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collections.ChainMap.__iter__ calls __getitem__ on underlying maps #86653

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ap mannequin opened this issue Nov 28, 2020 · 4 comments
Closed

collections.ChainMap.__iter__ calls __getitem__ on underlying maps #86653

ap mannequin opened this issue Nov 28, 2020 · 4 comments
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@rhettinger
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3.9 only security fixes 3.10 only security fixes performance Performance or resource usage stdlib Python modules in the Lib dir

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@ap
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Mannequin

ap mannequin commented Nov 28, 2020

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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    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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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
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    ap mannequin commented Nov 28, 2020

    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'}

    @ap ap mannequin added type-bug An unexpected behavior, bug, or error 3.7 (EOL) end of life 3.8 only security fixes 3.9 only security fixes 3.10 only security fixes stdlib Python modules in the Lib dir labels Nov 28, 2020
    @rhettinger rhettinger added performance Performance or resource usage and removed 3.7 (EOL) end of life 3.8 only security fixes 3.9 only security fixes type-bug An unexpected behavior, bug, or error labels Nov 28, 2020
    @rhettinger
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    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?

    @rhettinger rhettinger added the 3.9 only security fixes label Nov 28, 2020
    @rhettinger rhettinger self-assigned this Nov 28, 2020
    @rhettinger rhettinger added the 3.9 only security fixes label Nov 28, 2020
    @rhettinger rhettinger self-assigned this Nov 28, 2020
    @rhettinger
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    New changeset 0be9ce3 by Andreas Poehlmann in branch 'master':
    bpo-42487: don't call __getitem__ of underlying maps in ChainMap.__iter__ (GH-23534)
    0be9ce3

    @rhettinger
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    New changeset cf22aa3 by Miss Islington (bot) in branch '3.9':
    bpo-42487: don't call __getitem__ of underlying maps in ChainMap.__iter__ (GH-23534) (GH-23569)
    cf22aa3

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
    @zrothberg zrothberg mentioned this issue Oct 27, 2022
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