On 07/01/2018 19:55, Chris Angelico wrote:
Under what circumstances would you want "x != y" to be different from
"not (x == y)" ? How would this make for sane behaviour?
Presumably so that any behaviour any be programmed when overriding these
operators.
Maybe someone wants to do weird stuff with == that doesn't yield a true
or false result, so that you can't just reverse it for !=.
For example (perhaps this is similar to what was suggested in another post):
(10,20,30) == (10,20,40) yields (1,1,0)
(10,20,30) != (10,20,40) yields (0,0,1)
Although here, you would probably define 'not' so that 'not (1,1,0)'
does actually yield '(0,0,1)'.
So clearly I need a weirder example.
Even when
other things go weird with equality checks, that basic parallel is
always maintained:
z = float("nan")
z == z
False
z != z
True
Python gives us a "not in" operator that uses __contains__ and then
negates the result. There is no way for "x not in y" to be anything
different from "not (x in y)", as evidenced by the peephole optimizer:
dis.dis("x not in y")
1 0 LOAD_NAME 0 (x)
2 LOAD_NAME 1 (y)
4 COMPARE_OP 7 (not in)
6 RETURN_VALUE
dis.dis("not (x in y)")
1 0 LOAD_NAME 0 (x)
2 LOAD_NAME 1 (y)
4 COMPARE_OP 7 (not in)
I get '4 COMPARE OP 6 (in)' here. So they are distinct ops. 'not in'
doesn't just call 'in', then apply 'not'. Not here anyway.
--
bartc
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