Extract deserialization into _FromToml

stlt/config.py

@@ -1,6 +1,7 @@
 """Reading and writing user configuration and secrets."""
 from __future__ import annotations
 
+from abc import ABC
 from importlib import resources
 from pathlib import Path
 import typing as t
@@ -15,51 +16,61 @@
 DEFAULT_CONFIG_FILE = toml.loads(resources.read_text(assets, "config.toml"))
 
 
+_FromTomlType = t.TypeVar("_FromTomlType", bound="_FromToml")
+
+
+class _FromToml(ABC):
+    """Implements deserialization from `toml` into `attrs`."""
+
+    def __init__(self, *args: t.Any, **kwargs: t.Any) -> None:  # pragma: no cover
+        ...
+
+    @classmethod
+    def from_dict(cls: t.Type[_FromTomlType], data: t.Mapping) -> _FromTomlType:
+        """Deserialize some `data` into an `attrs`-based `cls`."""
+        kwargs = {}
+
+        for field in attr.fields(cls):
+            name = field.name
+            meta = field.metadata
+
+            try:
+                if section := meta.get("section", False):
+                    kwargs[name] = _nested_get(data, section)
+                else:
+                    kwargs[name] = data[name]
+            except KeyError as e:
+                if "section" in meta:
+                    err = f"Missing required section {e.args[0]}"
+                else:
+                    err = f"Missing required key {name}"
+                raise ConfigError(err)
+
+            if builder := meta.get("builder", False):
+                kwargs[name] = builder(kwargs[name])
+
+        return cls(**kwargs)
+
+
 @attr.s(slots=True)
-class OAuthConfig:
+class OAuthConfig(_FromToml):
     """Configuration data class for `SpotifyOAuth`."""
 
     client_id: str = attr.ib()
     client_secret: str = attr.ib()
     redirect_uri: str = attr.ib()
     scope: str = attr.ib()
 
-    @classmethod
-    def from_dict(cls, data: t.Mapping) -> OAuthConfig:
-        """Create an `OAuthConfig` from a `toml`-based mapping."""
-        fields = {}
-        for field in attr.fields(cls):
-            name = field.name
-            try:
-                fields[name] = data[name]
-            except KeyError:
-                err = f"Missing required key '{name}'"
-                raise ConfigError(err) from None
-        return cls(**fields)
-
 
 @attr.s
-class CacheConfig:
+class CacheConfig(_FromToml):
     """Configuration data class for the cache."""
 
     auth_cache: Path = attr.ib(converter=Path)
 
-    @classmethod
-    def from_dict(cls, data: t.Mapping) -> CacheConfig:
-        """Create a `CacheConfig` from a `toml`-based mapping."""
-        fields = {}
-        for field in attr.fields(cls):
-            name = field.name
-            try:
-                fields[name] = data[name]
-            except KeyError:
-                err = f"Missing required key '{name}'"
-                raise ConfigError(err) from None
-        return cls(**fields)
-
 
 @attr.s(slots=True)
-class Config:
+class Config(_FromToml):
     """Configuration data class for the project."""
 
     oauth: OAuthConfig = attr.ib(
@@ -70,22 +81,6 @@ class Config:
         metadata={"section": ["cache"], "builder": CacheConfig.from_dict}
     )
 
-    @classmethod
-    def from_dict(cls, data: t.Mapping) -> Config:
-        """Create a `Config` from a `toml`-based mapping."""
-        sections = {}
-        for field in attr.fields(cls):
-            name = field.name
-            meta = field.metadata
-            builder = meta["builder"]
-            section = meta["section"]
-            try:
-                sections[name] = builder(_nested_get(data, section))
-            except KeyError as e:
-                err = f"Missing required section '{e.args[0]}'"
-                raise ConfigError(err) from None
-        return cls(**sections)
-
 
 def ensure_config(config: Path) -> None:
     """Ensure that the `config` file exists and is valid."""