Beef up the assert_warnings test helper

2 files changed, 22 insertions, 1 deletions

diff --git a/tests/coveragetest.py b/tests/coveragetest.py
index 7fcc9f9..5f3457f 100644
--- a/tests/coveragetest.py
+++ b/tests/coveragetest.py
@@ -272,13 +272,17 @@ class CoverageTest(
         return cov
 
     @contextlib.contextmanager
-    def assert_warnings(self, cov, warnings):
+    def assert_warnings(self, cov, warnings, not_warnings=()):
         """A context manager to check that particular warnings happened in `cov`.
 
         `cov` is a Coverage instance.  `warnings` is a list of regexes.  Every
         regex must match a warning that was issued by `cov`.  It is OK for
         extra warnings to be issued by `cov` that are not matched by any regex.
 
+        `not_warnings` is a list of regexes that must not appear in the
+        warnings.  This is only checked if there are some positive warnings to
+        test for in `warnings`.
+
         If `warnings` is empty, then `cov` is not allowed to issue any
         warnings.
 
@@ -286,6 +290,8 @@ class CoverageTest(
         saved_warnings = []
         def capture_warning(msg, slug=None):            # pylint: disable=unused-argument
             """A fake implementation of Coverage._warn, to capture warnings."""
+            if slug:
+                msg = "%s (%s)" % (msg, slug)
             saved_warnings.append(msg)
 
         original_warn = cov._warn
@@ -303,6 +309,10 @@ class CoverageTest(
                             break
                     else:
                         self.fail("Didn't find warning %r in %r" % (warning_regex, saved_warnings))
+                for warning_regex in not_warnings:
+                    for saved in saved_warnings:
+                        if re.search(warning_regex, saved):
+                            self.fail("Found warning %r in %r" % (warning_regex, saved_warnings))
             else:
                 # No warnings expected. Raise if any warnings happened.
                 if saved_warnings:
diff --git a/tests/test_testing.py b/tests/test_testing.py
index d86207e..05bf0c9 100644
--- a/tests/test_testing.py
+++ b/tests/test_testing.py
@@ -114,6 +114,17 @@ class CoverageTestTest(CoverageTest):
             with self.assert_warnings(cov, ["Not me"]):
                 cov._warn("Hello there!")
 
+        # Try checking a warning that shouldn't appear: happy case.
+        with self.assert_warnings(cov, ["Hi"], not_warnings=["Bye"]):
+            cov._warn("Hi")
+
+        # But it should fail if the unexpected warning does appear.
+        warn_regex = r"Found warning 'Bye' in \['Hi', 'Bye'\]"
+        with self.assertRaisesRegex(AssertionError, warn_regex):
+            with self.assert_warnings(cov, ["Hi"], not_warnings=["Bye"]):
+                cov._warn("Hi")
+                cov._warn("Bye")
+
         # assert_warnings shouldn't hide a real exception.
         with self.assertRaises(ZeroDivisionError):
             with self.assert_warnings(cov, ["Hello there!"]):