ignore __all__ when not directly assigned (#675)

Author: asottile

pyflakes/checker.py

@@ -1291,7 +1291,14 @@ def handleNodeStore(self, node):
                 parent_stmt != node._pyflakes_parent and
                 not self.isLiteralTupleUnpacking(parent_stmt)):
             binding = Binding(name, node)
-        elif name == '__all__' and isinstance(self.scope, ModuleScope):
+        elif (
+                name == '__all__' and
+                isinstance(self.scope, ModuleScope) and
+                isinstance(
+                    node._pyflakes_parent,
+                    (ast.Assign, ast.AugAssign, ast.AnnAssign)
+                )
+        ):
             binding = ExportBinding(name, node._pyflakes_parent, self.scope)
         elif PY2 and isinstance(getattr(node, 'ctx', None), ast.Param):
             binding = Argument(name, self.getScopeNode(node))

pyflakes/test/test_imports.py

@@ -1057,6 +1057,12 @@ class foo:
             __all__ = ["bar"]
         ''', m.UnusedImport)
 
+    def test_ignored_when_not_directly_assigned(self):
+        self.flakes('''
+        import bar
+        (__all__,) = ("foo",)
+        ''', m.UnusedImport)
+
     def test_warningSuppressed(self):
         """
         If a name is imported and unused but is named in C{__all__}, no warning