src/snakia/field/field.py

@@ -1,7 +1,14 @@
 from __future__ import annotations
 
 from abc import ABC, abstractmethod
-from typing import TYPE_CHECKING, Any, Callable, Final, Generic, TypeVar, final
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Callable,
+    Generic,
+    TypeVar,
+    final,
+)
 
 from snakia.property.priv_property import PrivProperty
 from snakia.utils import inherit
@@ -11,10 +18,6 @@ R = TypeVar("R")
 
 
 class Field(ABC, PrivProperty[T], Generic[T]):
-    def __init__(self, default_value: T) -> None:
-        self.default_value: Final[T] = default_value
-        super().__init__(default_value)
-
     @abstractmethod
     def serialize(self, value: T, /) -> bytes:
         """Serialize a value
@@ -42,14 +45,18 @@ class Field(ABC, PrivProperty[T], Generic[T]):
         serialize: Callable[[Field[R], R], bytes],
         deserialize: Callable[[Field[R], bytes], R],
     ) -> type[Field[R]]:
-        return inherit(cls, {"serialize": serialize, "deserialize": deserialize})
+        return inherit(
+            cls, {"serialize": serialize, "deserialize": deserialize}
+        )
 
     @final
     @staticmethod
     def get_fields(class_: type[Any] | Any, /) -> dict[str, Field[Any]]:
         if not isinstance(class_, type):
             class_ = class_.__class__
-        return {k: v for k, v in class_.__dict__.items() if isinstance(v, Field)}
+        return {
+            k: v for k, v in class_.__dict__.items() if isinstance(v, Field)
+        }
 
     if TYPE_CHECKING:
 

src/snakia/property/priv_property.py

@@ -1,22 +1,43 @@
-from typing import Any, Generic, TypeVar
+from typing import Any, Callable, Final, Generic, TypeVar, overload
+from typing_extensions import Self
 
 T = TypeVar("T")
 
 
 class PrivProperty(Generic[T]):
-    __slots__ = "__name", "__default_value"
+    __slots__ = "__name", "__default_value", "__default_factory"
 
     __name: str
 
-    def __init__(self, default_value: T | None = None) -> None:
-        self.__default_value: T | None = default_value
+    @overload
+    def __init__(self) -> None: ...
+    @overload
+    def __init__(self, default_value: T) -> None: ...
+    @overload
+    def __init__(self, *, default_factory: Callable[[Self], T]) -> None: ...
+    def __init__(
+        self,
+        default_value: T | None = None,
+        default_factory: Callable[[Self], T] | None = None,
+    ) -> None:
+        self.__default_value: Final[T | None] = default_value
+        self.__default_factory: Final[Callable[[Self], T] | None] = (
+            default_factory
+        )
+
+    def _get_default(self: Self) -> T:
+        if self.__default_value is not None:
+            return self.__default_value
+        if self.__default_factory is not None:
+            return self.__default_factory(self)
+        raise ValueError("Either default_value or default_factory must be set")
 
     def __set_name__(self, owner: type, name: str) -> None:
         self.__name = f"_{owner.__name__}__{name}"
 
     def __get__(self, instance: Any, owner: type | None = None, /) -> T:
-        if self.__default_value:
-            return getattr(instance, self.__name, self.__default_value)
+        if not hasattr(instance, self.__name):
+            setattr(instance, self.__name, self._get_default())
         return getattr(instance, self.__name)  # type: ignore
 
     def __set__(self, instance: Any, value: T, /) -> None: