_file: str,
     compare: bool = False,
@@ -752,3 +767,27 @@ def create_config_obj(
                     raise ValueError(err_msg)
                 # Dump to file:
                 dump_method(path=outfile, cfg=config_obj)
+
+
+def print_config_section(config: dict, key_path: List[str], log: Logger) -> None:
+    """
+    Descends into the config via the given keys, then prints the contents of the located subtree as
+    key=value pairs, one per line.
+    """
+    keys = []
+    for section in key_path:
+        keys.append(section)
+        current_path = " -> ".join(keys)
+        try:
+            subconfig = config[section]
+        except KeyError:
+            _log_and_error(f"Bad config path: {current_path}", log)
+        if not isinstance(subconfig, dict):
+            _log_and_error(f"Value at {current_path} must be a dictionary", log)
+        config = subconfig
+    output_lines = []
+    for key, value in config.items():
+        if type(value) not in (bool, float, int, str):
+            _log_and_error(f"Non-scalar value {value} found at {current_path}", log)
+        output_lines.append(f"{key}={value}")
+    print("\n".join(sorted(output_lines)))

src/uwtools/tests/test_config.py

@@ -1,4 +1,4 @@
-# pylint: disable=duplicate-code,missing-function-docstring,redefined-outer-name
+# pylint: disable=duplicate-code,missing-function-docstring,protected-access,redefined-outer-name
 """
 Tests for uwtools.config module.
 """
@@ -22,6 +22,7 @@
 
 from uwtools import config, exceptions
 from uwtools.exceptions import UWConfigError
+from uwtools.logger import Logger
 from uwtools.tests.support import compare_files, fixture_path, line_in_lines, msg_in_caplog
 from uwtools.utils import cli_helpers
 
@@ -810,3 +811,68 @@ def test_YAMLConfig__load_unexpected_error(tmp_path):
         with raises(UWConfigError) as e:
             config.YAMLConfig(config_path=cfgfile)
         assert msg in str(e.value)
+
+
+def test_print_config_section_ini(capsys):
+    config_obj = config.INIConfig(fixture_path("simple3.ini"))
+    section = ["dessert"]
+    config.print_config_section(config_obj.data, section, log=Logger())
+    actual = capsys.readouterr().out
+    expected = """
+flavor={{flavor}}
+servings=0
+side=False
+type=pie
+""".lstrip()
+    assert actual == expected
+
+
+def test_print_config_section_ini_missing_section():
+    config_obj = config.INIConfig(fixture_path("simple3.ini"))
+    section = ["sandwich"]
+    msg = "Bad config path: sandwich"
+    with raises(UWConfigError) as e:
+        config.print_config_section(config_obj.data, section, log=Logger())
+    assert msg in str(e.value)
+
+
+def test_print_config_section_yaml(capsys):
+    config_obj = config.YAMLConfig(fixture_path("FV3_GFS_v16.yaml"))
+    section = ["sgs_tke", "profile_type"]
+    config.print_config_section(config_obj.data, section, log=Logger())
+    actual = capsys.readouterr().out
+    expected = """
+name=fixed
+surface_value=0.0
+""".lstrip()
+    assert actual == expected
+
+
+def test_print_config_section_yaml_for_nonscalar():
+    config_obj = config.YAMLConfig(fixture_path("FV3_GFS_v16.yaml"))
+    section = ["o3mr"]
+    with raises(UWConfigError) as e:
+        config.print_config_section(config_obj.data, section, log=Logger())
+    assert "Non-scalar value" in str(e.value)
+
+
+def test_print_config_section_yaml_list():
+    config_obj = config.YAMLConfig(fixture_path("srw_example.yaml"))
+    section = ["FV3GFS", "nomads", "file_names", "grib2", "anl"]
+    with raises(UWConfigError) as e:
+        config.print_config_section(config_obj.data, section, log=Logger())
+    assert "must be a dictionary" in str(e.value)
+
+
+def test_print_config_section_yaml_not_dict():
+    config_obj = config.YAMLConfig(fixture_path("FV3_GFS_v16.yaml"))
+    section = ["sgs_tke", "units"]
+    with raises(UWConfigError) as e:
+        config.print_config_section(config_obj.data, section, log=Logger())
+    assert "must be a dictionary" in str(e.value)
+
+
+def test__log_and_error():
+    with raises(UWConfigError) as e:
+        config._log_and_error("Must be scalar value", log=Logger())
+    assert "Must be scalar value" in str(e.value)