ch17: update from book draft

17-it-generator/tree/step1/tree.py

@@ -0,0 +1,10 @@
+def tree(cls):
+    yield cls.__name__, 0                        # <1>
+    for sub_cls in cls.__subclasses__():         # <2>
+        yield sub_cls.__name__, 1                # <3>
+
+
+if __name__ == '__main__':
+    for cls_name, level in tree(BaseException):
+        indent = ' ' * 4 * level                 # <4>
+        print(f'{indent}{cls_name}')