Hello,
This is inconsistent (and documented ;-). For the operations "or" and
"and" Python actually uses "__nonzero__". *Not* "__or__" and "__and__"
which are the bitwise operation "|" and "&".
They work as follows:
In "x and y" if "x.__nonzero__()" is False then the result is "x"
otherwise it is "y".
In "x or y" if "x.__nonzero__()" is True then the result is "x" and
otherwise it is "y".
Examples:
sage: 5 and 4 and 19
19
sage: 4 and 0 and 12
0
sage: 0 or 7
7
Similarly, even if X and Y are not defined you can write
sage: False and X and Y
False
The only way to fix Unknown would be to patch (non trivially) Python.
Boolean inherits from int... and it would be hard to have a third party
"Unknown" coherent with this inheritance. There was a slightly related
proposal, PEP 335, which was rejected with good reasons. We could
propose a new PEP for trooleans but it is not clear what should be the
concrete implementation.
Vincent
On 25/11/15 01:56, kcrisman wrote:
On social media:
sage: False or Unknown
Unknown
sage: Unknown or False
False
sage: False and Unknown
False
sage: Unknown and False
Unknown
It does seems somewhat inconsistent...
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