Add takes field

Author: hynek

docs/api.rst

@@ -93,21 +93,6 @@ Core
       >>> C([1, 2, 3])
       C(x=[1, 2, 3], y={1, 2, 3})
 
-.. autoclass:: Converter
-
-   For example:
-
-   .. doctest::
-
-      >>> def complicated(value, self_):
-      ...     return int(value) * self_.factor
-      >>> @define
-      ... class C:
-      ...     factor = 5  # not an *attrs* field
-      ...     x = field(converter=attrs.Converter(complicated, takes_self=True))
-      >>> C("42")
-      C(x=210)
-
 
 Exceptions
 ----------
@@ -622,6 +607,27 @@ Validators can be both globally and locally disabled:
 Converters
 ----------
 
+.. autoclass:: attrs.Converter
+
+   For example:
+
+   .. doctest::
+
+      >>> def complicated(value, self_, field):
+      ...     return int(value) * self_.factor + field.metadata["offset"]
+      >>> @define
+      ... class C:
+      ...     factor = 5  # not an *attrs* field
+      ...     x = field(
+      ...         metadata={"offset": 200},
+      ...         converter=attrs.Converter(
+      ...             complicated,
+      ...             takes_self=True, takes_field=True
+      ...     ))
+      >>> C("42")
+      C(x=410)
+
+
 .. module:: attrs.converters
 
 All objects from ``attrs.converters`` are also available from ``attr.converters`` (it's the same module in a different namespace).

src/attr/_make.py

@@ -2214,10 +2214,11 @@ def _setattr_with_converter(attr_name, value_var, has_on_setattr):
     Use the cached object.setattr to set *attr_name* to *value_var*, but run
     its converter first.
     """
-    return "_setattr('%s', %s(%s, self))" % (
+    return "_setattr('%s', %s(%s, self, attr_dict['%s']))" % (
         attr_name,
         _INIT_CONVERTER_PAT % (attr_name,),
         value_var,
+        attr_name,
     )
 
 
@@ -2240,10 +2241,11 @@ def _assign_with_converter(attr_name, value_var, has_on_setattr):
     if has_on_setattr:
         return _setattr_with_converter(attr_name, value_var, True)
 
-    return "self.%s = %s(%s, self)" % (
+    return "self.%s = %s(%s, self, attr_dict['%s'])" % (
         attr_name,
         _INIT_CONVERTER_PAT % (attr_name,),
         value_var,
+        attr_name,
     )
 
 
@@ -2273,10 +2275,11 @@ def fmt_setter_with_converter(attr_name, value_var, has_on_setattr):
                     attr_name, value_var, has_on_setattr
                 )
 
-            return "_inst_dict['%s'] = %s(%s, self)" % (
+            return "_inst_dict['%s'] = %s(%s, self, attr_dict['%s'])" % (
                 attr_name,
                 _INIT_CONVERTER_PAT % (attr_name,),
                 value_var,
+                attr_name,
             )
 
         return (
@@ -2351,7 +2354,7 @@ def _attrs_to_init_script(
         maybe_self = "self" if has_factory and a.default.takes_self else ""
 
         if a.converter and not isinstance(a.converter, Converter):
-            converter = Converter(a.converter, takes_self=False)
+            converter = Converter(a.converter)
         else:
             converter = a.converter
 
@@ -2989,44 +2992,61 @@ class Converter:
     """
     Stores a converter callable.
 
-    Allows for the wrapped converter to take additional arguments.
+    Allows for the wrapped converter to take additional arguments. The
+    arguments are passed in the order they are documented.
 
     :param Callable converter: A callable that converts a value.
     :param bool takes_self: Pass the partially initialized instance that is
-        being initialized as a positional argument. (default: `True`)
+        being initialized as a positional argument. (default: `False`)
+    :param bool takes_field: Pass the field definition (an `Attribute`) into
+        the converter as a positional argument. (default: `False`)
 
     .. versionadded:: 24.1.0
     """
 
-    __slots__ = ("converter", "takes_self", "_first_param_type", "__call__")
+    __slots__ = (
+        "converter",
+        "takes_self",
+        "takes_field",
+        "_first_param_type",
+        "__call__",
+    )
 
-    def __init__(self, converter, *, takes_self=True):
+    def __init__(self, converter, *, takes_self=False, takes_field=False):
         self.converter = converter
         self.takes_self = takes_self
-
-        ann = _AnnotationExtractor(converter)
-
-        self._first_param_type = ann.get_first_param_type()
+        self.takes_field = takes_field
 
         # Defining __call__ as a regular method leads to __annotations__ being
         # overwritten at a class level.
-        def __call__(value, inst):
-            if not self.takes_self:
-                return self.converter(value)
-
-            return self.converter(value, inst)
+        def __call__(value, inst, field):
+            return self.converter(
+                *{
+                    (False, False): (value,),
+                    (True, False): (value, inst),
+                    (False, True): (value, field),
+                    (True, True): (value, inst, field),
+                }[(takes_self, takes_field)]
+            )
 
+        ann = _AnnotationExtractor(converter)
         __call__.__annotations__.update(
             ann.get_annotations_for_converter_callable()
         )
         self.__call__ = __call__
 
+        self._first_param_type = ann.get_first_param_type()
+
     def __getstate__(self):
         """
         Return a dict containing only converter and takes_self -- the rest gets
         computed when loading.
         """
-        return {"converter": self.converter, "takes_self": self.takes_self}
+        return {
+            "converter": self.converter,
+            "takes_self": self.takes_self,
+            "takes_field": self.takes_field,
+        }
 
     def __setstate__(self, state):
         """
@@ -3048,7 +3068,7 @@ def __setstate__(self, state):
         init=True,
         inherited=False,
     )
-    for name in ("converter", "takes_self")
+    for name in ("converter", "takes_self", "takes_field")
 ]
 
 Converter = _add_hash(
@@ -3188,10 +3208,10 @@ def pipe(*converters):
     .. versionadded:: 20.1.0
     """
 
-    def pipe_converter(val, inst):
+    def pipe_converter(val, inst, field):
         for converter in converters:
             if isinstance(converter, Converter):
-                val = converter(val, inst)
+                val = converter(val, inst, field)
             else:
                 val = converter(val)
 
@@ -3212,4 +3232,4 @@ def pipe_converter(val, inst):
         if rt:
             pipe_converter.__annotations__["return"] = rt
 
-    return Converter(pipe_converter, takes_self=True)
+    return Converter(pipe_converter, takes_self=True, takes_field=True)

tests/test_converters.py

@@ -116,9 +116,9 @@ def test_success(self):
         """
         Succeeds if all wrapped converters succeed.
         """
-        c = pipe(str, Converter(to_bool, takes_self=False), bool)
+        c = pipe(str, Converter(to_bool), bool)
 
-        assert True is c("True", None) is c(True, None)
+        assert True is c("True", None, None) is c(True, None, None)
 
     def test_fail(self):
         """
@@ -128,11 +128,11 @@ def test_fail(self):
 
         # First wrapped converter fails:
         with pytest.raises(ValueError):
-            c(33, None)
+            c(33, None, None)
 
         # Last wrapped converter fails:
         with pytest.raises(ValueError):
-            c("33", None)
+            c("33", None, None)
 
     def test_sugar(self):
         """
@@ -153,7 +153,7 @@ def test_empty(self):
         """
         o = object()
 
-        assert o is pipe()(o, None)
+        assert o is pipe()(o, None, None)
 
 
 class TestToBool:

tests/test_make.py

@@ -1284,7 +1284,7 @@ class TestConverter:
     Tests for attribute conversion.
     """
 
-    def test_convert(self):
+    def test_converter(self):
         """
         Return value of converter is used as the attribute's value.
         """
@@ -1315,6 +1315,27 @@ class C:
 
         assert 84 == C(2).x
 
+    def test_converter_wrapped_takes_field(self):
+        """
+        When wrapped and passed `takes_field`, the converter receives the field
+        definition -- and the return value is used as the field's value.
+        """
+
+        def converter_with_field(v, field):
+            assert isinstance(field, attr.Attribute)
+            return v * field.metadata["x"]
+
+        @attr.define
+        class C:
+            x: int = attr.field(
+                converter=attr.Converter(
+                    converter_with_field, takes_field=True
+                ),
+                metadata={"x": 42},
+            )
+
+        assert 84 == C(2).x
+
     @given(integers(), booleans())
     def test_convert_property(self, val, init):
         """