[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Changes by Raymond Hettinger raymond.hettin...@gmail.com: -- resolution: - not a bug status: open - closed ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Raymond Hettinger added the comment: A possibility would be to add (disallowed by the Decimal standard) to the exception message. That is what InvalidOperation means ;-) -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Terry J. Reedy added the comment: Thank you Tim. A possibility would be to add (disallowed by the Decimal standard) to the exception message. -- nosy: +terry.reedy versions: -Python 3.6 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Stefan Krah added the comment: For the differences between the standard and IEEE 754-2008 we could link to: http://speleotrove.com/decimal/dascope.html In the long run, perhaps we should move to IEEE, because we're almost there (but that's a separate issue). -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Tim Peters added the comment: This is easy: Cowlishaw is wrong on this one, but nothing can be done about it ;-) Confusion arises because most people think of 0**0 as a value (where it certainly must be 1) while others seem to view it as some kind of shorthand for expressing a limit (as the base and/or exponent _approach_ 0, in which case there is no correct answer - it's an indeterminate form). It's in the spirit of 754 to take inputs at face value, viewing them as infinitely precise. So viewing 0**0 as anything other than 1 in this context is perverse. Centuries of history distilled to a few paragraphs here: http://en.wikipedia.org/wiki/Exponentiation#Zero_to_the_power_of_zero -- nosy: +tim.peters ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Steven D'Aprano added the comment: Mark Dickson wrote: I've talked to Mike Cowlishaw (the author of the specification) about this particular issue, and the spec is not likely to change on this point. I'm curious about the rationale for the decision. As I'm sure you're aware, in general 0**0 is treated as 1 by both a majority (I think) of mathematicians and programming languages. As Knuth puts it, the binomial theorem is too important to do otherwise. IEEE 754 treats it as 1, although the 2008 revision adds a second power function powr() which returns NAN if both arguments are 0. So I wonder why the decimal spec choose to do otherwise? (Not saying they're wrong to do so.) -- nosy: +steven.daprano ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Devin Jeanpierre added the comment: Does the spec have a handy list of differences to floats anywhere, or do you have to internalize the whole thing? -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Raymond Hettinger added the comment: this deserves to be spelled out in big red letters in the documentation for the decimal module, along with any other inconsistencies. I think you lost all sense of proportion here. The decimal module is obliged to follow the decimal spec (that is its reason for existence). The decimal module docs are already create a heavy mental load and their usability would not be improved shifting focus to corner case inconsistencies between types that haven't proven to be an issue in practice. If you were to go write a blog post about 0**0 versus Decimal(0)**0, I think you would find that no one cares. -- assignee: - rhettinger priority: normal - low ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
STINNER Victor added the comment: the spec is not likely to change on this point. In this case, we should just document the behaviour with a reference to the General Decimal Arithmetic Specification. -- nosy: +haypo ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Raymond Hettinger added the comment: The docs already reference the spec. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Mark Dickinson added the comment: This behavior seems to be required by the General Decimal Arithmetic Specification (http://speleotrove.com/decimal/daexcep.html ) Yes, exactly. The decimal module strictly follows that specification. I don't like the 0**0 - NaN result much either (especially when we also have inf**0 - 1), but it's what's specified. I've talked to Mike Cowlishaw (the author of the specification) about this particular issue, and the spec is not likely to change on this point. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Stefan Krah added the comment: The behavior is already documented (power function): at least one of x or y must be nonzero The decimal docs are already so large that it is probably a bad idea to add a warning. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Devin Jeanpierre added the comment: Yes, also, it is documented: https://docs.python.org/3/library/decimal.html#decimal.InvalidOperation Still, the status quo is bad. At the very least there should be clear documentation on how Decimal differs in behavior from floats and ints. (Other than the obvious, like 1/5 taking on a different value -- although explicitly mentioning that in the list might be a good idea.) BTW, 0**0=1 is not mathematically impure. It at one point was fairly well accepted as the right answer, since it's the one that tends to come out naturally . e.g. http://arxiv.org/abs/math/9205211 page 6 (ripples) . This might explain why ints and floats so casually evaluate 0**0 to 1. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
New submission from Devin Jeanpierre: import decimal x = 0 y = float(x) z = decimal.Decimal(x) x == y == z == x True x ** x 1 y**y 1.0 z**z Traceback (most recent call last): File stdin, line 1, in module File /usr/lib/python2.7/decimal.py, line 2216, in __pow__ return context._raise_error(InvalidOperation, '0 ** 0') File /usr/lib/python2.7/decimal.py, line 3872, in _raise_error raise error(explanation) decimal.InvalidOperation: 0 ** 0 This is PHP-like and confusing, and maybe not justified just by standards compliance. If it is justified by standards compliance, this deserves to be spelled out in big red letters in the documentation for the decimal module, along with any other inconsistencies. -- components: Library (Lib) messages: 233711 nosy: Devin Jeanpierre priority: normal severity: normal status: open title: Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0 versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4, Python 3.5, Python 3.6 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Changes by Devin Jeanpierre jeanpierr...@gmail.com: -- type: - behavior ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Ezio Melotti added the comment: In the code there is this comment: # 0**0 = NaN (!), x**0 = 1 for nonzero x (including +/-Infinity) and raising the error for this specific case seems intentional. -- nosy: +ezio.melotti, facundobatista, mark.dickinson, rhettinger, skrah versions: -Python 3.2, Python 3.3 ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Josh Rosenberg added the comment: Intentional, but really hard to justify from a consistency perspective. There appear to be several reasonable arguments to treat it as 1 regardless of the mathematical impurity ( https://www.math.hmc.edu/funfacts/ffiles/10005.3-5.shtml ), and since we clearly accept that for int and float, it seems reasonable to extend it to Decimal. -- nosy: +josh.r ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue23201] Decimal(0)**0 is an error, 0**0 is 1, but Decimal(0) == 0
Chris Rebert added the comment: This behavior seems to be required by the General Decimal Arithmetic Specification (http://speleotrove.com/decimal/daexcep.html ): The following exceptional conditions can occur: [...] Invalid operation This occurs and signals invalid-operation if: [...] * both operands of the power operation are zero signals invalid-operation apparently being mapped by default in Python to raise the InvalidOperation exception. -- nosy: +cvrebert ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue23201 ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
