Add assert_metric_no_diff (#386)

Author: asherf

django_prometheus/tests/test_testutils.py

@@ -6,6 +6,7 @@
 
 from django_prometheus.testutils import (
     PrometheusTestCaseMixin,
+    assert_metric_no_diff,
     assert_metric_not_equal,
     get_metric,
     get_metric_from_frozen_registry,
@@ -105,9 +106,7 @@ def test_registry_saving(self):
         assert 42 == get_metric_from_frozen_registry("some_gauge", frozen_registry)
         self.test_case.assertMetricDiff(frozen_registry, 99 - 42, "some_gauge", registry=self.registry)
         assert self.test_case.passes is True
-        self.test_case.assertMetricDiff(frozen_registry, 1, "some_gauge", registry=self.registry)
-        assert self.test_case.passes is False
-        self.test_case.passes = True
+        assert_metric_no_diff(frozen_registry, 1, "some_gauge", registry=self.registry)
 
         # Now test the same thing with a labelled metric.
         assert 1 == get_metric_from_frozen_registry(
@@ -126,12 +125,11 @@ def test_registry_saving(self):
             labelblue="indigo",
         )
         assert self.test_case.passes is True
-        self.test_case.assertMetricDiff(
+        assert_metric_no_diff(
             frozen_registry,
             1,
             "some_labelled_gauge",
             registry=self.registry,
             labelred="pink",
             labelblue="indigo",
         )
-        assert self.test_case.passes is False

django_prometheus/testutils.py

@@ -70,6 +70,32 @@ def assertMetricDiff(self, frozen_registry, expected_diff, metric_name, registry
         self.assertEqual(expected_diff, diff, assert_err)
 
 
+def assert_metric_no_diff(frozen_registry, expected_diff, metric_name, registry=REGISTRY, **labels):
+    """Asserts that metric_name{**labels} isn't changed by expected_diff between
+    the frozen registry and now. A frozen registry can be obtained
+    by calling save_registry, typically at the beginning of a test
+    case.
+    """
+    saved_value = get_metric_from_frozen_registry(metric_name, frozen_registry, **labels)
+    current_value = get_metric(metric_name, registry=registry, **labels)
+    assert current_value is not None, METRIC_DIFF_ERR_NONE_EXPLANATION % (
+        metric_name,
+        format_labels(labels),
+        saved_value,
+        current_value,
+    )
+    diff = current_value - (saved_value or 0.0)
+    assert_err = METRIC_DIFF_ERR_EXPLANATION % (
+        metric_name,
+        format_labels(labels),
+        diff,
+        expected_diff,
+        saved_value,
+        current_value,
+    )
+    assert expected_diff != diff, assert_err
+
+
 def assert_metric_not_equal(expected_value, metric_name, registry=REGISTRY, **labels):
     """Asserts that metric_name{**labels} == expected_value."""
     value = get_metric(metric_name, registry=registry, **labels)