New submission from Elias Zamaria <mikez...@gmail.com>:
Usually, a positive finite number modulo infinity is itself. But modding a
positive fraction by infinity produces nan:
>>> from fractions import Fraction
>>> from math import inf
>>> 3 % inf
3.0
>>> 3.5 % inf
3.5
>>> Fraction('1/3') % inf
nan
Likewise, a positive number modulo negative infinity is usually negative
infinity, a negative number modulo infinity is usually infinity, and a negative
number modulo negative infinity is usually itself, unless the number doing the
modding is a fraction, in which case it produces nan.
I think fractions should behave like other numbers in cases like these. I don't
think this comes up very often in practical situations, but it is inconsistent
behavior that may surprise people.
I looked at the fractions module. It seems like this can be fixed by putting
the following lines at the top of the __mod__ method of the Fraction class:
if b == math.inf:
if a >= 0:
return a
else:
return math.inf
elif b == -math.inf:
if a >= 0:
return -math.inf
else:
return a
If that is too verbose, it can also be fixed with these lines, although this is
less understandable IMO:
if math.isinf(b):
return a if (a >= 0) == (b > 0) else math.copysign(math.inf, b)
I noticed this in Python 3.6.4 on OS X 10.12.6. If anyone wants, I can come up
with a patch with some tests.
----------
components: Interpreter Core
messages: 313045
nosy: elias
priority: normal
severity: normal
status: open
title: Fraction modulo infinity should behave consistently with other numbers
versions: Python 3.6, Python 3.7, Python 3.8
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue32968>
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