Fix pyyaml converter crashing on non-enum str subclasses

HISTORY.md

@@ -13,6 +13,8 @@ Our backwards-compatibility policy can be found [here](https://github.com/python
 
 ## NEXT
 
+- The PyYAML preconf converter no longer fails with an `AttributeError` when unstructuring `str` subclasses that aren't enums (such as `markupsafe.Markup`); they are now unstructured into plain strings.
+  ([#804](https://github.com/python-attrs/cattrs/pull/804))
 - Dict unstructuring hook factories now inherit the converter's `omit_if_default` setting when `_cattrs_omit_if_default` is omitted. Explicit `True` or `False` values and per-attribute overrides retain their precedence.
   ([#238](https://github.com/python-attrs/cattrs/issues/238))
 - Support the [`frozendict`](https://docs.python.org/3.15/builtins/stdtypes.html#frozendict) built-in on Python 3.15+.

src/cattrs/preconf/pyyaml.py

@@ -1,6 +1,7 @@
 """Preconfigured converters for pyyaml."""
 
 from datetime import date, datetime
+from enum import Enum
 from functools import partial
 from typing import Any, TypeVar, Union
 
@@ -24,6 +25,15 @@ def validate_date(v: Any, _):
     return v
 
 
+def _unstructure_str(v: str) -> Any:
+    # `safe_dump` rejects str subclasses, so they have to become plain strings.
+    if v.__class__ is str:
+        return v
+    if isinstance(v, Enum):
+        return v.value
+    return str(v)
+
+
 class PyyamlConverter(Converter):
     def dumps(self, obj: Any, unstructure_as: Any = None, **kwargs: Any) -> str:
         return safe_dump(self.unstructure(obj, unstructure_as=unstructure_as), **kwargs)
@@ -43,16 +53,16 @@ def configure_converter(converter: BaseConverter) -> None:
     Configure the converter for use with the pyyaml library.
 
     * frozensets are serialized as lists
-    * string enums are converted into strings explicitly
+    * string enums and other str subclasses are converted into strings explicitly
     * datetimes and dates are validated
     * typed namedtuples are serialized as lists
 
     .. versionchanged:: 24.1.0
         Add support for typed namedtuples.
+    .. versionchanged:: NEXT
+        Non-enum str subclasses are unstructured into plain strings.
     """
-    converter.register_unstructure_hook(
-        str, lambda v: v if v.__class__ is str else v.value
-    )
+    converter.register_unstructure_hook(str, _unstructure_str)
 
     # datetime inherits from date, so identity unstructure hook used
     # here to prevent the date unstructure hook running.

tests/preconf/test_pyyaml.py

@@ -82,3 +82,21 @@ class A:
         assert (
             repr(exc_info.value.exceptions[1]) == "ValueError('Expected date, got 1')"
         )
+
+
+def test_pyyaml_str_subclass():
+    """Non-enum str subclasses are dumped as plain strings."""
+
+    class MyStr(str):
+        pass
+
+    @define
+    class A:
+        a: str
+
+    converter = make_converter()
+
+    unstructured = converter.unstructure(A(MyStr("x")))
+    assert unstructured == {"a": "x"}
+    assert unstructured["a"].__class__ is str
+    assert converter.loads(converter.dumps(A(MyStr("x"))), A) == A("x")