lweith
diff --git a/‎prometheus_client/metrics.py
Lines changed: 11 additions & 0 deletions b/‎prometheus_client/metrics.py
Lines changed: 11 additions & 0 deletions
diff --git a/‎tests/test_core.py
Lines changed: 32 additions & 0 deletions b/‎tests/test_core.py
Lines changed: 32 additions & 0 deletions
@@ -58,6 +58,13 @@ def _is_observable(self):
         # * the child of a labelled metric.
         return not self._labelnames or (self._labelnames and self._labelvalues)
 
+    def _raise_if_not_observable(self):
+        # Functions that mutate the state of the metric, for example incrementing
+        # a counter, will fail if the metric is not observable, because only if a
+        # metric is observable will the value be initialized.
+        if not self._is_observable():
+            raise ValueError('%s metric is missing label values' % str(self._type))
+
     def _is_parent(self):
         return self._labelnames and not self._labelvalues
 
@@ -250,6 +257,7 @@ def count_exceptions(self, exception=Exception):
         Increments the counter when an exception of the given
         type is raised up out of the code.
         """
+        self._raise_if_not_observable()
         return ExceptionCounter(self, exception)
 
     def _child_samples(self):
@@ -354,13 +362,15 @@ def track_inprogress(self):
         Increments the gauge when the code is entered,
         and decrements when it is exited.
         """
+        self._raise_if_not_observable()
         return InprogressTracker(self)
 
     def time(self):
         """Time a block of code or function, and set the duration in seconds.
 
         Can be used as a function decorator or context manager.
         """
+        self._raise_if_not_observable()
         return Timer(self.set)
 
     def set_function(self, f):
@@ -428,6 +438,7 @@ def time(self):
 
         Can be used as a function decorator or context manager.
         """
+        self._raise_if_not_observable()
         return Timer(self.observe)
 
     def _child_samples(self):
 
@@ -70,6 +70,14 @@ def test_block_decorator(self):
         self.assertTrue(raised)
         self.assertEqual(1, self.registry.get_sample_value('c_total'))
 
+    def test_count_exceptions_not_observable(self):
+        counter = Counter('counter', 'help', labelnames=('label',), registry=self.registry)
+
+        try:
+            counter.count_exceptions()
+        except ValueError as e:
+            self.assertIn('missing label values', str(e))
+
 
 class TestGauge(unittest.TestCase):
     def setUp(self):
@@ -150,6 +158,22 @@ def test_time_block_decorator(self):
             time.sleep(.001)
         self.assertNotEqual(0, self.registry.get_sample_value('g'))
 
+    def test_track_in_progress_not_observable(self):
+        g = Gauge('test', 'help', labelnames=('label',), registry=self.registry)
+
+        try:
+            g.track_inprogress()
+        except ValueError as e:
+            self.assertIn('missing label values', str(e))
+
+    def test_timer_not_observable(self):
+        g = Gauge('test', 'help', labelnames=('label',), registry=self.registry)
+
+        try:
+            g.time()
+        except ValueError as e:
+            self.assertIn('missing label values', str(e))
+
 
 class TestSummary(unittest.TestCase):
     def setUp(self):
@@ -230,6 +254,14 @@ def test_block_decorator(self):
             pass
         self.assertEqual(1, self.registry.get_sample_value('s_count'))
 
+    def test_timer_not_observable(self):
+        s = Summary('test', 'help', labelnames=('label',), registry=self.registry)
+
+        try:
+            s.time()
+        except ValueError as e:
+            self.assertIn('missing label values', str(e))
+
 
 class TestHistogram(unittest.TestCase):
     def setUp(self):