Fix issue 589 (#590)

* from parsyfiles development (https://github.com/smarie/python-simple-file-collection-parsing-framework):
* added new 'chain' validator to allow users to chain their custom validators with, for example, 'instance_of'
* added utility methods to get the declared attribute type (by inspecting its validators) and to check if it is optional

* from parsyfiles development (https://github.com/smarie/python-simple-file-collection-parsing-framework):
* added tests
* fixed bug with optional validator to pass tests :)

* from parsyfiles development (https://github.com/smarie/python-simple-file-collection-parsing-framework):
* completed doc api.rst
* small mod in tests

* Fixed issue with empty cell in closure when slots are used. Fixed #589

* Removed api.rst mods erroneously injected from another branch.

* Removed api.rst mods erroneously injected from another branch.

* noqa so that flake8 is happy

* Fixed lint error

* Added towncrier changelog entry

* Attempt to fix linter error

* Update tests/test_slots.py

* Last lint fix

* Update changelog.d/590.change.rst

Co-Authored-By: Hynek Schlawack <hs@ox.cx>

* Added extensive doc in the new test

* Update tests/test_slots.py

* last lint error

Co-authored-by: Sylvain MARIE <sylvain.marie@se.com>
Co-authored-by: Hynek Schlawack <hs@ox.cx>

changelog.d/590.change.rst

@@ -0,0 +1 @@
+Fixed a ``ValueError: Cell is empty`` bug that could happen in some rare edge cases.

src/attr/_make.py

@@ -641,7 +641,12 @@ class _ClassBuilder(object):
             if not closure_cells:  # Catch None or the empty list.
                 continue
             for cell in closure_cells:
-                if cell.cell_contents is self._cls:
+                try:
+                    match = cell.cell_contents is self._cls
+                except ValueError:  # ValueError: Cell is empty
+                    pass
+                else:
+                    if match:
                         set_closure_cell(cell, cls)
 
         return cls

tests/test_slots.py

@@ -543,3 +543,28 @@ def tests_weakref_does_not_add_with_weakref_attribute():
     w = weakref.ref(c)
 
     assert c is w()
+
+
+def test_slots_empty_cell():
+    """
+    Tests that no `ValueError: Cell is empty` exception is raised when
+    closure cells are present with no contents in a `slots=True` class.
+    (issue https://github.com/python-attrs/attrs/issues/589)
+
+    On Python 3, if a method mentions `__class__` or uses the no-arg `super()`,
+    the compiler will bake a reference to the class in the method itself as
+    `method.__closure__`. Since `attrs` replaces the class with a clone,
+    `_ClassBuilder._create_slots_class(self)` will rewrite these references so
+    it keeps working. This method was not properly covering the edge case where
+    the closure cell was empty, we fixed it and this is the non-regression
+    test.
+    """
+
+    @attr.s(slots=True)
+    class C(object):
+        field = attr.ib()
+
+        def f(self, a):
+            super(C, self).__init__()
+
+    C(field=1)