Fixed #21206 -- No longer run discovery if the test label doesn't point to a package or directory.

Thanks thepapermen for the report and Carl Meyer for the review.

django/test/runner.py

@@ -1,3 +1,4 @@
+from importlib import import_module
 import os
 from optparse import make_option
 import unittest
@@ -89,11 +90,11 @@ class DiscoverRunner(object):
                     break
                 kwargs['top_level_dir'] = top_level
 
-            if not (tests and tests.countTestCases()):
-                # if no tests found, it's probably a package; try discovery
+            if not (tests and tests.countTestCases()) and is_discoverable(label):
+                # Try discovery if path is a package or directory
                 tests = self.test_loader.discover(start_dir=label, **kwargs)
 
-                # make unittest forget the top-level dir it calculated from this
+                # Make unittest forget the top-level dir it calculated from this
                 # run, to support running tests from two different top-levels.
                 self.test_loader._top_level_dir = None
 
@@ -150,6 +151,20 @@ class DiscoverRunner(object):
         return self.suite_result(suite, result)
 
 
+def is_discoverable(label):
+    """
+    Check if a test label points to a python package or file directory.
+    """
+    try:
+        mod = import_module(label)
+    except ImportError:
+        pass
+    else:
+        return hasattr(mod, '__path__')
+
+    return os.path.isdir(os.path.abspath(label))
+
+
 def dependency_ordered(test_databases, dependencies):
     """
     Reorder test_databases into an order that honors the dependencies

tests/test_discovery_sample/tests_sample.py

@@ -13,3 +13,7 @@ class TestDjangoTestCase(DjangoTestCase):
 
     def test_sample(self):
         self.assertEqual(1, 1)
+
+
+class EmptyTestCase(TestCase):
+    pass

tests/test_runner/test_discover_runner.py

@@ -68,6 +68,34 @@ class DiscoverRunnerTest(TestCase):
 
         self.assertEqual(count, 3)
 
+    def test_empty_test_case(self):
+        count = DiscoverRunner().build_suite(
+            ["test_discovery_sample.tests_sample.EmptyTestCase"],
+        ).countTestCases()
+
+        self.assertEqual(count, 0)
+
+    def test_discovery_on_package(self):
+        count = DiscoverRunner().build_suite(
+            ["test_discovery_sample.tests"],
+        ).countTestCases()
+
+        self.assertEqual(count, 1)
+
+    def test_ignore_adjacent(self):
+        """
+        When given a dotted path to a module, unittest discovery searches
+        not just the module, but also the directory containing the module.
+
+        This results in tests from adjacent modules being run when they
+        should not. The discover runner avoids this behavior.
+        """
+        count = DiscoverRunner().build_suite(
+            ["test_discovery_sample.empty"],
+        ).countTestCases()
+
+        self.assertEqual(count, 0)
+
     def test_overrideable_test_suite(self):
         self.assertEqual(DiscoverRunner().test_suite, TestSuite)