Refs #32655 -- Improved error if iter_test_cases() is passed a string.

django/test/utils.py

@@ -242,6 +242,13 @@ def iter_test_cases(tests):
     The tests argument can also be an iterable of TestCase objects.
     """
     for test in tests:
+        if isinstance(test, str):
+            # Prevent an unfriendly RecursionError that can happen with
+            # strings.
+            raise TypeError(
+                f'Test {test!r} must be a test case or test suite not string '
+                f'(was found in {tests!r}).'
+            )
         if isinstance(test, TestCase):
             yield test
         else:

tests/test_runner/tests.py

@@ -36,7 +36,7 @@ class MySuite:
         yield from self.tests
 
 
-class TestSuiteTests(unittest.TestCase):
+class TestSuiteTests(SimpleTestCase):
     def build_test_suite(self, test_classes, suite=None, suite_class=None):
         if suite_class is None:
             suite_class = unittest.TestSuite
@@ -89,6 +89,14 @@ class TestSuiteTests(unittest.TestCase):
             'Tests1.test1', 'Tests1.test2', 'Tests2.test1', 'Tests2.test2',
         ])
 
+    def test_iter_test_cases_string_input(self):
+        msg = (
+            "Test 'a' must be a test case or test suite not string (was found "
+            "in 'abc')."
+        )
+        with self.assertRaisesMessage(TypeError, msg):
+            list(iter_test_cases('abc'))
+
     def test_iter_test_cases_iterable_of_tests(self):
         class Tests(unittest.TestCase):
             def test1(self):