On Thu, 11 Apr 2013, Timo wrote:
Op 11-04-13 12:41, w qj schreef:
I found this under Windows Python3
l="http://f/";
l[-1] is not '/'
False
and this under Linux Python3
l = "http://ff.f/";
l[-1]
'/'
l[-1] is not '/'
True
It's Looks like a python bug?
This looks like a "is not" versus "!=" thing. Someone (I think Steven
Apprano) posted a couple of days ago on this mailing list to only use "is"
and "is not" when comparing to None.
You're absolutely correct.
`is` compares the identity (basically id('/') == id('/') )
*sometimes* CPython will use a trick that caches smaller strings, so this
might work one time:
>>> x = '/'
>>> y = '/'
>>> x is y
True
But then the next time you do the same thing it could return False. Or on
a different OS. There's nothing (that I'm aware of) that will guarantee
either result in this case.
In the example case we were comparing that `l[-1]` referred to the same
spot in memory as the literal string `/`. And as the example showed,
sometimes it will be, other times it won't.
The takeaway is to use `is` when you want to compare identity, and `==`
when you want equaltiy. For example,
That car is *my* car.
The car I'm referring to is one specific car. But if I were to say...
My car is a Chevette.
That would be more like saying `car.model == 'Chevette'` - many different
cars may actually be a Chevette.
HTH,
Wayne
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