Fix converters on frozen classes

Fixes #76

This should be reasonably fast @Tinche?

CHANGELOG.rst

@@ -11,7 +11,8 @@ The third digit is only for regressions.
 Changes:
 ^^^^^^^^
 
-*none*
+- Converts now work with frozen classes.
+  `#76 <https://github.com/hynek/attrs/issues/76>`_
 
 
 ----

src/attr/_make.py

@@ -470,17 +470,20 @@ def validate(inst):
             a.validator(inst, a, getattr(inst, a.name))
 
 
-def _convert(inst):
+def _convert(inst, setattr_):
     """
     Convert all attributes on *inst* that have a converter.
 
+    Uses *setattr_* to set the attributes on the class.  Allows for
+    circumvention of frozen instances.
+
     Leaves all exceptions through.
 
     :param inst: Instance of a class with ``attrs`` attributes.
     """
     for a in inst.__class__.__attrs_attrs__:
         if a.convert is not None:
-            setattr(inst, a.name, a.convert(getattr(inst, a.name)))
+            setattr_(a.name, a.convert(getattr(inst, a.name)))
 
 
 def _attrs_to_script(attrs, frozen):
@@ -564,7 +567,10 @@ def fmt_setter(attr_name, value):
             lines.append(fmt_setter(attr_name, arg_name))
 
     if has_convert:
-        lines.append("_convert(self)")
+        if frozen is True:
+            lines.append("_convert(self, _setattr)")
+        else:
+            lines.append("_convert(self, self.__setattr__)")
     if attrs_to_validate:  # we can skip this if there are no validators.
         validators_for_globals["_config"] = _config
         lines.append("if _config._run_validators is False:")

tests/test_make.py

@@ -402,6 +402,13 @@ def validator(inst, attr, val):
         with pytest.raises(ZeroDivisionError):
             C(1, 2)
 
+    def test_frozen(self):
+        """
+        Converters circumvent immutability.
+        """
+        C = make_class("C", {"x": attr(convert=lambda v: int(v))}, frozen=True)
+        C("1")
+
 
 class TestValidate(object):
     """