fix: support string annotations in attrs.converters.optional

changelog.d/1629.change.md

@@ -0,0 +1 @@
+`attrs.converters.optional()` no longer raises a `TypeError` if the wrapped converter has string annotations, as created by quoted forward references or by using `from __future__ import annotations`.

src/attr/converters.py

@@ -4,6 +4,8 @@
 Commonly useful converters.
 """
 
+from typing import Optional
+
 from ._compat import _AnnotationExtractor
 from ._make import NOTHING, Converter, Factory, pipe
 
@@ -49,11 +51,13 @@ def optional_converter(val):
 
     t = xtr.get_first_param_type()
     if t:
-        optional_converter.__annotations__["val"] = t | None
+        # t can be a string -- because of a quoted forward reference or
+        # `from __future__ import annotations` -- and `str | None` blows up.
+        optional_converter.__annotations__["val"] = Optional[t]  # noqa: UP045
 
     rt = xtr.get_return_type()
     if rt:
-        optional_converter.__annotations__["return"] = rt | None
+        optional_converter.__annotations__["return"] = Optional[rt]  # noqa: UP045
 
     if isinstance(converter, Converter):
         return Converter(optional_converter, takes_self=True, takes_field=True)

tests/test_converters.py

@@ -5,6 +5,7 @@
 """
 
 import pickle
+import typing
 
 import pytest
 
@@ -170,6 +171,25 @@ def test_converter_instance(self):
 
         assert True is c("yes", None, None)
 
+    def test_string_annotations(self):
+        """
+        Converters whose annotations are strings -- because of quoted forward
+        references or because of `from __future__ import annotations` -- are
+        wrapped in `Optional` instead of blowing up.
+        """
+
+        def to_int(x: "int") -> "int":
+            return int(x)
+
+        c = optional(to_int)
+
+        assert 42 == c("42")
+        assert {
+            # Optional[] is the point: `t | None` crashes on strings.
+            "val": typing.Optional[typing.ForwardRef("int")],  # noqa: UP045
+            "return": typing.Optional[typing.ForwardRef("int")],  # noqa: UP045
+        } == c.__annotations__
+
 
 class TestDefaultIfNone:
     def test_missing_default(self):