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'}

I had a discussion with the release manager and we're good to backport to 3.9 but not 3.8.

Serhiy, would you also take a look at this?

New changeset 0be9ce305ff2b9e13ddcf15af8cfe28786afb36a by Andreas Poehlmann in branch 'master':
bpo-42487: don't call __getitem__ of underlying maps in ChainMap.__iter__ (GH-23534)
https://github.com/python/cpython/commit/0be9ce305ff2b9e13ddcf15af8cfe28786afb36a

New changeset cf22aa3bc698847459a6bf20c166aa80825b810a by Miss Islington (bot) in branch '3.9':
bpo-42487: don't call __getitem__ of underlying maps in ChainMap.__iter__ (GH-23534) (GH-23569)
https://github.com/python/cpython/commit/cf22aa3bc698847459a6bf20c166aa80825b810a