feat: Implement config loading bootstrap

app/__init__.py

@@ -1,6 +1,5 @@
-import inspect
 from logging.config import dictConfig
-from typing import Any, Dict, List, Mapping, Optional, Sequence, Type, cast
+from typing import Any, Dict, List, Mapping, Optional, Sequence, Type
 
 import yaml
 from yaml import Loader
@@ -11,7 +10,7 @@
     Config,
     ImproperlyConfiguredError,
     SettingInitializer,
-    import_string,
+    import_string_as_klass,
 )
 
 # =============================================================================
@@ -20,6 +19,8 @@
 
 _LOGGING_CONFIG_KEY = "LOGGING"
 
+_SETTINGS_INITIALIZERS_CONFIG_KEY = "SETTINGS_INITIALIZERS"
+
 _SUPPORTED_DATA_SOURCE_TYPES_CONFIG_KEY = "SUPPORTED_DATA_SOURCE_TYPES"
 
 _DEFAULT_CONFIG: Dict[str, Any] = {
@@ -43,9 +44,8 @@
         },
         "root": {"level": "INFO", "handlers": ["console"]},
     },
-    _SUPPORTED_DATA_SOURCE_TYPES_CONFIG_KEY: [
-        "app.imp.sql_data.SQLDataSourceType"
-    ],
+    _SETTINGS_INITIALIZERS_CONFIG_KEY: [],
+    _SUPPORTED_DATA_SOURCE_TYPES_CONFIG_KEY: [],
 }
 
 
@@ -73,13 +73,43 @@
 # =============================================================================
 
 
-def _load_config_file(config_file_path: str) -> Mapping[str, Any]:
+def _load_config_file(
+    config_file_path: str,
+) -> Mapping[str, Any]:  # pragma: no cover
     # TODO: Ensure that a valid config file path was given and if not raise an
     #  appropriate Exception.
     with open(config_file_path, "rb") as config_file:
         return yaml.load(config_file, Loader=Loader)
 
 
+def _load_settings_initializers(
+    initializers_dotted_paths: Sequence[str],
+) -> Sequence[SettingInitializer]:
+    initializers: List[SettingInitializer] = list()
+    for _initializer_dotted_path in initializers_dotted_paths:
+        try:
+            initializer_klass: Type[SettingInitializer]
+            initializer_klass = import_string_as_klass(
+                _initializer_dotted_path, SettingInitializer
+            )
+            initializers.append(initializer_klass())
+        except ImportError as exp:
+            raise ImproperlyConfiguredError(
+                message='"%s" does not seem to be a valid path.'
+                % _initializer_dotted_path
+            ) from exp
+        except TypeError as exp:
+            raise ImproperlyConfiguredError(
+                message=(
+                    'Invalid value, "%s" is either not class or is not a '
+                    'subclass of "app.lib.SettingInitializer".'
+                    % _initializer_dotted_path
+                )
+            ) from exp
+
+    return initializers
+
+
 # =============================================================================
 # DEFAULT SETTINGS INITIALIZERS
 # =============================================================================
@@ -130,22 +160,22 @@ def _dotted_path_to_data_source_type_klass(
         dotted_path: str,
     ) -> Type[DataSourceType]:
         try:
-            _module = import_string(dotted_path)
+            data_source_type_klass: Type[DataSourceType]
+            data_source_type_klass = import_string_as_klass(
+                dotted_path, DataSourceType
+            )
+            return data_source_type_klass
         except ImportError as exp:
             raise ImproperlyConfiguredError(
                 message='"%s" does not seem to be a valid path.' % dotted_path
             ) from exp
-
-        if not inspect.isclass(_module) or not issubclass(
-            _module, DataSourceType
-        ):  # type: ignore
+        except TypeError as exp:
             raise ImproperlyConfiguredError(
                 message=(
                     'Invalid value, "%s" is either not class or is not a '
                     'subclass of "app.core.DataSourceType".' % dotted_path
                 )
-            )
-        return cast(Type[DataSourceType], _module)
+            ) from exp
 
 
 # =============================================================================
@@ -174,11 +204,17 @@ def setup(
 
     # Load the application settings
     _settings_dict: Dict[str, Any] = dict(initial_settings or _DEFAULT_CONFIG)
-    if config_file_path:  # load config from a file when provided
+    # Load config from a file when provided
+    if config_file_path:  # pragma: no branch
         _settings_dict.update(_load_config_file(config_file_path))
 
     # Load initializers
     _initializers: List[Any] = list(settings_initializers or [])
+    _initializers.extend(
+        _load_settings_initializers(
+            _settings_dict.get(_SETTINGS_INITIALIZERS_CONFIG_KEY, tuple())
+        )
+    )
     _initializers.insert(0, _LoggingInitializer())
     _initializers.insert(1, _SupportedDataSourceTypesInitializer())
 

app/lib/__init__.py

@@ -2,7 +2,7 @@
 from .checkers import ensure_not_none, ensure_not_none_nor_empty
 from .config import *  # noqa: F401,F403
 from .config import __all__ as _all_config
-from .module_loading import import_string
+from .module_loading import import_string, import_string_as_klass
 from .tasks import *  # noqa: F401,F403
 from .tasks import __all__ as _all_tasks
 from .transports import *  # noqa: F401,F403
@@ -13,6 +13,7 @@
     "ensure_not_none",
     "ensure_not_none_nor_empty",
     "import_string",
+    "import_string_as_klass",
 ]
 __all__ += _all_config  # type: ignore
 __all__ += _all_tasks  # type: ignore

app/lib/module_loading.py

@@ -1,7 +1,20 @@
 # The contents of this module are copied from Django sources.
+import inspect
 import sys
 from importlib import import_module
 from types import ModuleType
+from typing import Type, TypeVar, cast
+
+# =============================================================================
+# TYPES
+# =============================================================================
+
+_T = TypeVar("_T")
+
+
+# =============================================================================
+# HELPERS
+# =============================================================================
 
 
 def _cached_import(module_path: str, class_name: str) -> ModuleType:
@@ -16,10 +29,15 @@ def _cached_import(module_path: str, class_name: str) -> ModuleType:
     return getattr(modules[module_path], class_name)
 
 
+# =============================================================================
+# IMPORT UTILITIES
+# =============================================================================
+
+
 def import_string(dotted_path: str) -> ModuleType:
     """
     Import a dotted module path and return the attribute/class designated by
-    the last name in the path. Raise ImportError if the import failed.
+    the last name in the path. Raise ``ImportError`` if the import failed.
 
     :param dotted_path: A dotted path to an attribute or class.
 
@@ -41,3 +59,33 @@ def import_string(dotted_path: str) -> ModuleType:
             'Module "%s" does not define a "%s" attribute/class'
             % (module_path, class_name)
         ) from err
+
+
+def import_string_as_klass(
+    dotted_path: str, target_klass: Type[_T]
+) -> Type[_T]:
+    """
+    Import a dotted module as the given class type. Raise ``ImportError`` if
+    the import failed and a ``TypeError`` if the imported module is not of the
+    given class type or derived from it.
+
+    :param dotted_path: A dotted path to a class.
+    :param target_klass: The class type that the imported module should have or
+        be derived from.
+
+    :return: The class designated by the last name in the path.
+
+    :raise ImportError: If the import fails for some reason.
+    :raise TypeError: If the imported module is not of the given class type or
+        derived from the given class.
+    """
+    _module = import_string(dotted_path)
+    if not inspect.isclass(_module) or not issubclass(  # noqa
+        _module, target_klass
+    ):
+        raise TypeError(
+            'Invalid value, "%s" is either not a class or a subclass of "%s".'
+            % (dotted_path, target_klass.__qualname__)
+        )
+
+    return cast(Type[target_klass], _module)

tests/lib/test_module_loading.py

@@ -1,9 +1,10 @@
 import pytest
 
-from app.lib import Config, import_string
+from app.core import Task
+from app.lib import Config, Consumer, import_string, import_string_as_klass
 
 
-def test_correct_return_value_on_valid_dotted_path() -> None:
+def test_correct_return_value_on_valid_dotted_path1() -> None:
     """
     Assert that when given a valid dotted path as input, ``import_string``
     returns the correct class or attribute.
@@ -14,7 +15,7 @@ def test_correct_return_value_on_valid_dotted_path() -> None:
     )  # noqa
 
 
-def test_correct_expected_behavior_on_invalid_dotted_path() -> None:
+def test_correct_expected_behavior_on_invalid_dotted_path1() -> None:
     """
     Assert that an ``ImportError`` is raised when an invalid dotted path is
     given as input to ``import_string``.
@@ -23,3 +24,28 @@ def test_correct_expected_behavior_on_invalid_dotted_path() -> None:
         import_string("aninvalidpath")
     with pytest.raises(ImportError):
         import_string("app.no_such")
+
+
+def test_correct_return_value_on_valid_dotted_path2() -> None:
+    """
+    Assert that when given a valid dotted path as input,
+    ``import_string_import_string_as_klass`` returns the correct class.
+    """
+    assert import_string_as_klass("app.lib.Config", Config) is Config
+    assert import_string_as_klass("app.lib.Consumer", Task) is Consumer
+
+
+def test_correct_expected_behavior_on_invalid_dotted_path2() -> None:
+    """
+    Assert that an ``ImportError`` or ``TypeError`` is raised when an invalid
+    dotted path is given as input to ``import_import_string_as_klass``.
+    """
+    with pytest.raises(ImportError):
+        import_string_as_klass("aninvalidpath", Config)
+    with pytest.raises(TypeError):
+        import_string_as_klass(
+            "app.lib.module_loading.import_string",
+            import_string,  # type: ignore
+        )
+    with pytest.raises(TypeError):
+        import_string_as_klass("app.lib.Config", Task)

tests/test_app.py

@@ -1,12 +1,97 @@
+from typing import Any, Dict, Mapping
+from unittest import TestCase
+
+import pytest
+
 import app
+from app.lib import ImproperlyConfiguredError, SettingInitializer
+
+# =============================================================================
+# HELPERS
+# =============================================================================
+
+
+def _test_config_factory() -> Mapping[str, Any]:
+    return {
+        "SETTINGS_INITIALIZERS": [
+            "tests.test_app.FakeDataSourceTypesConfigInitializer"
+        ],
+        "SUPPORTED_DATA_SOURCE_TYPES": ["app.imp.sql_data.SQLDataSourceType"],
+    }
+
+
+class FakeDataSourceTypesConfigInitializer(SettingInitializer):
+    """A fake settings initializers for testing."""
+
+    @property
+    def setting(self) -> str:
+        return "SUPPORTED_DATA_SOURCE_TYPES"
+
+    def execute(self, an_input: Any) -> Any:
+        # Do nothing
+        return an_input
+
+
+# =============================================================================
+# TESTS
+# =============================================================================
 
 
 def test_app_globals_are_set_after_successful_setup() -> None:
     assert app.settings is None
     assert app.registry is None
 
     # After setup, globals should be not None.
-    app.setup()
+    app.setup(initial_settings=_test_config_factory())
 
     assert app.settings is not None
     assert app.registry is not None
+
+
+class TestAppModule(TestCase):
+    """Tests for the app module."""
+
+    def setUp(self) -> None:
+        super().setUp()
+        self._default_config: Dict[str, Any] = dict()
+        self._some_state: int = 0
+
+    def test_valid_config_is_successful(self) -> None:
+        """Assert that a valid config will be executed successfully."""
+        app.setup(initial_settings=self._default_config)
+
+    def test_invalid_settings_initializers_config_causes_error(self) -> None:
+        """
+        Assert that invalid dotted paths or dotted paths to non
+        :config:`setting initializers <SettingInitializer>` on the
+        *SETTINGS_INITIALIZERS* setting results in errors.
+        """
+        config1: Dict[str, Any] = dict(self._default_config)
+        config2: Dict[str, Any] = dict(self._default_config)
+
+        config1["SETTINGS_INITIALIZERS"] = ["invalid_dotted_path"]
+        config2["SETTINGS_INITIALIZERS"] = ["app.core.Task"]
+        with pytest.raises(ImproperlyConfiguredError):
+            app.setup(initial_settings=config1)
+
+        # Not an initializer
+        with pytest.raises(ImproperlyConfiguredError):
+            app.setup(initial_settings=config2)
+
+    def test_invalid_data_source_types_config_causes_error(self) -> None:
+        """
+        Assert that invalid dotted paths or dotted paths to non
+        :config:`data source types <DataSourceType>` on the
+        *SUPPORTED_DATA_SOURCE_TYPES* setting result in errors.
+        """
+        config1: Dict[str, Any] = dict(self._default_config)
+        config2: Dict[str, Any] = dict(self._default_config)
+
+        config1["SUPPORTED_DATA_SOURCE_TYPES"] = ["invalid_dotted_path"]
+        config2["SUPPORTED_DATA_SOURCE_TYPES"] = ["app.core.Task"]
+        with pytest.raises(ImproperlyConfiguredError):
+            app.setup(initial_settings=config1)
+
+        # Not a data source type
+        with pytest.raises(ImproperlyConfiguredError):
+            app.setup(initial_settings=config2)