bf: yaml should be optional

Fixes: #245

src/con_duct/suite/ls.py

@@ -6,13 +6,18 @@
 import re
 from typing import Any, Dict, List, Optional
 from packaging.version import Version
+from con_duct.__main__ import DUCT_OUTPUT_PREFIX, SummaryFormatter
 
 try:
     import pyout  # type: ignore
 except ImportError:
     pyout = None
-import yaml
-from con_duct.__main__ import DUCT_OUTPUT_PREFIX, SummaryFormatter
+
+try:
+    import yaml  # type: ignore
+except ImportError:
+    yaml = None
+
 
 lgr = logging.getLogger(__name__)
 
@@ -72,10 +77,14 @@ def load_duct_runs(
                         f"is below minimum schema version {MINIMUM_SCHEMA_VERSION}."
                     )
                     continue
-                if eval_filter is not None and not (eval_results := eval(
-                    eval_filter, _flatten_dict(this), dict(re=re)
-                )):
-                    lgr.debug("Filtering out %s due to filter results matching: %s", this, eval_results)
+                if eval_filter is not None and not (
+                    eval_results := eval(eval_filter, _flatten_dict(this), dict(re=re))
+                ):
+                    lgr.debug(
+                        "Filtering out %s due to filter results matching: %s",
+                        this,
+                        eval_results,
+                    )
                     continue
 
                 loaded.append(this)
@@ -179,6 +188,8 @@ def ls(args: argparse.Namespace) -> int:
     elif args.format == "json_pp":
         print(json.dumps(output_rows, indent=2))
     elif args.format == "yaml":
+        if yaml is None:
+            raise RuntimeError("Install PyYaml yaml output")
         plain_rows = [dict(row) for row in output_rows]
         print(yaml.dump(plain_rows, default_flow_style=False))
     else: