Support attrs validators

This adds support for attrs validators. Validators are run after
decoding the instance has completed, but before calling
`__attrs_post_init__`. Since the builtin attrs validators include the
attribute name in the error message, we don't add the field to the error
path. This feels *a bit* weird, but is way easier since all of this can
be done in pure python.

When possible, it'd be better to use msgspec's builtin constraints for
handling validation. These will be much more performant, and integrate
better with the rest of the system.

Author: jcrist

docs/source/supported-types.rst

@@ -860,7 +860,13 @@ During decoding, any extra fields are ignored. An error is raised if a field's
 type doesn't match or if any required fields are missing.
 
 If the ``__attrs_pre_init__`` or ``__attrs_post_init__`` methods are defined on
-the class, they are called as part of the decoding process.
+the class, they are called as part of the decoding process. Likewise, if a
+class makes use of attrs' `validators
+<https://www.attrs.org/en/stable/examples.html#validators>`__, the validators
+will be called, and a `msgspec.ValidationError` raised on error. Note that
+attrs' `converters
+<https://www.attrs.org/en/stable/examples.html#conversion>`__ are not currently
+supported.
 
 When possible we recommend using `msgspec.Struct` instead of attrs_ types for
 specifying schemas - :doc:`structs` are faster, more ergonomic, and support

msgspec/_utils.py

@@ -239,6 +239,8 @@ def get_dataclass_info(obj):
     else:
         from attrs import NOTHING, Factory
 
+        fields_with_validators = []
+
         for field in cls.__attrs_attrs__:
             name = field.name
             typ = hints[name]
@@ -248,8 +250,7 @@ def get_dataclass_info(obj):
                     if default.takes_self:
                         raise NotImplementedError(
                             "Support for default factories with `takes_self=True` "
-                            "is not implemented. "
-                            "File a GitHub issue if you need "
+                            "is not implemented. File a GitHub issue if you need "
                             "this feature!"
                         )
                     defaults.append(default.factory)
@@ -260,14 +261,30 @@ def get_dataclass_info(obj):
             else:
                 required.append((name, typ, False))
 
+            if field.validator is not None:
+                fields_with_validators.append(field)
+
         required.extend(optional)
 
         pre_init = getattr(cls, "__attrs_pre_init__", None)
         post_init = getattr(cls, "__attrs_post_init__", None)
 
+        if fields_with_validators:
+            post_init = _wrap_attrs_validators(fields_with_validators, post_init)
+
     return cls, tuple(required), tuple(defaults), pre_init, post_init
 
 
+def _wrap_attrs_validators(fields, post_init):
+    def inner(obj):
+        for field in fields:
+            field.validator(obj, field, getattr(obj, field.name))
+        if post_init is not None:
+            post_init(obj)
+
+    return inner
+
+
 def rebuild(cls, kwargs):
     """Used to unpickle Structs with keyword-only fields"""
     return cls(**kwargs)

tests/test_common.py

@@ -3005,6 +3005,24 @@ def __attrs_pre_init__(self):
         with pytest.raises(ValueError, match="Oh no!"):
             proto.decode(proto.encode({"a": 1}), type=Example)
 
+    def test_decode_attrs_validators(self, proto):
+        def not2(self, attr, value):
+            if value == 2:
+                raise ValueError("Oh no!")
+
+        @attrs.define
+        class Example:
+            a: int = attrs.field(validator=[attrs.validators.gt(0), not2])
+
+        res = proto.decode(proto.encode({"a": 1}), type=Example)
+        assert res.a == 1
+
+        with pytest.raises(ValidationError):
+            res = proto.decode(proto.encode({"a": -1}), type=Example)
+
+        with pytest.raises(ValidationError, match="Oh no!"):
+            res = proto.decode(proto.encode({"a": 2}), type=Example)
+
     def test_decode_attrs_not_object(self, proto):
         @attrs.define
         class Example: