Hello,

I encountered an issue with collections.ChainMap, that was introduced when https://bugs.python.org/issue32792 got fixed.

Iterating a ChainMap will call __getitem__ on its underlying maps:


>>> from collections import UserDict, ChainMap
>>> class MyDict(UserDict):
...     def __getitem__(self, k):
...         print("expensive computation", k)
...         return super().__getitem__(k)
... 
>>> set(ChainMap(MyDict(a=1, b=2, c=3)))
expensive computation a
expensive computation b
expensive computation c
{'c', 'b', 'a'}