gh-150165: Clarify membership test semantics for hash-based containers and object identity#150166
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KRRT7 wants to merge 1 commit into
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gh-150165: Clarify membership test semantics for hash-based containers and object identity#150166KRRT7 wants to merge 1 commit into
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…tainers and object identity Split the membership test equivalence in §6.10.2 to distinguish sequence types (linear scan) from hash-based containers (hash lookup). Add a note acknowledging that the x is e check has observable consequences for NaN and state-dependent __eq__, and that object identity is unspecified for non-literal expressions. Also add a clarifying sentence in §6.2.3.1 noting that non-literal expressions make no identity guarantees.
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As discussed on discuss.python.org.
Changes
§6.10.2 Membership test operations:
§6.2.3.1 Literals and object identity:
Motivation
This came from a real bug: Nuitka/Nuitka#3889. Nuitka's constant blob cached ``float('nan')`` as a singleton, making ``nan2 in {nan1}`` return ``True`` instead of ``False``. Both were spec-compliant because the spec doesn't address whether calls return the same object — that ambiguity is what this PR addresses.
Related issues