Feature: to_dict(key, duplicated=reducer)

Allow to pass reducer to to_dict.
This makes it possible to select items according to specific conditions when keys are duplicated.
For example, it will be possible to take out the largest item and take out the smallest item.

richset/__init__.py

@@ -18,6 +18,7 @@
 T = TypeVar("T")
 S = TypeVar("S")
 Key = TypeVar("Key", bound=Hashable)
+OnDuplicateActions = Literal["error", "first", "last"]
 
 
 @dataclass(frozen=True)
@@ -93,7 +94,7 @@ def to_dict(
         self,
         key: Callable[[T], Key],
         *,
-        duplicated: Literal["error", "first", "last"] = "error",
+        duplicated: OnDuplicateActions | Callable[[list[T]], T] = "error",
     ) -> dict[Key, T]:
         """Returns a dictionary mapping keys to values.
 
@@ -112,7 +113,7 @@ def to_dict(
         elif duplicated == "last":
             return {k: v[-1] for k, v in base.items()}
         else:
-            raise ValueError("invalid duplicated value")
+            return {k: duplicated(v) for k, v in base.items()}
 
     def to_dict_of_list(
         self,

tests/test_conversions.py

@@ -167,9 +167,21 @@ def test_richset_to_dict() -> None:
         "jane": Something(2, "jane"),
     }
 
-    with pytest.raises(ValueError) as err:
-        rs.to_dict(lambda r: r.name, duplicated="invalid")  # type: ignore
-    assert str(err.value) == "invalid duplicated value"
+    # test with a custom duplicate key function
+    assert rs.to_dict(
+        lambda r: r.name,
+        duplicated=lambda items: min(items, key=lambda item: item.id),
+    ) == {
+        "john": Something(1, "john"),
+        "jane": Something(2, "jane"),
+    }
+    assert rs.to_dict(
+        lambda r: r.name,
+        duplicated=lambda items: max(items, key=lambda item: item.id),
+    ) == {
+        "john": Something(3, "john"),
+        "jane": Something(2, "jane"),
+    }
 
 
 def test_richset_to_dict_of_list() -> None: