NG: convert on setattr by default

Not doing that from the get-go was an oversight.

Fixes #835

changelog.d/835.breaking.rst

@@ -0,0 +1,4 @@
+When using ``@attr.define``, converters are now run by default when setting an attribute on an instance -- additionally to validators.
+I.e. the new default is ``on_setattr=[attr.setters.convert, attr.setters.validate]``.
+
+This is unfortunately a breaking change, but it was an oversight, impossible to raise a ``DeprecationWarning`` about, and it's better to fix it now while the APIs are very fresh with few users.

docs/api.rst

@@ -707,13 +707,15 @@ The most notable differences are:
 - *auto_exc=True*
 - *auto_detect=True*
 - *eq=True*, but *order=False*
-- Validators run when you set an attribute (*on_setattr=attr.setters.validate*).
+- Converters and validators are run when you set an attribute (*on_setattr=[attr.setters.convert, attr.setters.validate*]).
 - Some options that aren't relevant to Python 3 have been dropped.
 
 Please note that these are *defaults* and you're free to override them, just like before.
 
 Since the Python ecosystem has settled on the term ``field`` for defining attributes, we have also added `attr.field` as a substitute for `attr.ib`.
 
+.. versionchanged:: 21.3.0 Converters are also run ``on_setattr``.
+
 .. note::
 
    `attr.s` and `attr.ib` (and their serious business cousins) aren't going anywhere.

src/attr/_next_gen.py

@@ -35,8 +35,10 @@ def define(
     match_args=True,
 ):
     r"""
-    The only behavioral differences are the handling of the *auto_attribs*
-    option:
+    Define an ``attrs`` class.
+
+    The behavioral differences to `attr.s` are the handling of the
+    *auto_attribs* option:
 
     :param Optional[bool] auto_attribs: If set to `True` or `False`, it behaves
        exactly like `attr.s`. If left `None`, `attr.s` will try to guess:
@@ -46,9 +48,11 @@ def define(
        2. Otherwise it assumes *auto_attribs=False* and tries to collect
           `attr.ib`\ s.
 
-    and that mutable classes (``frozen=False``) validate on ``__setattr__``.
+    and that mutable classes (``frozen=False``) convert and validate on
+    ``__setattr__``.
 
     .. versionadded:: 20.1.0
+    .. versionchanged:: 21.3.0 Converters are also run ``on_setattr``.
     """
 
     def do_it(cls, auto_attribs):
@@ -86,9 +90,9 @@ def wrap(cls):
 
         had_on_setattr = on_setattr not in (None, setters.NO_OP)
 
-        # By default, mutable classes validate on setattr.
+        # By default, mutable classes convert & validate on setattr.
         if frozen is False and on_setattr is None:
-            on_setattr = setters.validate
+            on_setattr = [setters.convert, setters.validate]
 
         # However, if we subclass a frozen class, we inherit the immutability
         # and disable on_setattr.

tests/test_next_gen.py

@@ -308,3 +308,28 @@ class MyException(Exception):
 
         assert "foo" == ei.value.x
         assert ei.value.__cause__ is None
+
+    def test_converts_and_validates_by_default(self):
+        """
+        If no on_setattr is set, assume setters.convert, setters.validate.
+        """
+
+        @attr.define
+        class C:
+            x: int = attr.field(converter=int)
+
+            @x.validator
+            def _v(self, _, value):
+                if value < 10:
+                    raise ValueError("must be >=10")
+
+        inst = C(10)
+
+        # Converts
+        inst.x = "11"
+
+        assert 11 == inst.x
+
+        # Validates
+        with pytest.raises(ValueError, match="must be >=10"):
+            inst.x = "9"