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

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collections.ChainMap.__iter__ calls __getitem__ on underlying maps#86653
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rhettinger
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3.10 (EOL)end of life3.9 (EOL)end of lifeperformancePerformance or resource usagestdlibStandard Library Python modules in the Lib/ directory

Description

@ap
ap
mannequin
opened on 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']

    Activity

    1. ap commented on Nov 28, 2020

      @ap
      apmannequin
      on Nov 28, 2020
      MannequinAuthor

      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'}
    2. ap
      added
      type-bugAn unexpected behavior, bug, or error
      3.7 (EOL)end of life
      3.8 (EOL)end of life
      3.9 (EOL)end of life
      3.10 (EOL)end of life
      stdlibStandard Library Python modules in the Lib/ directory
      on Nov 28, 2020
    3. rhettinger
      removed
      3.7 (EOL)end of life
      3.8 (EOL)end of life
      on Nov 28, 2020

    4. 3 remaining items

    5. rhettinger
      added
      performancePerformance or resource usage
      and removed
      3.7 (EOL)end of life
      3.8 (EOL)end of life
      3.9 (EOL)end of life
      type-bugAn unexpected behavior, bug, or error
      on Nov 28, 2020

    6. rhettinger commented on Nov 28, 2020

      @rhettinger
      rhettinger
      on Nov 28, 2020
      Contributor

      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?

    7. rhettinger
      added
      3.9 (EOL)end of life
      on Nov 28, 2020
    8. rhettinger
      self-assigned this
      on Nov 28, 2020
    9. rhettinger
      added
      3.9 (EOL)end of life
      on Nov 28, 2020
    10. rhettinger
      self-assigned this
      on Nov 28, 2020

    11. rhettinger commented on Nov 30, 2020

      @rhettinger
      rhettinger
      on Nov 30, 2020
      Contributor

      New changeset 0be9ce3 by Andreas Poehlmann in branch 'master':
      bpo-42487: don't call __getitem__ of underlying maps in ChainMap.__iter__ (GH-23534)
      0be9ce3

    12. rhettinger commented on Nov 30, 2020

      @rhettinger
      rhettinger
      on Nov 30, 2020
      Contributor

      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

    13. rhettinger
      closed this as completedon Nov 30, 2020
    14. rhettinger
      closed this as completedon Nov 30, 2020
    15. ezio-melotti
      transferred this issue fromon Apr 10, 2022
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