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gh-135575: MutableMapping.update: use .items() in the Mapping case. #135576

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gh-135575: MutableMapping.update: use .items() in the Mapping case. #135576

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randolf-scholz
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@randolf-scholz randolf-scholz commented Jun 16, 2025 •
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@randolf-scholz randolf-scholz force-pushed the mutablemapping_performance branch from b78241b to d6f4c82 Compare June 16, 2025 16:21
eendebakpt
eendebakpt reviewed Jun 16, 2025
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Misc/NEWS.d/next/Core_and_Builtins/2025-06-16-18-16-41.gh-issue-135575._N5rTH.rst Outdated
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Improved performance of ``MutableMapping.update``.
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Suggested change
Improved performance of ``MutableMapping.update``.
Improved performance of :meth:`MutableMapping.update`.

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I actually tried this one first, but CI complained that it cannot find the RST-reference.

https://github.com/python/cpython/compare/b78241be0ce1147a9b79b50edf2b8ddb95852f37..d6f4c823eaaa30642984f3a30e02f26c00112f23

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Ah sorry about that. It probably should be something like :meth:`collections.abc.MutableMapping.update`.

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I don't think this should change. This is Guido's code and he designed it to only depend on the abstract methods required by the API rather than downstream mixins. Also, this would be a change in behavior if items() has been overridden to have different behavior than you expect.

In general, the ABC mixin methods have been designed as plainly as possible to serve as a specification or guide to implementers. They are not in the optimization business.

@rhettinger rhettinger self-assigned this Jun 16, 2025
@randolf-scholz
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There are already plenty of places where one mixin depends on other mixins:

  • Mapping.__eq__(other) uses self.items() and other.items().
  • MutableSequence.remove uses mixin .index.
  • MutableSequence.extend uses mixin .append.
  • MutableSequence.__iadd__ uses mixin .extend.
  • MutableSet.clear uses mixin .pop.
  • MutableSet.__ixor__ uses mixin .clear.

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Nevertheless, it would be a change in behavior and it alters Guido's intentional design choices.

@rhettinger rhettinger closed this Jun 17, 2025
@gvanrossum-ms
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Thanks @rhettinger. The methods here exist to specify the semantics, not as a performant basis for subclasses.

@randolf-scholz
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@rhettinger Does the same reasoning also apply to #135312?

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