Use an explicit comparison to None for the converter of a field (#1374)

Make attrs work with a converter instance that does not evaluate True

Author: filbranden

src/attr/_make.py

@@ -2021,7 +2021,7 @@ def _attrs_to_init_script(
         has_factory = isinstance(a.default, Factory)
         maybe_self = "self" if has_factory and a.default.takes_self else ""
 
-        if a.converter and not isinstance(a.converter, Converter):
+        if a.converter is not None and not isinstance(a.converter, Converter):
             converter = Converter(a.converter)
         else:
             converter = a.converter

tests/test_converters.py

@@ -112,6 +112,25 @@ def wrapped(_, __, ___) -> float:
         assert float is c.__call__.__annotations__["return"]
         assert None is c2.__call__.__annotations__.get("return")
 
+    def test_falsey_converter(self):
+        """
+        Passing a false-y instance still produces a valid converter.
+        """
+
+        class MyConv:
+            def __bool__(self):
+                return False
+
+            def __call__(self, value):
+                return value * 2
+
+        @attr.s
+        class C:
+            a = attrib(converter=MyConv())
+
+        c = C(21)
+        assert 42 == c.a
+
 
 class TestOptional:
     """