Make Converter a kind of adapter, fix converters.pipe

src/attr/_make.py

@@ -2712,7 +2712,6 @@ class Converter:
         "takes_field",
         "_first_param_type",
         "_global_name",
-        "__call__",
     )
 
     def __init__(self, converter, *, takes_self=False, takes_field=False):
@@ -2724,6 +2723,21 @@ def __init__(self, converter, *, takes_self=False, takes_field=False):
             converter
         ).get_first_param_type()
 
+    def __call__(self, value: object, instance: object, field: Attribute):
+        """
+        Call the converter with the appropriate subset of *instance* and
+        *field*.
+        """
+        return self.converter(
+            value,
+            *{
+                (False, False): (),
+                (True, False): (instance,),
+                (False, True): (field,),
+                (True, True): (instance, field),
+            }[self.takes_self, self.takes_field],
+        )
+
     @staticmethod
     def _get_global_name(attr_name: str) -> str:
         """
@@ -2943,18 +2957,7 @@ def pipe(*converters):
 
     def pipe_converter(val, inst, field):
         for c in converters:
-            if isinstance(c, Converter):
-                val = c.converter(
-                    val,
-                    *{
-                        (False, False): (),
-                        (True, False): (c.takes_self,),
-                        (False, True): (c.takes_field,),
-                        (True, True): (c.takes_self, c.takes_field),
-                    }[c.takes_self, c.takes_field],
-                )
-            else:
-                val = c(val)
+            val = c(val, inst, field) if isinstance(c, Converter) else c(val)
 
         return val
 

src/attr/setters.py

@@ -4,7 +4,6 @@
 Commonly used hooks for on_setattr.
 """
 
-
 from . import _config
 from .exceptions import FrozenAttributeError
 
@@ -63,7 +62,13 @@ def convert(instance, attrib, new_value):
     """
     c = attrib.converter
     if c:
-        return c(new_value)
+        # This can be removed once we drop 3.8 and use attrs.Converter instead.
+        from ._make import Converter
+
+        if not isinstance(c, Converter):
+            return c(new_value)
+
+        return c(new_value, instance, attrib)
 
     return new_value
 

tests/test_setattr.py

@@ -109,16 +109,34 @@ def test_pipe(self):
         used. They can be supplied using the pipe functions or by passing a
         list to on_setattr.
         """
+        taken = None
+
+        def takes_all(val, instance, attrib):
+            nonlocal taken
+            taken = val, instance, attrib
+
+            return val
 
         s = [setters.convert, lambda _, __, nv: nv + 1]
 
         @attr.s
         class Piped:
-            x1 = attr.ib(converter=int, on_setattr=setters.pipe(*s))
+            x1 = attr.ib(
+                converter=[
+                    attr.Converter(
+                        takes_all, takes_field=True, takes_self=True
+                    ),
+                    int,
+                ],
+                on_setattr=setters.pipe(*s),
+            )
             x2 = attr.ib(converter=int, on_setattr=s)
 
         p = Piped("41", "22")
 
+        assert ("41", p) == taken[:-1]
+        assert "x1" == taken[-1].name
+
         assert 41 == p.x1
         assert 22 == p.x2
 
@@ -417,3 +435,19 @@ def test_docstring(self):
             "Method generated by attrs for class WithOnSetAttrHook."
             == WithOnSetAttrHook.__setattr__.__doc__
         )
+
+    def test_setattr_converter_piped(self):
+        """
+        If a converter is used, it is piped through the on_setattr hooks.
+
+        Regression test for https://github.com/python-attrs/attrs/issues/1327
+        """
+
+        @attr.define  # converter on setattr is implied in NG
+        class C:
+            x = attr.field(converter=[int])
+
+        c = C("1")
+        c.x = "2"
+
+        assert 2 == c.x