Fix #293, B024 now skips classes with class attribute declarations

bugbear.py

@@ -612,7 +612,7 @@ def check_for_b023(self, loop_node):
             if reassigned_in_loop.issuperset(err.vars):
                 self.errors.append(err)
 
-    def check_for_b024_and_b027(self, node: ast.ClassDef):
+    def check_for_b024_and_b027(self, node: ast.ClassDef):  # noqa: C901
         """Check for inheritance from abstract classes in abc and lack of
         any methods decorated with abstract*"""
 
@@ -661,6 +661,12 @@ def is_str_or_ellipsis(node):
         has_abstract_method = False
 
         for stmt in node.body:
+            # https://github.com/PyCQA/flake8-bugbear/issues/293
+            # Ignore abc's that declares a class attribute that must be set
+            if isinstance(stmt, (ast.AnnAssign, ast.Assign)):
+                has_abstract_method = True
+                continue
+
             # only check function defs
             if not isinstance(stmt, (ast.FunctionDef, ast.AsyncFunctionDef)):
                 continue

tests/b024.py

@@ -110,3 +110,20 @@ def method(self):
 class keyword_abc_2(metaclass=abc.ABC):  # safe
     def method(self):
         foo()
+
+
+class abc_set_class_variable_1(ABC):  # safe
+    foo: int
+
+
+class abc_set_class_variable_2(ABC):  # safe
+    foo = 2
+
+
+class abc_set_class_variable_3(ABC):  # safe
+    foo: int = 2
+
+
+# this doesn't actually declare a class variable, it's just an expression
+class abc_set_class_variable_4(ABC):  # error
+    foo

tests/test_bugbear.py

@@ -364,6 +364,7 @@ def test_b024(self):
             B024(58, 0, vars=("MetaBase_1",)),
             B024(69, 0, vars=("abc_Base_1",)),
             B024(74, 0, vars=("abc_Base_2",)),
+            B024(128, 0, vars=("abc_set_class_variable_4",)),
         )
         self.assertEqual(errors, expected)