Thanks so much Ben for documenting all these examples. I've been frustrated by the inconsistencies, but hasn't realized all of those you note.

It would be a breaking change, but I'd really vastly prefer if almost all of those OverflowErrors and others were simply infinities. That's much closer to the spirit of IEEE-754.

The tricky case is 1./0. Division is such an ordinary operation, and it's so easy to get zero in a variable accidentally. That one still feels like an exception, but yes 1/1e-323 vs. 1/1e-324 would them remain a sore spot.

Likewise, a bunch of operations really should be NaN that are exceptions now.

On Mon, Sep 14, 2020, 5:26 PM Ben Rudiak-Gould <> wrote:
On Mon, Sep 14, 2020 at 9:36 AM Stephen J. Turnbull <> 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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