Newtypes fix (#381)

* Newtypes fix

* Rework test

Author: Tinche

HISTORY.md

@@ -6,6 +6,8 @@
 - _cattrs_ is now linted with [Ruff](https://beta.ruff.rs/docs/).
 - Fix TypedDicts with periods in their field names.
   ([#376](https://github.com/python-attrs/cattrs/issues/376) [#377](https://github.com/python-attrs/cattrs/pull/377))
+- Optimize and improve unstructuring of `Optional` (unions of one type and `None`).
+  ([#380](https://github.com/python-attrs/cattrs/issues/380) [#381](https://github.com/python-attrs/cattrs/pull/381))
 - Fix `format_exception` and `transform_error` type annotations.
 
 ## 23.1.2 (2023-06-02)

src/cattrs/converters.py

@@ -881,13 +881,17 @@ def __init__(
             is_frozenset,
             lambda cl: self.gen_unstructure_iterable(cl, unstructure_to=frozenset),
         )
+        self.register_unstructure_hook_factory(
+            is_optional, self.gen_unstructure_optional
+        )
         self.register_unstructure_hook_factory(
             is_typeddict, self.gen_unstructure_typeddict
         )
         self.register_unstructure_hook_factory(
             lambda t: get_newtype_base(t) is not None,
             lambda t: self._unstructure_func.dispatch(get_newtype_base(t)),
         )
+
         self.register_structure_hook_factory(is_annotated, self.gen_structure_annotated)
         self.register_structure_hook_factory(is_mapping, self.gen_structure_mapping)
         self.register_structure_hook_factory(is_counter, self.gen_structure_counter)
@@ -938,6 +942,17 @@ def gen_unstructure_attrs_fromdict(
             cl, self, _cattrs_omit_if_default=self.omit_if_default, **attrib_overrides
         )
 
+    def gen_unstructure_optional(self, cl: Type[T]) -> Callable[[T], Any]:
+        """Generate an unstructuring hook for optional types."""
+        union_params = cl.__args__
+        other = union_params[0] if union_params[1] is NoneType else union_params[1]
+        handler = self._unstructure_func.dispatch(other)
+
+        def unstructure_optional(val, _handler=handler):
+            return None if val is None else _handler(val)
+
+        return unstructure_optional
+
     def gen_structure_typeddict(self, cl: Any) -> Callable[[Dict], Dict]:
         """Generate a TypedDict structure function.
 

tests/test_newtypes.py

@@ -3,13 +3,13 @@
 
 import pytest
 
-from cattrs import BaseConverter
+from cattrs import Converter
 
 PositiveIntNewType = NewType("PositiveIntNewType", int)
 BigPositiveIntNewType = NewType("BigPositiveIntNewType", PositiveIntNewType)
 
 
-def test_newtype_structure_hooks(genconverter: BaseConverter):
+def test_newtype_structure_hooks(genconverter: Converter):
     """NewTypes should work with `register_structure_hook`."""
 
     assert genconverter.structure("0", int) == 0
@@ -39,7 +39,7 @@ def test_newtype_structure_hooks(genconverter: BaseConverter):
     assert genconverter.structure("51", BigPositiveIntNewType) == 51
 
 
-def test_newtype_unstructure_hooks(genconverter: BaseConverter):
+def test_newtype_unstructure_hooks(genconverter: Converter):
     """NewTypes should work with `register_unstructure_hook`."""
 
     assert genconverter.unstructure(0, int) == 0

tests/test_optionals.py

@@ -0,0 +1,41 @@
+from typing import NewType, Optional
+
+import pytest
+from attrs import define
+
+from cattrs._compat import is_py310_plus
+
+
+def test_newtype_optionals(genconverter):
+    """Newtype optionals should work."""
+    Foo = NewType("Foo", str)
+
+    genconverter.register_unstructure_hook(Foo, lambda v: v.replace("foo", "bar"))
+
+    @define
+    class ModelWithFoo:
+        total_foo: Foo
+        maybe_foo: Optional[Foo]
+
+    assert genconverter.unstructure(ModelWithFoo(Foo("foo"), Foo("is it a foo?"))) == {
+        "total_foo": "bar",
+        "maybe_foo": "is it a bar?",
+    }
+
+
+@pytest.mark.skipif(not is_py310_plus, reason="3.10+ union syntax")
+def test_newtype_modern_optionals(genconverter):
+    """Newtype optionals should work."""
+    Foo = NewType("Foo", str)
+
+    genconverter.register_unstructure_hook(Foo, lambda v: v.replace("foo", "bar"))
+
+    @define
+    class ModelWithFoo:
+        total_foo: Foo
+        maybe_foo: Foo | None
+
+    assert genconverter.unstructure(ModelWithFoo(Foo("foo"), Foo("is it a foo?"))) == {
+        "total_foo": "bar",
+        "maybe_foo": "is it a bar?",
+    }