From ddf1a876b23b189f81fc308a4436d98e20e70d34 Mon Sep 17 00:00:00 2001
From: Luke Jefferies <lukejefferiesuk@gmail.com>
Date: Mon, 18 Aug 2025 11:45:03 +0100
Subject: [PATCH] Fixed #36533 -- Fix startapp to allow empty directories as
 valid targets.

---
 django/core/management/templates.py | 15 +++++++-
 tests/admin_scripts/tests.py        | 60 +++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+), 1 deletion(-)

diff --git a/django/core/management/templates.py b/django/core/management/templates.py
index ea2c4a294f..a9669161da 100644
--- a/django/core/management/templates.py
+++ b/django/core/management/templates.py
@@ -105,7 +105,20 @@ class TemplateCommand(BaseCommand):
         else:
             top_dir = os.path.abspath(os.path.expanduser(target))
             if app_or_project == "app":
-                self.validate_name(os.path.basename(top_dir), "directory")
+                dir_name = os.path.basename(top_dir)
+
+                # Non empty directory already exists
+                if os.path.exists(top_dir):
+                    if os.listdir(top_dir):
+                        raise CommandError(
+                            f"{top_dir} already exists. Overlaying an app into an "
+                            "existing directory won't replace conflicting files."
+                        )
+                # Does not exist so validate new name
+                else:
+                    self.validate_name(dir_name, "directory")
+
+            # Create directory if it doesn't exist
             if not os.path.exists(top_dir):
                 try:
                     os.makedirs(top_dir)
diff --git a/tests/admin_scripts/tests.py b/tests/admin_scripts/tests.py
index 97d2d7cf7e..56eb1f4c73 100644
--- a/tests/admin_scripts/tests.py
+++ b/tests/admin_scripts/tests.py
@@ -3197,6 +3197,66 @@ class StartApp(AdminScriptTestCase):
         )
         self.assertFalse(os.path.exists(testapp_dir))
 
+    def test_existing_empty_directory_allows_create(self):
+        """
+        Ensure that an existing empty directory
+        does not trigger a false importable module check.
+        """
+        custom_dir = os.path.join(self.test_dir, "destination")
+        os.makedirs(custom_dir, exist_ok=True)
+
+        args = ["startapp", "my_app", custom_dir]
+        _, err = self.run_django_admin(args)
+        self.assertNoOutput(err)
+        self.assertTrue(os.path.exists(os.path.join(custom_dir, "apps.py")))
+
+    def test_custom_directory_allows_create(self):
+        """
+        Ensure destination can be created as a new directory to regression
+        test ticket #36533
+        """
+        custom_dir = os.path.join(self.test_dir, "destination")
+        args = ["startapp", "my_app", custom_dir]
+        _, err = self.run_django_admin(args)
+        self.assertNoOutput(err)
+        self.assertTrue(os.path.exists(os.path.join(custom_dir, "apps.py")))
+
+    def test_importable_python_module_errors(self):
+        """
+        Ensure folder named 'os' can not be created as the name
+        of an app
+        """
+        args = ["startapp", "example", "os"]
+        _, err = self.run_django_admin(args)
+        self.assertIn("CommandError: 'os'", err)
+
+    def test_existing_directory_matching_python_modules_errors(self):
+        """
+        Double checks that an importable module still throws an error if
+        the user created a directory first with a matching name
+        """
+        os.makedirs("os", exist_ok=True)
+        args = ["startapp", "example", "os"]
+        _, err = self.run_django_admin(args)
+        self.assertIn("CommandError: 'os'", err)
+
+    def test_existing_non_empty_directory_overlay_error(self):
+        """
+        Ensure that an existing non empty directory
+        triggers an overlaying app warning.
+        """
+        custom_dir = os.path.join(self.test_dir, "non_empty_dir")
+        os.makedirs(custom_dir, exist_ok=True)
+
+        with open(os.path.join(custom_dir, "dummy.txt"), "w") as f:
+            f.write("test")
+
+        args = ["startapp", "my_app", custom_dir]
+        _, err = self.run_django_admin(args)
+        self.assertIn(
+            "already exists. Overlaying an app into an existing directory", err
+        )
+
 
 class DiffSettings(AdminScriptTestCase):
     """Tests for diffsettings management command."""