feat: strategy for using class methods (#405)

Author: pohlt

docs/strategies.md

@@ -172,7 +172,7 @@ Without the application of the strategy, in both unstructure and structure opera
 
 ```{note}
 The handling of subclasses is an opt-in feature for two main reasons:
-- Performance. While small and probably negligeable in most cases the subclass handling incurs more function calls and has a performance impact. 
+- Performance. While small and probably negligeable in most cases the subclass handling incurs more function calls and has a performance impact.
 - Customization. The specific handling of subclasses can be different from one situation to the other. In particular there is not apparent universal good defaults for disambiguating the union type. Consequently the decision is left to the user.
 ```
 
@@ -258,3 +258,68 @@ Child(a=1, b='foo')
 ```{versionadded} 23.1.0
 
 ```
+
+
+
+### Using Class-Specific Structure and Unstructure Methods
+
+_Found at {py:func}`cattrs.strategies.use_class_methods`._
+
+The following strategy can be applied for both structuring and unstructuring (also simultaneously).
+
+If a class requires special handling for (un)structuring, you can add a dedicated (un)structuring
+method:
+
+```{doctest} class_methods
+
+>>> from attrs import define
+>>> from cattrs import Converter
+>>> from cattrs.strategies import use_class_methods
+
+>>> @define
+... class MyClass:
+...     a: int
+...
+...     @classmethod
+...     def _structure(cls, data: dict):
+...         return cls(data["b"] + 1)  # expecting "b", not "a"
+...
+...     def _unstructure(self):
+...         return {"c": self.a - 1}  # unstructuring as "c", not "a"
+
+>>> converter = Converter()
+>>> use_class_methods(converter, "_structure", "_unstructure")
+>>> print(converter.structure({"b": 42}, MyClass))
+MyClass(a=43)
+>>> print(converter.unstructure(MyClass(42)))
+{'c': 41}
+```
+
+Any class without a `_structure` or `_unstructure` method will use the default strategy for
+structuring or unstructuring, respectively. Feel free to use other names.
+
+If you want to (un)structured nested objects, just append a converter parameter
+to your (un)structuring methods and you will receive the converter there:
+
+```{doctest} class_methods
+
+>>> @define
+... class Nested:
+...     m: MyClass
+...
+...     @classmethod
+...     def _structure(cls, data: dict, conv):
+...         return cls(conv.structure(data["n"], MyClass))
+...
+...     def _unstructure(self, conv):
+...         return {"n": conv.unstructure(self.m)}
+
+>>> print(converter.structure({"n": {"b": 42}}, Nested))
+Nested(m=MyClass(a=43))
+>>> print(converter.unstructure(Nested(MyClass(42))))
+{'n': {'c': 41}}
+```
+
+```{versionadded} 23.2.0
+
+```

src/cattrs/strategies/__init__.py

@@ -1,5 +1,6 @@
 """High level strategies for converters."""
+from ._class_methods import use_class_methods
 from ._subclasses import include_subclasses
 from ._unions import configure_tagged_union
 
-__all__ = ["configure_tagged_union", "include_subclasses"]
+__all__ = ["configure_tagged_union", "include_subclasses", "use_class_methods"]

src/cattrs/strategies/_class_methods.py

@@ -0,0 +1,64 @@
+"""Strategy for using class-specific (un)structuring methods."""
+
+from inspect import signature
+from typing import Any, Callable, Optional, Type, TypeVar
+
+from cattrs import BaseConverter
+
+T = TypeVar("T")
+
+
+def use_class_methods(
+    converter: BaseConverter,
+    structure_method_name: Optional[str] = None,
+    unstructure_method_name: Optional[str] = None,
+) -> None:
+    """
+    Configure the converter such that dedicated methods are used for (un)structuring
+    the instance of a class if such methods are available. The default (un)structuring
+    will be applied if such an (un)structuring methods cannot be found.
+
+    :param converter: The `Converter` on which this strategy is applied. You can use
+        :class:`cattrs.BaseConverter` or any other derived class.
+    :param structure_method_name: Optional string with the name of the class method
+        which should be used for structuring. If not provided, no class method will be
+        used for structuring.
+    :param unstructure_method_name: Optional string with the name of the class method
+        which should be used for unstructuring. If not provided, no class method will
+        be used for unstructuring.
+
+    If you want to (un)structured nested objects, just append a converter parameter
+    to your (un)structuring methods and you will receive the converter there.
+
+    .. versionadded:: 23.2.0
+    """
+
+    if structure_method_name:
+
+        def make_class_method_structure(cl: Type[T]) -> Callable[[Any, Type[T]], T]:
+            fn = getattr(cl, structure_method_name)
+            n_parameters = len(signature(fn).parameters)
+            if n_parameters == 1:
+                return lambda v, _: fn(v)
+            if n_parameters == 2:
+                return lambda v, _: fn(v, converter)
+            raise TypeError("Provide a class method with one or two arguments.")
+
+        converter.register_structure_hook_factory(
+            lambda t: hasattr(t, structure_method_name), make_class_method_structure
+        )
+
+    if unstructure_method_name:
+
+        def make_class_method_unstructure(cl: Type[T]) -> Callable[[T], T]:
+            fn = getattr(cl, unstructure_method_name)
+            n_parameters = len(signature(fn).parameters)
+            if n_parameters == 1:
+                return fn
+            if n_parameters == 2:
+                return lambda self_: fn(self_, converter)
+            raise TypeError("Provide a method with no or one argument.")
+
+        converter.register_unstructure_hook_factory(
+            lambda t: hasattr(t, unstructure_method_name), make_class_method_unstructure
+        )

tests/strategies/test_class_methods.py

@@ -0,0 +1,92 @@
+import itertools
+from typing import Union
+
+import pytest
+from attrs import define
+from hypothesis import given
+from hypothesis.strategies import integers
+
+from cattrs import BaseConverter
+from cattrs.strategies import use_class_methods
+
+
+@define
+class Base:
+    a: int
+
+
+class Structure(Base):
+    @classmethod
+    def _structure(cls, data: dict):
+        return cls(data["b"])  # expecting "b", not "a"
+
+
+class Unstructure(Base):
+    def _unstructure(self):
+        return {"c": self.a}  # unstructuring as "c", not "a"
+
+
+class Both(Structure, Unstructure):
+    pass
+
+
+@pytest.fixture
+def get_converter(converter: BaseConverter):
+    def aux(structure: str, unstructure: str) -> BaseConverter:
+        use_class_methods(converter, structure, unstructure)
+        return converter
+
+    return aux
+
+
+@pytest.mark.parametrize(
+    "cls,structure_method,unstructure_method",
+    itertools.product(
+        [Structure, Unstructure, Both],
+        ["_structure", "_undefined", None],
+        ["_unstructure", "_undefined", None],
+    ),
+)
+def test_not_nested(get_converter, structure_method, unstructure_method, cls) -> None:
+    converter = get_converter(structure_method, unstructure_method)
+
+    assert converter.structure(
+        {
+            "b"
+            if structure_method == "_structure" and hasattr(cls, "_structure")
+            else "a": 42
+        },
+        cls,
+    ) == cls(42)
+
+    assert converter.unstructure(cls(42)) == {
+        "c"
+        if unstructure_method == "_unstructure" and hasattr(cls, "_unstructure")
+        else "a": 42
+    }
+
+
+@given(integers(1, 5))
+def test_nested_roundtrip(depth):
+    @define
+    class Nested:
+        a: Union["Nested", None]
+        c: int
+
+        @classmethod
+        def _structure(cls, data, conv):
+            b = data["b"]
+            return cls(None if b is None else conv.structure(b, cls), data["c"])
+
+        def _unstructure(self, conv):
+            return {"b": conv.unstructure(self.a), "c": self.c}
+
+        @staticmethod
+        def create(depth: int) -> Union["Nested", None]:
+            return None if depth == 0 else Nested(Nested.create(depth - 1), 42)
+
+    structured = Nested.create(depth)
+
+    converter = BaseConverter()
+    use_class_methods(converter, "_structure", "_unstructure")
+    assert structured == converter.structure(converter.unstructure(structured), Nested)