Is duck typing float or Decimal worth the bother? Barring that it could be
done with some isinstance() checks (in the user code, not in math.isnan()).
Otherwise, your only resort is a PEP, like the final comment of that bug
says.
On Fri, Dec 27, 2019 at 18:16 David Mertz <me...@gnosis.cx> wrote:
> 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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--Guido (mobile)
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