[Python-Dev] Re: marshal / unmarshal

[Tim Peters]

...

[mwh]
>>> I recall stories of machines that stored the bytes of long in some
>>> crazy order like that.  I think Python would already be broken on such
>>> a system, but, also, don't care.

[Tim]
>> Python does very little that depends on internal native byte order,
>> and C hides it in the absence of casting abuse.

[mwh]
> This surely does:
>
> PyObject *
> PyLong_FromLongLong(PY_LONG_LONG ival)
> {
>        PY_LONG_LONG bytes = ival;
>        int one = 1;
>        return _PyLong_FromByteArray(
>                (unsigned char *)&bytes,
>                               SIZEOF_LONG_LONG, IS_LITTLE_ENDIAN, 1);
> }

Yes, that's "casting abuse'.  Python does very little of that.  If it
becomes necessary, it's straightforward but long-winded to rewrite the
above in wholly portable C (peel the bytes out of ival,
least-signficant first, via shifting and masking 8 times; "ival &
0xff" is the least-significant byte regardless of memory storage
order; etc).  BTW, the IS_LITTLE_ENDIAN macro also relies on casting
abuse, and more deeply than does the visible cast there.
 
> It occurs that in the IEEE case, special values can be detected with
> reliablity -- by picking the exponent field out by force

Right, that works for NaNs and infinities; signed zeroes are a bit
trickier to detect.

> -- and a warning emitted or exception raised.  Good idea?  Hard to say, to me.

It's not possible to _create_ a NaN or infinity from finite operands
in 754 without signaling some exceptional condition.  Once you have
one, though, there's generally nothing exceptional about _using_ it. 
Sometimes there is, like +Inf - +Inf or Inf / Inf, but not generally. 
Using a quiet NaN never signals; using a signaling NaN almost always
signals.

So packing a nan or inf shouldn't complain.  On a 754 box, unpacking
one shouldn't complain either.  Unpacking a nan or inf on a non-754
box probably should complain, since there's in general nothing it can
be unpacked _to_ that makes any sense ("errors should never pass
silently").