collections.ChainMap.__iter__ calls __getitem__ on underlying maps

[ap]

BPO	42487
Nosy	@rhettinger, @ambv, @serhiy-storchaka, @miss-islington, @ap--
PRs	bpo-42487: don't call __getitem__ of underlying maps in ChainMap.__iter__ #23534 [3.9] bpo-42487: don't call __getitem__ of underlying maps in ChainMap.__iter__ (GH-23534) #23569

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2020-11-30.21:23:39.144>
created_at = <Date 2020-11-28.00:50:30.644>
labels = ['library', '3.9', '3.10', 'performance']
title = 'collections.ChainMap.__iter__ calls __getitem__ on underlying maps'
updated_at = <Date 2020-11-30.21:23:39.144>
user = 'https://github.com/ap'

bugs.python.org fields:

activity = <Date 2020-11-30.21:23:39.144>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2020-11-30.21:23:39.144>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2020-11-28.00:50:30.644>
creator = 'ap--'
dependencies = []
files = []
hgrepos = []
issue_num = 42487
keywords = ['patch']
message_count = 4.0
messages = ['381971', '381974', '382159', '382164']
nosy_count = 5.0
nosy_names = ['rhettinger', 'lukasz.langa', 'serhiy.storchaka', 'miss-islington', 'ap--']
pr_nums = ['23534', '23569']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue42487'
versions = ['Python 3.9', 'Python 3.10']

[ap]

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'}