Hi,
In Sage 6.10.beta7:
sage: bool(x == 0)
False
sage: bool(x != 0)
True
sage: x = SR.var('x', domain='real')
sage: bool(x == 0)
False
sage: bool(x != 0)
False
The latest output differs from what one would get from Sage 6.10.beta6 (and
any previous version of Sage), which would have been True, i.e. consistent
with the second output.
It seems that this change has been introduced in #19312
<http://trac.sagemath.org/ticket/19312>. This introduces a kind of
non-binary logic for undecidable expressions. A priori I am not against it,
but IMHO this should be fully advertised, since having x != 0 different
from not (x==0), i.e. having
sage: bool(x != 0) is bool(not (x == 0))
False
may break a lot of codes (for sure it broke mine!). Also it is quite
annoying that the last output is not consistent with the second one, simply
because x has been declared real. Note that declaring x complex does not
restore the consistency:
sage: forget()
sage: x = SR.var('x', domain='complex')
sage: bool(x != 0)
False
Only having domain equal to None does:
sage: forget()
sage: x = SR.var('x')
sage: bool(x != 0)
True
>From the discussion in #19312 <http://trac.sagemath.org/ticket/19312> and
in #19040 <http://trac.sagemath.org/ticket/19040>, I understand that this
change of logic is a feature and not a bug, and I can easily fix my code by
replacing x != 0 by not (x==0). However, such an important change may
surprise many users and IMHO it should be stressed during the release of
Sage 6.10.
Eric.
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