Make FIRST_PARTY_APPS handling match the behaviour of INSTALLED_APPS

Supercedes #62.

HISTORY.rst

@@ -2,6 +2,11 @@
 History
 =======
 
+* Make ``FIRST_PARTY_APPS`` handling match the behaviour of ``INSTALLED_APPS``.
+
+  Thanks to Martin Bächtold for the report in `Pull Request #62
+  <https://github.com/adamchainz/django-linear-migrations/pull/62>`__.
+
 * Stop distributing tests to reduce package size. Tests are not intended to be
   run outside of the tox setup in the repository. Repackagers can use GitHub's
   tarballs per tag.

src/django_linear_migrations/apps.py

@@ -1,11 +1,14 @@
 import pkgutil
+from functools import lru_cache
 from importlib import import_module, reload
 from pathlib import Path
 
 from django.apps import AppConfig, apps
 from django.conf import settings
 from django.core.checks import Error, Tags, register
+from django.core.signals import setting_changed
 from django.db.migrations.loader import MigrationLoader
+from django.dispatch import receiver
 from django.utils.functional import cached_property
 
 from django_linear_migrations.compat import is_namespace_module
@@ -19,9 +22,23 @@ def ready(self):
         register(Tags.models)(check_max_migration_files)
 
 
+@lru_cache(maxsize=1)
+def get_first_party_app_labels():
+    if not settings.is_overridden("FIRST_PARTY_APPS"):
+        return None
+    return {AppConfig.create(name).label for name in settings.FIRST_PARTY_APPS}
+
+
+@receiver(setting_changed)
+def reset_first_party_app_labels(*, setting, **kwargs):
+    if setting == "FIRST_PARTY_APPS":
+        get_first_party_app_labels.cache_clear()
+
+
 def is_first_party_app_config(app_config):
-    if settings.is_overridden("FIRST_PARTY_APPS"):
-        return app_config.label in settings.FIRST_PARTY_APPS
+    first_party_labels = get_first_party_app_labels()
+    if first_party_labels is not None:
+        return app_config.label in first_party_labels
 
     # Check if it seems to be installed in a virtualenv
     path = Path(app_config.path)

tests/test_apps.py

@@ -0,0 +1,31 @@
+from django.apps import apps
+from django.test import SimpleTestCase
+from django.test.utils import override_settings
+
+from django_linear_migrations.apps import is_first_party_app_config
+
+
+class IsFirstPartyAppConfigTests(SimpleTestCase):
+    @override_settings(FIRST_PARTY_APPS=[])
+    def test_empty(self):
+        app_config = apps.get_app_config("testapp")
+
+        assert not is_first_party_app_config(app_config)
+
+    @override_settings(FIRST_PARTY_APPS=["django_linear_migrations"])
+    def test_not_named(self):
+        app_config = apps.get_app_config("testapp")
+
+        assert not is_first_party_app_config(app_config)
+
+    @override_settings(FIRST_PARTY_APPS=["tests.testapp"])
+    def test_named_by_path(self):
+        app_config = apps.get_app_config("testapp")
+
+        assert is_first_party_app_config(app_config)
+
+    @override_settings(FIRST_PARTY_APPS=["tests.testapp.apps.TestAppConfig"])
+    def test_named_by_app_config_path(self):
+        app_config = apps.get_app_config("testapp")
+
+        assert is_first_party_app_config(app_config)

tests/testapp/__init__.py

@@ -0,0 +1,4 @@
+import django
+
+if django.VERSION < (3, 2):
+    default_app_config = "tests.testapp.apps.TestAppConfig"

tests/testapp/apps.py

@@ -0,0 +1,6 @@
+from django.apps import AppConfig
+
+
+class TestAppConfig(AppConfig):
+    name = "tests.testapp"
+    verbose_name = "Test App"