Convert transformed attrs to AttrsClass (#824)

* Convert transformed attrs to AttrsClass

Fixes: #821

* Add cangelog entry

* Only call AttrsClass once

* Calm mypy by inline the AttrsClass call

* Defer AttrsClass creation as long as possible

Co-authored-by: Hynek Schlawack <hs@ox.cx>

changelog.d/824.changes.rst

@@ -0,0 +1 @@
+Attributes transformed via ``field_transformer`` are wrapped with ``AttrsClass`` again.

src/attr/_make.py

@@ -581,15 +581,11 @@ def _transform_attrs(
             cls, {a.name for a in own_attrs}
         )
 
-    attr_names = [a.name for a in base_attrs + own_attrs]
-
-    AttrsClass = _make_attr_tuple_class(cls.__name__, attr_names)
-
     if kw_only:
         own_attrs = [a.evolve(kw_only=True) for a in own_attrs]
         base_attrs = [a.evolve(kw_only=True) for a in base_attrs]
 
-    attrs = AttrsClass(base_attrs + own_attrs)
+    attrs = base_attrs + own_attrs
 
     # Mandatory vs non-mandatory attr order only matters when they are part of
     # the __init__ signature and when they aren't kw_only (which are moved to
@@ -608,7 +604,13 @@ def _transform_attrs(
 
     if field_transformer is not None:
         attrs = field_transformer(cls, attrs)
-    return _Attributes((attrs, base_attrs, base_attr_map))
+
+    # Create AttrsClass *after* applying the field_transformer since it may
+    # add or remove attributes!
+    attr_names = [a.name for a in attrs]
+    AttrsClass = _make_attr_tuple_class(cls.__name__, attr_names)
+
+    return _Attributes((AttrsClass(attrs), base_attrs, base_attr_map))
 
 
 if PYPY:

tests/test_hooks.py

@@ -117,6 +117,22 @@ class Sub(Base):
 
         assert attr.asdict(Sub(2)) == {"y": 2}
 
+    def test_attrs_attrclass(self):
+        """
+        The list of attrs returned by a field_transformer is converted to
+        "AttrsClass" again.
+
+        Regression test for #821.
+        """
+
+        @attr.s(auto_attribs=True, field_transformer=lambda c, a: list(a))
+        class C:
+            x: int
+
+        fields_type = type(attr.fields(C))
+        assert fields_type.__name__ == "CAttributes"
+        assert issubclass(fields_type, tuple)
+
 
 class TestAsDictHook:
     def test_asdict(self):