extend

src/tests/test_iterables.py

@@ -592,6 +592,18 @@ def test_new_many_iterables(self) -> None:
         assert isinstance(collection, Collection)
         assert len(collection) == 3
 
+    def test_new_post_init(self) -> None:
+        class SubCollection(Collection[_Item]):
+            @classmethod
+            @override
+            def check_items(cls, items: Iterable[_Item]) -> None:
+                if any(item.n >= 1 for item in items):
+                    msg = "n >= 1 is not permitted"
+                    raise ValueError(msg)
+
+        with raises(ValueError, match="n >= 1 is not permitted"):
+            _ = SubCollection(map(_Item, range(3)))
+
     def test_or_singleton(self) -> None:
         collection = Collection(map(_Item, range(3)))
         result = collection | _Item(3)

src/utilities/iterables.py

@@ -499,6 +499,7 @@ class Collection(frozenset[_TSupportsHashAndSort]):
 
     def __new__(cls, *item_or_items: MaybeIterable[_TSupportsHashAndSort]) -> Self:
         items = list(chain(*map(always_iterable, item_or_items)))
+        cls.check_items(items)
         return super().__new__(cls, items)
 
     def __init__(self, *item_or_items: MaybeIterable[_TSupportsHashAndSort]) -> None:
@@ -574,6 +575,10 @@ def __sub__(
         other = cast(Iterable[_TSupportsHashAndSort], other)
         return self.__sub__(type(self)(other))
 
+    @classmethod
+    def check_items(cls, items: Iterable[_TSupportsHashAndSort], /) -> None:
+        _ = items
+
     def filter(self, func: Callable[[_TSupportsHashAndSort], bool], /) -> Self:
         return type(self)(filter(func, self))