ch11-24: clean up by @eumiro & sync with Atlas

17-it-generator/tree/4steps/tree_step3.py

@@ -0,0 +1,10 @@
+def tree(cls, level=0):
+    yield cls.__name__, level
+    for sub_cls in cls.__subclasses__():
+        yield from tree(sub_cls, level + 1)
+
+
+if __name__ == '__main__':
+    for cls_name, level in tree(BaseException):
+        indent = ' ' * 4 * level
+        print(f'{indent}{cls_name}')