Add support for isolating apps in tests

Fix #1253.

Author: bluetech

docs/changelog.rst

@@ -15,6 +15,9 @@ Improvements
 ^^^^^^^^^^^^
 
 * The :ref:`multiple databases <multi-db>` support added in v4.3.0 is no longer considered experimental.
+* Added :func:`@pytest.mark.django_isolate_apps <pytest.mark.django_isolate_apps>`
+  for isolating Django's app registry in pytest tests, and a
+  :fixture:`django_isolated_apps` fixture to access the isolated Apps registry instance if needed.
 
 v4.11.1 (2025-04-03)
 --------------------

docs/helpers.rst

@@ -132,6 +132,29 @@ dynamically in a hook or fixture.
          assert b'Success!' in client.get('/some_url_defined_in_test_urls/').content
 
 
+``pytest.mark.django_isolate_apps`` - isolate the app registry
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. decorator:: pytest.mark.django_isolate_apps(*app_labels)
+
+   Isolate models defined within the marked tests into their own isolated apps registry.
+   See :func:`Isolating apps <django.test.utils.isolate_apps>` for when this might be useful.
+
+   :type app_labels: str
+   :param app_labels:
+     One or more application labels to include in the isolated registry.
+
+   The :fixture:`django_isolated_apps` fixture provides access to the isolated
+   apps registry instance, if needed.
+
+   Example usage::
+
+     @pytest.mark.django_isolate_apps("myapp")
+     def test_something(django_isolated_apps):
+         assert django_isolated_apps.is_installed("myapp")
+         assert not django_isolated_apps.is_installed("otherapp")
+
+
 ``pytest.mark.ignore_template_errors`` - ignore invalid template variables
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -317,6 +340,14 @@ resolves to the user model's :attr:`~django.contrib.auth.models.CustomUser.USERN
 Use this fixture to make pluggable apps testable regardless what the username field
 is configured to be in the containing Django project.
 
+.. fixture:: django_isolated_apps
+
+``django_isolated_apps`` - isolated app registry
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Access the isolated app registry created by
+:func:`@pytest.mark.django_isolate_apps([...]) <pytest.mark.django_isolate_apps>`.
+
 .. fixture:: db
 
 ``db``

pytest_django/plugin.py

@@ -57,6 +57,7 @@
     from typing import Any, NoReturn
 
     import django
+    import django.apps.registry
 
 
 SETTINGS_MODULE_ENV = "DJANGO_SETTINGS_MODULE"
@@ -294,6 +295,10 @@ def pytest_load_initial_conftests(
         "a string specifying the module of a URL config, e.g. "
         '"my_app.test_urls".',
     )
+    early_config.addinivalue_line(
+        "markers",
+        "django_isolate_apps(*app_labels): isolate Django's app registry for this test.",
+    )
     early_config.addinivalue_line(
         "markers",
         "ignore_template_errors(): ignore errors from invalid template "
@@ -669,6 +674,39 @@ def _django_set_urlconf(request: pytest.FixtureRequest) -> Generator[None, None,
         set_urlconf(None)
 
 
+@pytest.fixture(autouse=True)
+def _django_isolate_apps(
+    request: pytest.FixtureRequest,
+) -> Generator[django.apps.registry.Apps, None, None]:
+    """Apply the @pytest.mark.django_isolate_apps marker if present, internal to pytest-django."""
+    marker: pytest.Mark | None = request.node.get_closest_marker("django_isolate_apps")
+    if not marker:
+        yield None
+        return
+
+    skip_if_no_django()
+
+    from django.test.utils import isolate_apps
+
+    app_labels = validate_django_isolate_apps(marker)
+
+    with isolate_apps(*app_labels) as apps:
+        yield apps
+
+
+@pytest.fixture
+def django_isolated_apps(
+    _django_isolate_apps: django.apps.registry.Apps | None,
+) -> django.apps.registry.Apps:
+    """Access the isolated Apps registry instance for tests marked with
+    @pytest.mark.django_isolate_apps(...)."""
+    if _django_isolate_apps is None:
+        raise pytest.UsageError(
+            "The django_isolated_apps fixture requires @pytest.mark.django_isolate_apps([...])."
+        )
+    return _django_isolate_apps
+
+
 @pytest.fixture(autouse=True, scope="session")
 def _fail_for_invalid_template_variable() -> Generator[None, None, None]:
     """Fixture that fails for invalid variables in templates.
@@ -887,3 +925,14 @@ def apifun(urls: list[str]) -> list[str]:
         return urls
 
     return apifun(*marker.args, **marker.kwargs)
+
+
+def validate_django_isolate_apps(marker: pytest.Mark) -> tuple[str, ...]:
+    """Validate the django_isolate_apps marker."""
+
+    def apifun(*app_labels: str) -> tuple[str, ...]:
+        if not app_labels:
+            raise ValueError("@pytest.mark.django_isolate_apps requires at least one app label")
+        return app_labels
+
+    return apifun(*marker.args, **marker.kwargs)

tests/test_isolate_apps.py

@@ -0,0 +1,100 @@
+from .helpers import DjangoPytester
+
+
+def test_django_isolate_apps_marker(django_pytester: DjangoPytester) -> None:
+    django_pytester.create_test_module(
+        """
+        import pytest
+        from django.apps import apps
+
+
+        @pytest.mark.django_isolate_apps("tpkg.app")
+        def test_isolated_registry_fixture(django_isolated_apps):
+            assert django_isolated_apps.is_installed("tpkg.app")
+            assert not django_isolated_apps.is_installed("django.contrib.auth")
+
+
+        @pytest.mark.django_isolate_apps("tpkg.app", "django.contrib.auth")
+        def test_isolated_registry_multiple_apps(django_isolated_apps):
+            assert django_isolated_apps.is_installed("tpkg.app")
+            assert django_isolated_apps.is_installed("django.contrib.auth")
+
+
+        @pytest.mark.django_isolate_apps("tpkg.app")
+        class TestIsolatedRegistryClass:
+            def test_first(self, django_isolated_apps):
+                assert django_isolated_apps.is_installed("tpkg.app")
+
+            def test_second(self, django_isolated_apps):
+                assert not django_isolated_apps.is_installed("django.contrib.auth")
+
+
+        @pytest.mark.django_isolate_apps("tpkg.app")
+        def test_global_registry_is_unchanged():
+            assert apps.is_installed("django.contrib.auth")
+        """
+    )
+
+    result = django_pytester.runpytest_subprocess("-v")
+    result.assert_outcomes(passed=5)
+
+
+def test_django_isolate_apps_is_functional(django_pytester: DjangoPytester) -> None:
+    django_pytester.create_test_module(
+        """
+        import pytest
+        from django.db import models
+        from django.apps import apps
+
+
+        @pytest.mark.django_db
+        @pytest.mark.django_isolate_apps("tpkg.app")
+        def test_model_is_registered(django_isolated_apps):
+            class MyModel(models.Model):
+                class Meta:
+                    app_label = "app"
+
+            assert django_isolated_apps.get_model('app', 'MyModel') is not None
+
+            with pytest.raises(LookupError):
+                apps.get_model('app', 'MyModel')
+        """
+    )
+    result = django_pytester.runpytest_subprocess("-v")
+    result.assert_outcomes(passed=1)
+
+
+def test_django_isolate_apps_marker_requires_labels(django_pytester: DjangoPytester) -> None:
+    django_pytester.create_test_module(
+        """
+        import pytest
+
+
+        @pytest.mark.django_isolate_apps()
+        def test_isolated_registry_requires_labels():
+            pass
+        """
+    )
+
+    result = django_pytester.runpytest_subprocess("-v")
+    result.assert_outcomes(errors=1)
+    result.stdout.fnmatch_lines(
+        ["*ValueError: @pytest.mark.django_isolate_apps requires at least one app label*"]
+    )
+
+
+def test_django_isolated_apps_fixture_requires_marker(django_pytester: DjangoPytester) -> None:
+    django_pytester.create_test_module(
+        """
+        def test_requires_marker(django_isolated_apps):
+            pass
+        """
+    )
+
+    result = django_pytester.runpytest_subprocess("-v")
+    result.assert_outcomes(errors=1)
+    result.stdout.fnmatch_lines(
+        [
+            "*UsageError: The django_isolated_apps fixture requires @pytest.mark.django_isolate_apps*"
+        ]
+    )