Usefulness of the "not in" operator

[Jussi Piitulainen]

Mel writes:

> Steven D'Aprano wrote:
> 
> > candide wrote:
> > 
> >> So what is the usefulness of the "not in" operator ? Recall what Zen of
> >> Python tells
> >> 
> >> There should be one-- and preferably only one --obvious way to do it.
> > 
> > And "not in" is the obvious way to do it.
> > 
> > 
> > "If the key is not in the ignition, you won't be able to start the car."
> > 
> > "If not the key is in the ignition, you won't be able to start the car."
> > 
> > 
> > Who like that second one speaks?
> 
> :)  
> "If the key is not in the ignition, you will be able to start the car, not."

Oh, be consistent.

"If not the key is in the ignition, not you will be able to start the car."

But both negations can be avoided by modus tollens.

"If you are able to start the car, the key is in the ignition."

And one could express "x not in s" as "(x in s) implies False" without
making the "not" explicit if "implies" was in the language. (I know
about <= but I also witnessed an unpleasant thread in another
newsgroup where people insisted that <= should not be defined for
truth values at all, and I also happen to like Python's "not in".)