Strip leftover attr.ib() sentinels when these= is passed

changelog.d/621.change.md

@@ -0,0 +1 @@
+Passing `these=` no longer leaves leftover `attr.ib()` sentinels on the class.

docs/examples.md

@@ -112,6 +112,9 @@ This is useful in times when you want to enhance classes that are not yours (nic
 SomethingFromSomeoneElse(x=1)
 ```
 
+If *these* is passed, *attrs* does not collect `attr.ib()` / `field()` definitions from the class body.
+Leftover sentinels are stripped so they cannot leak as public attributes.
+
 [Subclassing is bad for you](https://www.youtube.com/watch?v=3MNVP9-hglc) (except when doing [strict specialization](https://hynek.me/articles/python-subclassing-redux/)), but *attrs* will still do what you'd hope for:
 
 ```{doctest}

src/attr/_make.py

@@ -839,6 +839,13 @@ def _patch_original_class(self):
                     # same name by using only a type annotation.
                     with contextlib.suppress(AttributeError):
                         delattr(cls, name)
+        else:
+            # `these=` does not collect class-body `attr.ib()`s as fields, but
+            # the sentinels would otherwise leak as public attributes (#621).
+            for name, value in list(vars(cls).items()):
+                if isinstance(value, _CountingAttr):
+                    with contextlib.suppress(AttributeError):
+                        delattr(cls, name)
 
         # Attach our dunder methods.
         for name, value in self._cls_dict.items():

tests/test_make.py

@@ -384,6 +384,24 @@ class C:
 
         assert "C(a=1, b=2)" == repr(C())
 
+    def test_these_strips_leftover_counting_attrs(self):
+        """
+        `these=` does not collect class-body attrs as fields, but leftover
+        `attr.ib()` sentinels must not leak as public attributes.
+
+        Regression test for #621.
+        """
+
+        @attr.s(these={"a": attr.ib()})
+        class C:
+            b = attr.ib(default=0)
+
+        inst = C(5)
+
+        assert inst.a == 5
+        assert "b" not in vars(C)
+        assert not isinstance(getattr(inst, "b", None), _CountingAttr)
+
     def test_multiple_inheritance_old(self):
         """
         Old multiple inheritance attribute collection behavior is retained.