Fixed #35728 -- Computed error messages in assertions only on test failures.

Performance regression in 1dae65dc63ae84be5002c37b4ddae0b9220e8808.

Thanks to Adam Johnson for the report.

1 files changed, 9 insertions, 0 deletions

diff --git a/tests/test_utils/tests.py b/tests/test_utils/tests.py
index 494a0ea8d3..37e87aa102 100644
--- a/tests/test_utils/tests.py
+++ b/tests/test_utils/tests.py
@@ -1083,6 +1083,15 @@ class InHTMLTests(SimpleTestCase):
         with self.assertRaisesMessage(AssertionError, msg):
             self.assertNotInHTML("<b>Hello</b>", haystack=haystack)
 
+    def test_assert_not_in_html_msg_prefix(self):
+        haystack = "<p>Hello</p>"
+        msg = (
+            "1 != 0 : Prefix: '<p>Hello</p>' unexpectedly found in the following "
+            f"response\n{haystack!r}"
+        )
+        with self.assertRaisesMessage(AssertionError, msg):
+            self.assertNotInHTML("<p>Hello</p>", haystack=haystack, msg_prefix="Prefix")
+
 
 class JSONEqualTests(SimpleTestCase):
     def test_simple_equal(self):