On Mon, Sep 14, 2020 at 9:36 AM Stephen J. Turnbull <
turnbull.stephen...@u.tsukuba.ac.jp> wrote:
> Christopher Barker writes:
> > IEEE 754 is a very practical standard -- it was well designed, and is
> > widely used and successful. It is not perfect, and in certain use
> cases, it
> > may not be the best choice. But it's a really good idea to keep to that
> > standard by default.
>
I feel the same way; I really wish Python was better about following IEEE
754.
I agree, but Python doesn't. It raises on some infs (generally
> speaking, true infinities), and returns inf on others (generally
> speaking, overflows).
>
It seems to be very inconsistent. From testing just now:
* math.lgamma(0) raises "ValueError: math domain error"
* math.exp(1000) raises "OverflowError: math range error"
* math.e ** 1000 raises "OverflowError: (34, 'Result too large')"
* (math.e ** 500) * (math.e ** 500) returns inf
* sum([1e308, 1e308]) returns inf
* math.fsum([1e308, 1e308]) raises "OverflowError: intermediate overflow in
fsum"
* math.fsum([1e308, inf, 1e308]) returns inf
* math.fsum([inf, 1e308, 1e308]) raises "OverflowError: intermediate
overflow in fsum"
* float('1e999') returns inf
* float.fromhex('1p1024') raises "OverflowError: hexadecimal value too
large to represent as a float"
I get the impression that little planning has gone into this. There's no
consistency in the OverflowError messages. 1./0. raises ZeroDivisionError
which isn't a subclass of OverflowError. lgamma(0) raises a ValueError,
which isn't even a subclass of ArithmeticError. The function has a pole at
0 with a well-defined two-sided limit of +inf. If it isn't going to return
+inf then it ought to raise ZeroDivisionError, which should obviously be a
subclass of OverflowError.
Because of the inconsistent handling of overflow, many functions aren't
even monotonic. exp(2*x) returns a float for x <= 709.782712893384, raises
OverflowError for 709.782712893384 < x <= 8.98846567431158e+307, and
returns a float for x > 8.98846567431158e+307.
1./0. is not a true infinity. It's the reciprocal of a number that may have
underflowed to zero. It's totally inconsistent to return inf for 1/1e-323
and raise an exception for 1/1e-324, as Python does.
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