Is 'everything' a refrence or isn't it?

Fredrik Lundh fredrik at pythonware.com
Sun Jan 15 21:48:39 CET 2006


Donn Cave wrote:

> | > What's the value of None? Would you say "it has no value" or "it's value
> | > is None" or something else?
> |
> | it has no value (or if you prefer, the set of values is empty).
>
> Dang, I would have said it does have a value.
>
> For one thing, in
>
>      if a:
>          print 'more or less True'
>      else:
>          print 'evidently not'
>
> None doesn't follow the default, so it seems like "if" has discovered
> some sort of value in there.

the "if" statement happens to use object identity to determine truth values
for certain special objects, including None:

    int
    PyObject_IsTrue(PyObject *v)
    {
        int res;
        if (v == Py_True)
            return 1;
        if (v == Py_False)
            return 0;
        if (v == Py_None)
            return 0;
        /... query object .../
    }

in the more general case, it's up to the object's type implementation to deter-
mine its "truth value", in response to a __nonzero__ or __len__ query.

maybe we need a new word for "virtual values" that are not really part of an
object's internal state ?

</F>






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