Add unconnected database support

Also adds a helper for doing nested config parsing since we're getting
more and more

src/config/__init__.py

@@ -4,7 +4,8 @@ from typing import Any
 
 from .logging import Logging
 from .email import Email
-from .parse import ConfigParseError, assert_key_of_type
+from .database import Database
+from .parse import assert_key_of_type, parse_nested_config
 
 
 @dataclass
@@ -13,6 +14,7 @@ class Config:
     port: int | None
     logging: Logging
     email: Email
+    database: Database
 
     @classmethod
     def from_dict(cls, config: dict[str, Any]) -> 'Config':
@@ -20,17 +22,13 @@ class Config:
         assert_key_of_type(config, 'email', dict)
         assert_key_of_type(config, 'host', str)
         assert_key_of_type(config, 'port', int)
-        try:
-            log_config = Logging.from_dict(config['logging'])
-        except ConfigParseError as e:
-            raise ConfigParseError(['logging', *e.keypath], e.issue) from e
+        log_config = parse_nested_config(config, 'logging', Logging.from_dict)
+        email_config = parse_nested_config(config, 'email', Email.from_dict)
+        db_config = parse_nested_config(config, 'database', Database.from_dict)
 
-        try:
-            email_config = Email.from_dict(config['email'])
-        except ConfigParseError as e:
-            raise ConfigParseError(['email', *e.keypath], e.issue) from e
-
-        return Config(host=config['host'], port=config['port'], email=email_config, logging=log_config)
+        return Config(
+            host=config['host'], port=config['port'], email=email_config, logging=log_config, database=db_config
+        )
 
 
 def parse_config(filename: str) -> Config:

src/config/database.py

@@ -0,0 +1,16 @@
+from src.config.parse import assert_key_of_type
+from typing import Any
+from dataclasses import dataclass
+
+
+@dataclass
+class Database:
+    url: str
+    echo: bool
+
+    @classmethod
+    def from_dict(cls, config: dict[str, Any]) -> 'Database':
+        assert_key_of_type(config, 'url', str)
+        assert_key_of_type(config, 'echo', bool)
+
+        return Database(url=config['url'], echo=config['echo'])

src/config/parse.py

@@ -1,4 +1,4 @@
-from typing import Any
+from typing import Any, TypeVar, Callable
 
 
 class ConfigParseError(Exception):
@@ -18,3 +18,14 @@ def assert_key_of_type(config: dict[str, Any], key: str, kind: Any):
 
     if not isinstance(config[key], kind):
         raise ConfigParseError([key], f'type of "{key}" was {type(config[key])}, expected {kind}')
+
+
+T = TypeVar('T')
+
+
+def parse_nested_config(config: dict[str, Any], key: str, parse: Callable[[dict[str, Any]], T]) -> T:
+    assert_key_of_type(config, key, dict)
+    try:
+        return parse(config[key])
+    except ConfigParseError as e:
+        raise ConfigParseError([key, *e.keypath], e.issue)