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dict items views have set like operations even when the values are not hashable. #111622

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EvanTheB opened this issue Nov 1, 2023 · 5 comments
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dict items views have set like operations even when the values are not hashable. #111622

EvanTheB opened this issue Nov 1, 2023 · 5 comments
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docs Documentation in the Doc dir interpreter-core (Objects, Python, Grammar, and Parser dirs)

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@EvanTheB
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EvanTheB commented Nov 1, 2023 •
edited by bedevere-app bot
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Documentation

Keys views are set-like since their entries are unique and :term:hashable. If all values are hashable, so that (key, value) pairs are unique and hashable, then the items view is also set-like. (Values views are not treated as set-like since the entries are generally not unique.) For set-like views, all of the operations defined for the abstract base class :class:collections.abc.Set are available (for example, ==, <, or ^). While using set operators, set-like views accept any iterable as the other operand, unlike sets which only accept sets as the input.

> {1:2, 3:4}.items() >= {1:2}.items()
True
> {1:[2], 3:4}.items() >= {1:[2]}.items()
True
> set({1:[2], 3:4}.items())
TypeError: unhashable type: 'list'

It seems like this documentation is not accurate. dict.items() does have set operations even when the values are not hashable.

See also: https://stackoverflow.com/questions/77399592/set-like-behaviour-of-dict-items-for-non-hashable-values

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@EvanTheB EvanTheB added the docs Documentation in the Doc dir label Nov 1, 2023
@terryjreedy terryjreedy added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Nov 1, 2023
@terryjreedy
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My guess is the same as in 'Blcknight's answer: it was realized that items is read only, making hash((key,value)) = hash(key) is sufficient, making the caveat in the doc unnecessary. To be sure, someone should check that the items for the example dict works for all of the supported operations. And check the dict items code and the git blame history of revisions of that code.

@pochmann
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pochmann commented Nov 2, 2023 •
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Not all set operations actually work, for example {0: []}.items() | set() fails with TypeError: unhashable type: 'list'.

@rhettinger
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Perhaps replace with second sentence with:

Item views also have set-like operations since the (key, value) pairs are unique
and the keys are hashable. If the values in the items view are hashable as well,
then the items views can interoperate with other sets.

terryjreedy added a commit to terryjreedy/cpython that referenced this issue Nov 14, 2023
@terryjreedy
pythongh-111622: Fix doc for items views
b5397c0 
They are set-like even when some values are not hashable,
but work even better when all are.
@bedevere-app bedevere-app bot mentioned this issue Nov 14, 2023
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@terryjreedy
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#112051 is mostly Raymond's suggestion.

terryjreedy added a commit that referenced this issue Nov 14, 2023
@terryjreedy
gh-111622: Fix doc for items views (#112051)
e31d65e 
They are set-like even when some values are not hashable,
but work even better when all are.
miss-islington pushed a commit to miss-islington/cpython that referenced this issue Nov 14, 2023
@terryjreedy @miss-islington
pythongh-111622: Fix doc for items views (pythonGH-112051)
d9d099b 
They are set-like even when some values are not hashable,
but work even better when all are.
(cherry picked from commit e31d65e)

Co-authored-by: Terry Jan Reedy <[email protected]>
@bedevere-app bedevere-app bot mentioned this issue Nov 14, 2023
Merged
terryjreedy added a commit that referenced this issue Nov 14, 2023
@miss-islington @terryjreedy
[3.12] gh-111622: Fix doc for items views (GH-112051) (#112052)
18b88fe 
They are set-like even when some values are not hashable,
but work even better when all are.
(cherry picked from commit e31d65e)

Co-authored-by: Terry Jan Reedy <[email protected]>
@terryjreedy
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3.11 backport failed (conflict)

aisk pushed a commit to aisk/cpython that referenced this issue Feb 11, 2024
@terryjreedy @aisk
pythongh-111622: Fix doc for items views (python#112051)
2113d5a 
They are set-like even when some values are not hashable,
but work even better when all are.
Glyphack pushed a commit to Glyphack/cpython that referenced this issue Sep 2, 2024
@terryjreedy @Glyphack
pythongh-111622: Fix doc for items views (python#112051)
1599588 
They are set-like even when some values are not hashable,
but work even better when all are.
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