Re: [Python-ideas] Re: Magnitude and ProtoMagnitude ABCs — primarily for argument validation

[Chris Angelico]

On Tue, Mar 10, 2020 at 11:16 PM Marco Sulla via Python-list
<python-list at python.org> wrote:
> Raising a NanError seems to me the only way to eliminate the NaN
> problem. Indeed NaN was created for languages like C, that does not
> support exceptions.

People keep saying this - that nan exists to avoid exceptions - but
that doesn't take signalling nans into account. Even in C, they are
expected to raise an exception or equivalent.

Python raises an exception for float division by zero because Python
does its best to treat smallish integers and integral floats as
equivalent. (Yes, I'm using "smallish" to mean "less than nine
quadrillion"... programmers have weird ideas about magnitude
sometimes.) Apart from indexing, the values 3 and 3.0 should be able
to be used pretty much interchangeably. They compare equal (which, in
turn, implies that they have the same hash), they behave very
similarly in arithmetic, etc, etc, etc. It makes good sense for
division by 0 and division by 0.0 to both result in the same
exception. It also makes sense for integer division to raise rather
than return a float (since "x // y" between two ints will never
otherwise be a float). So it'd be nastily inconsistent to return a NaN
for float division by zero.

ChrisA