[New-bugs-announce] [issue36507] frozenset type breaks ZFC

[George Shuklin]

New submission from George Shuklin <george.shuklin at gmail.com>:

ZFC (https://en.wikipedia.org/wiki/Zermelo%E2%80%93Fraenkel_set_theory) defines numbers as nested empty sets.

0 is {}
1 is {{}}
2 is {{{}}}

Sets can not be nested in python (as they are mutable), so next best type is frozen set. Unfortunately, nested sets are equal to each other no matter how deep they are nested. This behavior means that 3==2, and it breaks all set operations for ZFC.

Minimal example:

frozenset({frozenset()})
>>> x=frozenset()
>>> y=frozenset(frozenset())
>>> x is y
True

----------
components: Interpreter Core
messages: 339340
nosy: george-shuklin
priority: normal
severity: normal
status: open
title: frozenset type breaks ZFC
type: behavior
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36507>
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