list_from_dict initial try

Author: Tinche

src/cattrs/strategies/__init__.py

@@ -1,10 +1,12 @@
 """High level strategies for converters."""
 
 from ._class_methods import use_class_methods
+from ._listfromdict import configure_list_from_dict
 from ._subclasses import include_subclasses
 from ._unions import configure_tagged_union, configure_union_passthrough
 
 __all__ = [
+    "configure_list_from_dict",
     "configure_tagged_union",
     "configure_union_passthrough",
     "include_subclasses",

src/cattrs/strategies/_listfromdict.py

@@ -0,0 +1,40 @@
+"""The list-from-dict implementation."""
+
+from collections.abc import Mapping
+from typing import Any, TypeVar, get_args
+
+from .. import BaseConverter, SimpleStructureHook
+from ..dispatch import UnstructureHook
+
+T = TypeVar("T")
+
+
+def configure_list_from_dict(
+    seq_type: list[T], field: str, converter: BaseConverter
+) -> tuple[SimpleStructureHook[Mapping, T], UnstructureHook]:
+    """
+    Configure a list subtype to be structured and unstructured using a dictionary.
+
+    List elements have to be an attrs class or a dataclass. One field of the element
+    type is extracted into a dictionary key; the rest of the data is stored under that
+    key.
+
+    """
+    arg_type = get_args(seq_type)[0]
+
+    arg_structure_hook = converter.get_structure_hook(arg_type, cache_result=False)
+
+    def structure_hook(
+        value: Mapping, type: Any = seq_type, _arg_type=arg_type
+    ) -> list[T]:
+        return [arg_structure_hook(v | {field: k}, _arg_type) for k, v in value.items()]
+
+    arg_unstructure_hook = converter.get_unstructure_hook(arg_type, cache_result=False)
+
+    def unstructure_hook(val: list[T]) -> dict:
+        return {
+            (unstructured := arg_unstructure_hook(v)).pop(field): unstructured
+            for v in val
+        }
+
+    return structure_hook, unstructure_hook

tests/strategies/test_from_from_dict.py

@@ -0,0 +1,22 @@
+"""Tests for the list-from-dict strategy."""
+
+from attrs import define
+
+from cattrs import BaseConverter
+from cattrs.strategies import configure_list_from_dict
+
+
+@define
+class A:
+    a: int
+    b: str
+
+
+def test_simple_roundtrip(converter: BaseConverter):
+    hook, hook2 = configure_list_from_dict(list[A], "a", converter)
+
+    structured = [A(1, "2"), A(3, "4")]
+    unstructured = hook2(structured)
+    assert unstructured == {1: {"b": "2"}, 3: {"b": "4"}}
+
+    assert hook(unstructured) == structured