Implement NoneType structuring.

This is useful to have for custom structure hooks of union types
that are also optional.

HISTORY.md

@@ -26,6 +26,8 @@
   ([#398](https://github.com/python-attrs/cattrs/issues/398) [#399](https://github.com/python-attrs/cattrs/pull/399))
 - Broaden loads' type definition for the preconf orjson converter.
   ([#400](https://github.com/python-attrs/cattrs/pull/400))
+- Implement structuring for `NoneType`.
+  ([#406](https://github.com/python-attrs/cattrs/pull/408))
 
 ## 23.1.2 (2023-06-02)
 

docs/structuring.md

@@ -90,6 +90,20 @@ In case the conversion isn't possible, the resulting exception is propagated out
 
 ```
 
+### `NoneType`
+
+[`NoneType`](https://docs.python.org/3/library/types.html#types.NoneType) is
+the type of `None`. Prior to Python 3.10, `NoneType` is just `type(None)`.
+
+```python
+>>> cattrs.structure(None, NoneType)
+>>> cattrs.structure(1, NoneType)
+Traceback (most recent call last):
+...
+ValueError: 1 is not None
+```
+
+
 ## Collections and Other Generics
 
 ### Optionals

src/cattrs/converters.py

@@ -222,6 +222,7 @@ def __init__(
                 (float, self._structure_call),
                 (Enum, self._structure_call),
                 (Path, self._structure_call),
+                (NoneType, self._structure_nonetype),
             ]
         )
 
@@ -445,6 +446,12 @@ def _structure_call(obj: Any, cl: Type[T]) -> Any:
         """
         return cl(obj)
 
+    @staticmethod
+    def _structure_nonetype(val, cl):
+        if val is not None:
+            raise ValueError(f"{val} is not None")
+        return val
+
     @staticmethod
     def _structure_simple_literal(val, type):
         if val not in type.__args__:

tests/test_structure.py

@@ -387,3 +387,10 @@ class B:
         A("foo"),
         B("bar"),
     ]
+
+
+def test_structuring_nonetype():
+    converter = BaseConverter()
+
+    assert converter.structure(None, NoneType) is None
+    converter.structure(1, NoneType)