New submission from Vedran Čačić:
During the discussion about http://bugs.python.org/issue27539, we found the
following weird behavior:
>>> from fractions import Fraction as F
>>> F(2, 3, 4)
Fraction(2, 3)
It looks like the third argument is simply disregarded, but it's not so: if
it's false, the fraction is not normalized, and it breaks a lot of arithmetic
assumptions.
>>> F(4, 2, 0)
Fraction(4, 2)
>>> _ == 2
False
The secret is additional argument "_normalize" to Fraction.__new__, which was
added for optimization in various cases (where we know that numerator and
denominator are relatively prime, so the gcd needn't be computed). That
argument was obviously meant for internal use only (its name starts with '_'),
so accepting the above forms can be viewed as a bug. Although cases can be made
for being able to set that argument from the outside, surely in most cases
people passing 3 arguments are doing it by accident, not on purpose.
That bug is easily fixed, by declaring the argument as keyword only. The line
84 in lib/fractions.py should be changed from
- def __new__(cls, numerator=0, denominator=None, _normalize=True):
to
+ def __new__(cls, numerator=0, denominator=None, *, _normalize=True):
That way, those who know what they are doing, can still pass the _normalize
argument, but it's much harder to accidentally pass it for people who don't
know nor care about it.
Of course, all the code from that file already passes _normalize by keyword, so
no other code should be changed.
----------
messages: 273422
nosy: veky
priority: normal
severity: normal
status: open
title: fractions.Fraction with 3 arguments: error passes silently
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Python tracker <[email protected]>
<http://bugs.python.org/issue27832>
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