Great work! I had not known if those corners.
Have you tried with NumPy zero-D arrays or PyTorch Values? Both of those
should have sensible answers.
On Fri, Dec 27, 2019, 7:42 PM Steven D'Aprano <st...@pearwood.info> wrote:
> By the way, it's not as easy as you might think to test whether a value
> is a NAN or not.
>
> There are currently three ways to test whether a value is a NAN. (Four
> if you include cmath.isnan.)
>
> 1. math.isnan(x) but it can fail in annoying ways.
>
>
> # No int is a NAN so this should return False
> py> math.isnan(10**1000)
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> OverflowError: int too large to convert to float
>
> # Decimal signalling NANs are NANs so this should return True
> py> x = Decimal('snan')
> py> math.isnan(x)
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> ValueError: cannot convert signaling NaN to float
>
>
> 2. Decimal.is_nan() but it only works with decimals. However it does
> work with signalling NANs.
>
>
> 3. Back before math.isnan existed, the idiomatic way to check for NANs
> was to test whether they were equal to themselves. Assuming that x is a
> kind of numeric type, we should expect that `x == x` except for NANs.
> But this too fails for signalling NANs:
>
>
> # Using the default Decimal context
> py> x == x
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> decimal.InvalidOperation: [<class 'decimal.InvalidOperation'>]
>
>
> So there is no good way to check for an arbitrary NAN that duck-types
> floats, Decimals, ints, Fractions and other numeric types.
>
> I have come up with this:
>
>
> def isnan(x, _isnan=math.isnan):
> try:
> return _isnan(x)
> except ValueError:
> # Could be a Decimal signalling NAN.
> try:
> return x.is_nan()
> except AttributeError:
> return x != x
> except OverflowError:
> return False
> except TypeError:
> from cmath import isnan
> return isnan(x)
>
>
> but I fear that there could be other landmines waiting, other odd corner
> cases I haven't thought of.
>
> See also
>
> https://bugs.python.org/issue27975
>
>
>
> --
> Steven
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