Bug report

Bug description:

import struct
f = struct.unpack(">d", bytes.fromhex("7FF0000000080001"))[0]
print(f)  # nan

f2 = struct.unpack(">f", struct.pack(">f", f))[0]
print(f2)  # inf

Repeating this with FFF0000000080001 you'll see nan turn into -inf.

This only happens in Python 3.14+ (not 3.13). I'm pretty sure that this fix is causing it: #130317

Preserving the signaling/quieting bit is a great idea, I find. But since the mantissa is being truncated, one must ensure that the narrower width mantissa doesn't end up being all 0 bits as that represents inf instead of nan.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-141042: fix sNaN's packing for mixed floating-point formats #141107
• [3.14] gh-141042: fix sNaN's packing for mixed floating-point formats (GH-141107) #141459