[issue4114] struct returns incorrect 4 byte float

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

[Robert]
> I have to disagree.  It seems entirely reasonable to expect that
> unpack should return the same value passed to pack.

Robert:  notice that a *Python* float (a *64-bit* C double internally) is here being stored as a *32-bit* float, losing precision.  So no, it's not at all reasonable to expect that unpack should return the same value passed to pack---it's mathematically impossible for it to do so.  There are (around) 2**64 distinct Python floats, and only 2**32 ways to pack them using '<f'.

When packing / unpacking using '<d', it *is* reasonable to expect the value to be recovered exactly, and as far as I know that's always what happens (barring peculiarities like NaN payloads not being reproduced exactly).

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue4114>
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