Tweak tagged unions (#443)

* Tweak tagged unions

* Tagged unions: Fix to not change input dict

* Old style unions

HISTORY.md

@@ -18,8 +18,12 @@
   ([#405](https://github.com/python-attrs/cattrs/pull/405))
 - The `omit` parameter of {py:func}`cattrs.override` is now of type `bool | None` (from `bool`).
   `None` is the new default and means to apply default _cattrs_ handling to the attribute, which is to omit the attribute if it's marked as `init=False`, and keep it otherwise.
-- Converters can now be initialized with custom fallback hook factories for un/structuring.
+- Converters can now be initialized with [custom fallback hook factories](https://catt.rs/en/latest/converters.html#fallback-hook-factories) for un/structuring.
   ([#331](https://github.com/python-attrs/cattrs/issues/311) [#441](https://github.com/python-attrs/cattrs/pull/441))
+- Add support for `date` to preconfigured converters.
+  ([#420](https://github.com/python-attrs/cattrs/pull/420))
+- Add support for `datetime.date`s to the PyYAML preconfigured converter.
+  ([#393](https://github.com/python-attrs/cattrs/issues/393))
 - Fix {py:func}`format_exception() <cattrs.v.format_exception>` parameter working for recursive calls to {py:func}`transform_error <cattrs.transform_error>`.
   ([#389](https://github.com/python-attrs/cattrs/issues/389))
 - [_attrs_ aliases](https://www.attrs.org/en/stable/init.html#private-attributes-and-aliases) are now supported, although aliased fields still map to their attribute name instead of their alias by default when un/structuring.
@@ -44,10 +48,8 @@
   ([#412](https://github.com/python-attrs/cattrs/issues/412))
 - Fix certain cases of structuring `Annotated` types.
   ([#418](https://github.com/python-attrs/cattrs/issues/418))
-- Add support for `date` to preconfigured converters.
-  ([#420](https://github.com/python-attrs/cattrs/pull/420))
-- Add support for `datetime.date`s to the PyYAML preconfigured converter.
-  ([#393](https://github.com/python-attrs/cattrs/issues/393))
+- Fix the [tagged union strategy](https://catt.rs/en/stable/strategies.html#tagged-unions-strategy) to work with `forbid_extra_keys`.
+  ([#402](https://github.com/python-attrs/cattrs/issues/402) [#443](https://github.com/python-attrs/cattrs/pull/443))
 - Use [PDM](https://pdm.fming.dev/latest/) instead of Poetry.
 - _cattrs_ is now linted with [Ruff](https://beta.ruff.rs/docs/).
 - Remove some unused lines in the unstructuring code.
@@ -68,7 +70,8 @@
 
 ## 23.1.0 (2023-05-30)
 
-- Introduce the `tagged_union` strategy. ([#318](https://github.com/python-attrs/cattrs/pull/318) [#317](https://github.com/python-attrs/cattrs/issues/317))
+- Introduce the [`tagged_union` strategy](https://catt.rs/en/stable/strategies.html#tagged-unions-strategy).
+  ([#318](https://github.com/python-attrs/cattrs/pull/318) [#317](https://github.com/python-attrs/cattrs/issues/317))
 - Introduce the `cattrs.transform_error` helper function for formatting validation exceptions. ([258](https://github.com/python-attrs/cattrs/issues/258) [342](https://github.com/python-attrs/cattrs/pull/342))
 - Add support for [`typing.TypedDict` and `typing_extensions.TypedDict`](https://peps.python.org/pep-0589/).
   ([#296](https://github.com/python-attrs/cattrs/issues/296) [#364](https://github.com/python-attrs/cattrs/pull/364))

src/cattrs/strategies/_unions.py

@@ -88,7 +88,8 @@ def unstructure_tagged_union(
         def structure_tagged_union(
             val: dict, _, _tag_to_cl=tag_to_hook, _tag_name=tag_name
         ) -> union:
-            return _tag_to_cl[val[_tag_name]](val)
+            val = val.copy()
+            return _tag_to_cl[val.pop(_tag_name)](val)
 
     else:
 
@@ -101,7 +102,8 @@ def structure_tagged_union(
             _default=default,
         ) -> union:
             if _tag_name in val:
-                return _tag_to_hook[val[_tag_name]](val)
+                val = val.copy()
+                return _tag_to_hook[val.pop(_tag_name)](val)
             return _dh(val, _default)
 
     converter.register_unstructure_hook(union, unstructure_tagged_union)

tests/strategies/test_tagged_unions.py

@@ -2,7 +2,7 @@
 
 from attrs import define
 
-from cattrs import BaseConverter
+from cattrs import BaseConverter, Converter
 from cattrs.strategies import configure_tagged_union
 
 
@@ -102,3 +102,39 @@ def test_default_member_validation(converter: BaseConverter) -> None:
 
     # A.a should be coerced to an int.
     assert converter.structure({"_type": "A", "a": "1"}, union) == A(1)
+
+
+def test_forbid_extra_keys():
+    """The strategy works when converters forbid extra keys."""
+
+    @define
+    class A:
+        pass
+
+    @define
+    class B:
+        pass
+
+    c = Converter(forbid_extra_keys=True)
+    configure_tagged_union(Union[A, B], c)
+
+    data = c.unstructure(A(), Union[A, B])
+    c.structure(data, Union[A, B])
+
+
+def test_forbid_extra_keys_default():
+    """The strategy works when converters forbid extra keys."""
+
+    @define
+    class A:
+        pass
+
+    @define
+    class B:
+        pass
+
+    c = Converter(forbid_extra_keys=True)
+    configure_tagged_union(Union[A, B], c, default=A)
+
+    data = c.unstructure(A(), Union[A, B])
+    c.structure(data, Union[A, B])