[Python-Dev] the not operator (and the __not__ special method)
Marcelo Matus
mmatus@dinha.acms.arizona.edu
2002年10月03日 15:30:34 -0700
Thanks to everybody for your fast and helpful answer.
Clearly I was confused with the similarity between the names 'and': __and__
and 'or' : __or__, and I was thinking they were related, but that is
not the case.
For now, I can implement my logical classes by using the bitwise
operators '&,|,~' .
I forgot to say, but what I want to do is something like
x = vfncs.Identity() # a class instance
b = x + 2 # another
class instance
cond = (sin(x) <= 0.5 ) & ( b > 0.5 ) # another class instance
print x(0.3) ==> 0.3
print b(0.3) ==> 2.3
print cond(0.3) ==> 1
where x, b and cond are "virtual functions" than can be evaluated like
lambda functions after they are defined. The difference is that the are
build from a C++
library, and they can be passed back to the C++ code where
they will be evaluated directly.
Anyway, given that now python has the "rich comparison" engine, where the
operator <=,>,<,>= can return anything you want (and thanks for that,
because
I can do what I am doing now) and more boolean capabilities are been
included
(I read that the boolean type is coming soon), I guess it will be also
good to
have the special methods for the logical operations 'and', 'or' and 'not',
(something like __land__, __lor__ and __lnot__ to distinguish them from
the bitwise versions __and__ and __or__).
So, if you overload the proper logical methods, you can write something
like
cond = not (sin(x) > 0.5 ) and ( b <= 0.5 )
where 'not' and 'and', like '>' and '<=', wil return anything you want.
Marcelo
Brian Quinlan wrote:
>>shows that python doesn't call the __not__ special method
>>in a 'not' operator statement.
>>>>>>Python calls the special __nonzero__ method so check the truth value of
>an object.
>>>>>Another question, I notice than "a or b" and "a and b" are
>>not equivalent to "a | b" and "a & b", since the last ones call
>>the methods __or__ and __and__ if they are defined, but
>>the "literal forms" never call them. Is this intentional?, if
>>that is the case, I guess a big and clear note is needed
>>in the documentation.
>>>>>>"and" and "or" are logical operators, while "|" and "&" are bitwise
>operators.
>>>>>>>13 & 14
>>>>>>>>>12
>>>>>>13 and 14
>>>>>>>>>14
>>>>>And just for symmetry considerations, given that python has the
>>pairs (and, &) and (or, |), does anybody considering to introduce
>>the ! operator, so you can have the equivalent (not, !) too?
>>>>>>There is a bitwise not operator: ~
>>>>>>>~2
>>>>>>>>>-3
>>Cheers,
>Brian
>>>