util/yaml: warn on duplicate keys in mappings

When adding drivers/resources to an environment config or resources to
an exporter config, users can stumble upon redundant keys. The resulting
errors are not obvious, the last key simply overrides any previous
entries with the same key.

In order to make these config errors more obvious, emit a warning when
duplicate keys are detected in mappings.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

labgrid/util/yaml.py

@@ -2,6 +2,8 @@
 This module contains the custom YAML load and dump functions and associated
 loader and dumper
 """
+
+import warnings
 from collections import OrderedDict, UserString
 from string import Template
 
@@ -16,7 +18,19 @@ class Dumper(yaml.SafeDumper):
     pass
 
 
+def _check_duplicate_dict_keys(loader, node):
+    seen_keys = []
+    for key_node, _ in node.value:
+        key = loader.construct_scalar(key_node)
+        if key in seen_keys:
+            warnings.warn(
+                f"{loader.name}: previous entry with duplicate YAML dictionary key '{key}' overwritten", UserWarning
+            )
+        seen_keys.append(key)
+
+
 def _dict_constructor(loader, node):
+    _check_duplicate_dict_keys(loader, node)
     return OrderedDict(loader.construct_pairs(node))