fix: let config class specify default app handler, engine maker

this avoids the need for a config subclass to use `setdefault()` hacks
to specify default app handler for instance, since that approach must
compete with config extensions who also may wish to do that.

similar concept for the engine maker; notably the rattail project
needs to override this function somewhat and we need a way to allow
for that without (re-)introducing the app handler here.

src/wuttjamaican/conf.py

@@ -126,6 +126,23 @@ class WuttaConfig:
        it's useful, but in practice you should not update it directly;
        instead use :meth:`setdefault()`.
 
+    .. attribute:: default_app_handler_spec
+
+       Spec string for the default app handler, if config does not
+       specify to use another.
+
+       The true default for this is ``'wuttjamaican.app:AppHandler'``
+       (aka.  :class:`~wuttjamaican.app.AppHandler`).
+
+    .. attribute:: default_engine_maker_spec
+
+       Spec string for the default engine maker function, if config
+       does not specify to use another.
+
+       The true default for this is
+       ``'wuttjamaican.db.conf:make_engine_from_config'`` (aka.
+       :func:`~wuttjamaican.db.conf.make_engine_from_config()`).
+
     .. attribute:: files_read
 
        List of all INI config files which were read on app startup.
@@ -165,6 +182,8 @@ class WuttaConfig:
 
        See also :ref:`where-config-settings-come-from`.
     """
+    default_app_handler_spec = 'wuttjamaican.app:AppHandler'
+    default_engine_maker_spec = 'wuttjamaican.db.conf:make_engine_from_config'
 
     def __init__(
             self,
@@ -572,11 +591,22 @@ class WuttaConfig:
         """
         if not hasattr(self, '_app'):
             spec = self.get(f'{self.appname}.app.handler', usedb=False,
-                            default='wuttjamaican.app:AppHandler')
+                            default=self.default_app_handler_spec)
             factory = load_object(spec)
             self._app = factory(self)
         return self._app
 
+    def get_engine_maker(self):
+        """
+        Returns a callable to be used for constructing SQLAlchemy
+        engines fromc config.
+
+        Which callable is used depends on
+        :attr:`default_engine_maker_spec` but by default will be
+        :func:`wuttjamaican.db.conf.make_engine_from_config()`.
+        """
+        return load_object(self.default_engine_maker_spec)
+
 
 class WuttaConfigExtension:
     """

src/wuttjamaican/db/conf.py

@@ -68,16 +68,18 @@ def get_engines(config, prefix):
     else:
         keys = ['default']
 
+    make_engine = config.get_engine_maker()
+
     engines = OrderedDict()
     cfg = config.get_dict(prefix)
     for key in keys:
         key = key.strip()
         try:
-            engines[key] = make_engine_from_config(cfg, prefix=f'{key}.')
+            engines[key] = make_engine(cfg, prefix=f'{key}.')
         except KeyError:
             if key == 'default':
                 try:
-                    engines[key] = make_engine_from_config(cfg, prefix='sqlalchemy.')
+                    engines[key] = make_engine(cfg, prefix='sqlalchemy.')
                 except KeyError:
                     pass
     return engines

tests/test_conf.py

@@ -10,6 +10,7 @@ import sqlalchemy as sa
 from wuttjamaican import conf
 from wuttjamaican.exc import ConfigurationError
 from wuttjamaican.db import Session
+from wuttjamaican.db.conf import make_engine_from_config
 from wuttjamaican.app import AppHandler
 from wuttjamaican.testing import FileConfigTestCase
 
@@ -386,12 +387,43 @@ configure_logging = true
         self.assertEqual(config.get_list('foo.bar'), ['hello', 'world'])
 
     def test_get_app(self):
+
+        # default handler
         config = conf.WuttaConfig()
+        self.assertEqual(config.default_app_handler_spec, 'wuttjamaican.app:AppHandler')
         app = config.get_app()
-        # make sure we get the true default handler class
         self.assertIsInstance(app, AppHandler)
+        # nb. make extra sure we didn't get a subclass
         self.assertIs(type(app), AppHandler)
 
+        # custom default handler
+        config = conf.WuttaConfig()
+        config.default_app_handler_spec = 'tests.test_conf:CustomAppHandler'
+        app = config.get_app()
+        self.assertIsInstance(app, CustomAppHandler)
+
+    def test_get_engine_maker(self):
+
+        # default func
+        config = conf.WuttaConfig()
+        self.assertEqual(config.default_engine_maker_spec, 'wuttjamaican.db.conf:make_engine_from_config')
+        make_engine = config.get_engine_maker()
+        self.assertIs(make_engine, make_engine_from_config)
+
+        # custom default func
+        config = conf.WuttaConfig()
+        config.default_engine_maker_spec = 'tests.test_conf:custom_make_engine_from_config'
+        make_engine = config.get_engine_maker()
+        self.assertIs(make_engine, custom_make_engine_from_config)
+
+
+class CustomAppHandler(AppHandler):
+    pass
+
+
+def custom_make_engine_from_config(*args, **kwargs):
+    pass
+
 
 class TestWuttaConfigExtension(TestCase):