"is not" operator?

[MRAB]

sturlamolden wrote:
> What happens here? Does Python (2.6.5) have an "is not" operator?
> 
>>>> a = 5
>>>> print (a is not False)
> True
>>>> print (a is (not False))
> False
>>>> print (not (a is False))
> True
> 
> It seems "y is not x" fits well with spoken English, but it is also a
> bit surprising that "y is not x" does not mean "y is (not x)" but "not
> (y is x)". Why does Python reorder is and not operators, and what are
> the formal rules for this behavior?
> 
In English the negative comes after the verb (or the auxiliary, if there
is one), so:

     x is not y

If you wanted to abbreviate:

     x is in y

the natural result would be:

     x in y

and the opposite:

     x is not in y:

would be abbreviated to:

     x not in y

The resulting inconsistency in Python is somewhat unfortunate, but in
practice it's not a problem because:

     not y

returns either False or True, and how often would you write:

     x is False

or:

     x is True

?