Implement @converter decorator

.gitignore

@@ -3,6 +3,7 @@
 .cache
 .coverage*
 .hypothesis
+.idea/
 .mypy_cache
 .pytest_cache
 .tox
@@ -11,3 +12,5 @@ dist
 docs/_build/
 htmlcov
 pip-wheel-metadata
+Pipfile
+Pipfile.lock

src/attr/_make.py

@@ -2384,6 +2384,7 @@ def from_counting_attr(cls, name, ca, type=None):
             not in (
                 "name",
                 "validator",
+                "converter",
                 "default",
                 "type",
                 "inherited",
@@ -2392,6 +2393,7 @@ def from_counting_attr(cls, name, ca, type=None):
         return cls(
             name=name,
             validator=ca._validator,
+            converter=ca._converter,
             default=ca._default,
             type=type,
             cmp=None,
@@ -2500,7 +2502,7 @@ class _CountingAttr(object):
         "init",
         "metadata",
         "_validator",
-        "converter",
+        "_converter",
         "type",
         "kw_only",
         "on_setattr",
@@ -2568,7 +2570,7 @@ def __init__(
         self.counter = _CountingAttr.cls_counter
         self._default = default
         self._validator = validator
-        self.converter = converter
+        self._converter = converter
         self.repr = repr
         self.eq = eq
         self.order = order
@@ -2593,6 +2595,20 @@ def validator(self, meth):
             self._validator = and_(self._validator, meth)
         return meth
 
+    def converter(self, meth):
+        """
+        Decorator that adds *meth* to the list of converters.
+
+        Returns *meth* unchanged.
+
+        .. versionadded:: # TODO: TBD
+        """
+        if self._converter is None:
+            self._converter = meth
+        else:
+            self._converter = pipe(self._converter, meth)
+        return meth
+
     def default(self, meth):
         """
         Decorator that allows to set the default for an attribute.

tests/test_make.py

@@ -120,6 +120,36 @@ def v2(self, _, __):
 
         assert _AndValidator((v, v2)) == a._validator
 
+    def test_converter_decorator_single(self):
+        """
+        If _CountingAttr.converter is used as a decorator and there is no
+        decorator set, the decorated method is used as the converter.
+        """
+        a = attr.ib()
+
+        @a.converter
+        def c():
+            pass
+
+        assert c == a._converter
+
+    def test_converter_decorator(self):
+        """
+        If _CountingAttr.converter is used as a decorator and there is already
+        a decorator set, the decorators are composed using `pipe`.
+        """
+
+        def c(val):
+            return ('c', val)
+
+        a = attr.ib(converter=c)
+
+        @a.converter
+        def c2(val):
+            return ('c2', val)
+
+        assert ('c2', ('c', 'foo')) == a._converter('foo')
+
     def test_default_decorator_already_set(self):
         """
         Raise DefaultAlreadySetError if the decorator is used after a default