[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[Stefan Krah]

Stefan Krah  added the comment:

Mark, do you think that we should document the other oddity as well
or should we close this?

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status: open -> pending

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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[miss-islington]

miss-islington  added the comment:


New changeset dac8a3a15647d016842e2ae7284cefa27ce5e838 by Miss Islington (bot) 
in branch '3.8':
bpo-41205: Document Decimal power 0 to the 0 (GH-21386)
https://github.com/python/cpython/commit/dac8a3a15647d016842e2ae7284cefa27ce5e838


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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[miss-islington]

miss-islington  added the comment:


New changeset 8831162464138eb0267f4967b85710247ee95fde by Miss Islington (bot) 
in branch '3.9':
bpo-41205: Document Decimal power 0 to the 0 (GH-21386)
https://github.com/python/cpython/commit/8831162464138eb0267f4967b85710247ee95fde


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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[miss-islington]

Change by miss-islington :


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pull_request: https://github.com/python/cpython/pull/21547

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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[miss-islington]

Change by miss-islington :


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pull_request: https://github.com/python/cpython/pull/21548

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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[Stefan Krah]

Stefan Krah  added the comment:


New changeset 10e466448f67850ed7bb2e2a4e7f017f2b050cad by Srinivas Reddy 
Thatiparthy (శ్రీనివాస్  రెడ్డి తాటిపర్తి) in branch 'master':
bpo-41205: Document Decimal power 0 to the 0 (GH-21386)
https://github.com/python/cpython/commit/10e466448f67850ed7bb2e2a4e7f017f2b050cad


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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[శ్రీనివాస్ రెడ్డి తాటిపర్తి]

Change by Srinivas  Reddy Thatiparthy(శ్రీనివాస్ రెడ్డి తాటిపర్తి) 
:


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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[Mark Dickinson]

Mark Dickinson  added the comment:

@JD-Veiga: would you be willing to work on a PR?

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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[Stefan Krah]

Stefan Krah  added the comment:

Agreed, and it's even slightly worse with the ROUND_FLOOR special case:

>>> c.rounding = ROUND_FLOOR
>>> +Decimal("-0")
Decimal('-0')
>>> 

That's why there are three slightly different methods for applying the
context:

   1) context.create_decimal() ==> special case for NaN payloads.

   2) context.apply() (private method) ==> no special cases at all.

   3) Decimal.plus() ==> ROUND_FLOOR special case.

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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[Mark Dickinson]

Mark Dickinson  added the comment:

I agree that the result is surprising and should be noted in the docs; we 
shouldn't expect all the decimal users to be familiar with the specification. 
Some of the other differences with float arithmetic *are* noted (e.g., the 
behaviour of `//` and `%`), but apparently not this one. We do document the 
behaviour of these corner cases for `math.pow`.

The part that I find really odd is that the specification says that 
`Decimal("inf")**Decimal(0)` is `1`. Treating one of 0**0 and inf**0 as 
undefined but not the other seems inconsistent to me. We'd probably want to 
note the behaviour of inf**0 in the docs, too. (I tried to argue with Mike 
Cowlishaw about this, but I lost.)

I'm not sure whether just noting this in the "power" documentation is enough: 
it's not clear to me that users using the ** operator would think to check the 
docs for the "power" method.

The other weirdly non-IEEE 754 behaviour (again, mandated by the standard) that 
bugs me from time to time is that the unary `-` and `+` operators will never 
give negative zero: Decimal("-0.0") and -Decimal("0.0") are not the same thing. 
That one's probably worth noting, too.

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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[JD-Veiga]

JD-Veiga  added the comment:

Well, it is the way that it has been defined in `decimal` package and in the 
document that inspired this package (IBM's General Decimal Arithmetic 
Specification, The General Decimal Arithmetic Specification: 
http://speleotrove.com/decimal/decarith.html).

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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[Tim Peters]

Tim Peters  added the comment:

Huh! I thought everyone in Standards World gave up by now, and agreed 0**0 
should be 1.

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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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[issue41205] Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which is 1)

[JD-Veiga]

New submission from JD-Veiga :

Hi,

I would like to propose a minor addition to `decimal` package documentation 
page (https://docs.python.org/3/library/decimal.html). I think that we should 
add a paragraph in `context.power(x, y, modulo=None)` stating that `Decimal(0) 
** Decimal(0)` returns `Decimal('NaN')` instead of `1` --as `0 ** 0` does-- or 
`1.0` --in case of `0.0 ** 0.0`.

Indeed, `0 ** 0` is `NaN` is mentioned as example of operations raising 
`InvalidOperation` exceptions.

However, I think that this is not enough to indicate the different behaviour of 
decimal versus int and float numbers.

Moreover, in the case of `%` and `//` operators, there are clear remarks on 
these differences (See: “There are some small differences between arithmetic on 
Decimal objects and arithmetic on integers and floats [...]” in the page).

Thank you and sorry for the inconvenience.

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assignee: docs@python
components: Documentation
messages: 372969
nosy: JD-Veiga, docs@python
priority: normal
severity: normal
status: open
title: Documentation Decimal power 0 to the 0 is Nan (versus 0 to the 0 which 
is 1)
type: enhancement
versions: Python 3.10, Python 3.8, Python 3.9

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