Validate Enum arguments before registering the collector

prometheus_client/metrics.py

@@ -769,6 +769,10 @@ def __init__(self,
                  _labelvalues: Optional[Sequence[str]] = None,
                  states: Optional[Sequence[str]] = None,
                  ):
+        if name in labelnames:
+            raise ValueError(f'Overlapping labels for Enum metric: {name}')
+        if not states:
+            raise ValueError(f'No states provided for Enum metric: {name}')
         super().__init__(
             name=name,
             documentation=documentation,
@@ -779,10 +783,6 @@ def __init__(self,
             registry=registry,
             _labelvalues=_labelvalues,
         )
-        if name in labelnames:
-            raise ValueError(f'Overlapping labels for Enum metric: {name}')
-        if not states:
-            raise ValueError(f'No states provided for Enum metric: {name}')
         self._kwargs['states'] = self._states = states
 
     def _metric_init(self) -> None:

tests/test_core.py

@@ -595,6 +595,19 @@ def test_overlapping_labels(self):
         with pytest.raises(ValueError):
             Enum('e', 'help', registry=None, labelnames=['e'])
 
+    def test_failed_init_does_not_pollute_registry(self):
+        registry = CollectorRegistry()
+        # A validation failure in __init__ must not leave a half-built collector
+        # registered: otherwise the name stays permanently taken and any later
+        # scrape of the registry crashes on the missing _states attribute.
+        with pytest.raises(ValueError):
+            Enum('task_state', 'help', states=None, registry=registry)
+        with pytest.raises(ValueError):
+            Enum('task_state', 'help', states=['a'], labelnames=['task_state'], registry=registry)
+        # The name is still free, so a correct definition registers and scrapes.
+        Enum('task_state', 'help', states=['a', 'b'], registry=registry)
+        self.assertEqual(1, registry.get_sample_value('task_state', {'task_state': 'a'}))
+
 
 class TestMetricWrapper(unittest.TestCase):
     def setUp(self):