Fix assigning-non-slot false positive with setattr (#5457)

* Fix assigning-non-slot false positive with setattr

Previously, if a class was slotted and overrode `__setattr__`,
`assigning-non-slot` would be issued when assigning to attributes.  With
`__setattr__` defined, we cannot infer if it is an error to assign to an
attribute, so we suppress the error.

Fix #3793

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

5 files changed, 47 insertions, 0 deletions

diff --git a/CONTRIBUTORS.txt b/CONTRIBUTORS.txt
index f50f8e470..275225424 100644
--- a/CONTRIBUTORS.txt
+++ b/CONTRIBUTORS.txt
@@ -585,6 +585,8 @@ contributors:
 
 * Felix von Drigalski (felixvd): contributor
 
+* Jake Lishman (jakelishman): contributor
+
 * Philipp Albrecht (pylbrecht): contributor
 
 * Allan Chandler (allanc65): contributor
diff --git a/ChangeLog b/ChangeLog
index c2f77182e..069868d98 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -16,6 +16,11 @@ Release date: TBA
 
   Closes #85, #2615
 
+* Fixed a false positive for ``assigning-non-slot`` when the slotted class
+  defined ``__setattr__``.
+
+  Closes #3793
+
 * ``used-before-assignment`` now assumes that assignments in except blocks
   may not have occurred and warns accordingly.
 
diff --git a/doc/whatsnew/2.13.rst b/doc/whatsnew/2.13.rst
index da595d225..31b6d4e36 100644
--- a/doc/whatsnew/2.13.rst
+++ b/doc/whatsnew/2.13.rst
@@ -51,6 +51,11 @@ Other Changes
 
   Closes #85, #2615
 
+* Fix a false positive for ``assigning-non-slot`` when the slotted class
+  defined ``__setattr__``.
+
+  Closes #3793
+
 * ``used-before-assignment`` now assumes that assignments in except blocks
   may not have occurred and warns accordingly.
 
diff --git a/pylint/checkers/classes.py b/pylint/checkers/classes.py
index 4bbc45f20..478197b0f 100644
--- a/pylint/checkers/classes.py
+++ b/pylint/checkers/classes.py
@@ -1487,6 +1487,14 @@ a metaclass class method.",
             return
         if "__slots__" not in klass.locals or not klass.newstyle:
             return
+        # If `__setattr__` is defined on the class, then we can't reason about
+        # what will happen when assigning to an attribute.
+        if any(
+            base.locals.get("__setattr__")
+            for base in klass.mro()
+            if base.qname() != "builtins.object"
+        ):
+            return
 
         # If 'typing.Generic' is a base of bases of klass, the cached version
         # of 'slots()' might have been evaluated incorrectly, thus deleted cache entry.
diff --git a/tests/functional/a/assign/assigning_non_slot.py b/tests/functional/a/assign/assigning_non_slot.py
index cf673692f..2cd1483e0 100644
--- a/tests/functional/a/assign/assigning_non_slot.py
+++ b/tests/functional/a/assign/assigning_non_slot.py
@@ -173,3 +173,30 @@ class Cls(Generic[TYPE]):
 
     def __init__(self, value):
         self.value = value
+
+
+class ClassDefiningSetattr(object):
+    __slots__ = ["foobar"]
+
+    def __init__(self):
+        self.foobar = {}
+
+    def __setattr__(self, name, value):
+        if name == "foobar":
+            super().__setattr__(name, value)
+        else:
+            self.foobar[name] = value
+
+
+class ClassWithParentDefiningSetattr(ClassDefiningSetattr):
+    __slots__ = []
+
+
+def dont_emit_for_defined_setattr():
+    inst = ClassDefiningSetattr()
+    # This should not emit because we can't reason about what happens with
+    # classes defining __setattr__
+    inst.non_existent = "non-existent"
+
+    child = ClassWithParentDefiningSetattr()
+    child.non_existent = "non-existent"