test: ensure child metrics retain parent namespace/subsystem/unit

Author: hazel-shen

prometheus_client/metrics.py

@@ -109,6 +109,12 @@ def __init__(self: T,
                  registry: Optional[CollectorRegistry] = REGISTRY,
                  _labelvalues: Optional[Sequence[str]] = None,
                  ) -> None:
+
+        self._original_name = name
+        self._namespace = namespace
+        self._subsystem = subsystem
+        self._unit = unit       
+
         self._name = _build_full_name(self._type, name, namespace, subsystem, unit)
         self._labelnames = _validate_labelnames(self, labelnames)
         self._labelvalues = tuple(_labelvalues or ())
@@ -177,18 +183,22 @@ def labels(self: T, *labelvalues: Any, **labelkwargs: Any) -> T:
         with self._lock:
             if labelvalues not in self._metrics:
 
-                child_kwargs = dict(self._kwargs) if self._kwargs else {}
+                original_name = getattr(self, '_original_name', self._name)
+                namespace = getattr(self, '_namespace', '')
+                subsystem = getattr(self, '_subsystem', '')
+                unit = getattr(self, '_unit', '')
 
+                child_kwargs = dict(self._kwargs) if self._kwargs else {}
                 for k in ('namespace', 'subsystem', 'unit'):
                     child_kwargs.pop(k, None)
 
                 self._metrics[labelvalues] = self.__class__(
-                    self._name,
+                    original_name,
                     documentation=self._documentation,
                     labelnames=self._labelnames,
-                    namespace="",
-                    subsystem="",  
-                    unit="",
+                    namespace=namespace,
+                    subsystem=subsystem,  
+                    unit=unit,
                     _labelvalues=labelvalues,
                     **child_kwargs
                 )

tests/test_multiprocess.py

@@ -467,6 +467,45 @@ def __init__(
             f"found double-built name(s): {[n for n in all_names if n.startswith(bad_prefix)]}"
         )
 
+    def test_child_preserves_parent_context_for_subclasses(self):
+        """
+        Ensure child metrics preserve parent's namespace/subsystem/unit information
+        so that subclasses can correctly use these parameters in their logic.
+        """
+        class ContextAwareCounter(Counter):
+            def __init__(self,
+                         name,
+                         documentation,
+                         labelnames=(),
+                         namespace="",
+                         subsystem="",
+                         unit="",
+                         **kwargs):
+                self.context = {
+                    'namespace': namespace,
+                    'subsystem': subsystem,
+                    'unit': unit
+                }
+                super().__init__(name, documentation,
+                                 labelnames=labelnames,
+                                 namespace=namespace,
+                                 subsystem=subsystem,
+                                 unit=unit,
+                                 **kwargs)
+
+        parent = ContextAwareCounter('m', 'help',
+                                     labelnames=['status'],
+                                     namespace='prod',
+                                     subsystem='api',
+                                     unit='seconds',
+                                     registry=None)
+
+        child = parent.labels(status='200')
+
+        # Verify that child retains parent's context
+        self.assertEqual(child.context['namespace'], 'prod')
+        self.assertEqual(child.context['subsystem'], 'api')
+        self.assertEqual(child.context['unit'], 'seconds')
 
 
 class TestMmapedDict(unittest.TestCase):