Add cattrs support for TypeVar with default (PEP696) (#512)

* Add support for TypeVar with default (PEP696)

* Add changelog entry

* Fix test

Author: jason-myers-klaviyo

HISTORY.md

@@ -33,6 +33,8 @@ can now be used as decorators and have gained new features.
   ([#426](https://github.com/python-attrs/cattrs/issues/426) [#477](https://github.com/python-attrs/cattrs/pull/477))
 - Add support for [PEP 695](https://peps.python.org/pep-0695/) type aliases.
   ([#452](https://github.com/python-attrs/cattrs/pull/452))
+- Add support for [PEP 696](https://peps.python.org/pep-0696/) `TypeVar`s with defaults.
+  ([#512](https://github.com/python-attrs/cattrs/pull/512))
 - Add support for named tuples with type metadata ([`typing.NamedTuple`](https://docs.python.org/3/library/typing.html#typing.NamedTuple)).
   ([#425](https://github.com/python-attrs/cattrs/issues/425) [#491](https://github.com/python-attrs/cattrs/pull/491))
 - The `include_subclasses` strategy now fetches the member hooks from the converter (making use of converter defaults) if overrides are not provided, instead of generating new hooks with no overrides.

src/cattrs/gen/_generics.py

@@ -33,9 +33,27 @@ def generate_mapping(cl: type, old_mapping: dict[str, type] = {}) -> dict[str, t
             if not hasattr(base, "__args__"):
                 continue
             base_args = base.__args__
-            if not hasattr(base.__origin__, "__parameters__"):
+            if hasattr(base.__origin__, "__parameters__"):
+                base_params = base.__origin__.__parameters__
+            elif any(
+                getattr(base_arg, "__default__", None) is not None
+                for base_arg in base_args
+            ):
+                # TypeVar with a default e.g. PEP 696
+                # https://www.python.org/dev/peps/pep-0696/
+                # Extract the defaults for the TypeVars and insert
+                # them into the mapping
+                mapping_params = [
+                    (base_arg, base_arg.__default__)
+                    for base_arg in base_args
+                    # Note: None means no default was provided, since
+                    # TypeVar("T", default=None) sets NoneType as the default
+                    if getattr(base_arg, "__default__", None) is not None
+                ]
+                base_params, base_args = zip(*mapping_params)
+            else:
                 continue
-            base_params = base.__origin__.__parameters__
+
             for param, arg in zip(base_params, base_args):
                 mapping[param.__name__] = arg
 

tests/test_generics_696.py

@@ -0,0 +1,50 @@
+"""Tests for generics under PEP 696 (type defaults)."""
+from typing import Generic
+
+import pytest
+from attrs import define, fields
+from typing_extensions import TypeVar
+
+from cattrs.errors import StructureHandlerNotFoundError
+from cattrs.gen import generate_mapping
+
+T = TypeVar("T")
+TD = TypeVar("TD", default=str)
+
+
+def test_structure_typevar_default(genconverter):
+    """Generics with defaulted TypeVars work."""
+
+    @define
+    class C(Generic[T]):
+        a: T
+
+    c_mapping = generate_mapping(C)
+    atype = fields(C).a.type
+    assert atype.__name__ not in c_mapping
+
+    with pytest.raises(StructureHandlerNotFoundError):
+        # Missing type for generic argument
+        genconverter.structure({"a": "1"}, C)
+
+    c_mapping = generate_mapping(C[str])
+    atype = fields(C[str]).a.type
+    assert c_mapping[atype.__name__] == str
+
+    assert genconverter.structure({"a": "1"}, C[str]) == C("1")
+
+    @define
+    class D(Generic[TD]):
+        a: TD
+
+    d_mapping = generate_mapping(D)
+    atype = fields(D).a.type
+    assert d_mapping[atype.__name__] == str
+
+    # Defaults to string
+    assert d_mapping[atype.__name__] == str
+    assert genconverter.structure({"a": "1"}, D) == D("1")
+
+    # But allows other types
+    assert genconverter.structure({"a": "1"}, D[str]) == D("1")
+    assert genconverter.structure({"a": 1}, D[int]) == D(1)