sameness/identity

Xavier Defrang xavier at perceval.net
Mon Oct 1 12:22:52 CEST 2001


On Mon, 1 Oct 2001, Markus Schaber wrote:

> Just to confuse you some more:
> 
> >>> a = 3
> >>> b = 3
> >>> a is b
> 1
> >>> a = 10000
> >>> b = 10000
> >>> a is b
> 0

Damn, this is weird. :)
After reading that, I wrote a small script :

<snip>
a = 0
b = 0
while a is b:
    a += 1
    b += 1
print id(a), a
print id(b), b
</snip>

Here's the output with Python 2.1:

<snip>
136907204 100
136907168 100
</snip>

That '100' definetly looks like some hardcoded limit, has anybody a clear
explanation for this?  Why the interpreter behaves differently for numbers
lower than 100 and the others?  I guess it's for optimization purpose
since most algorithms deal with small numerical values but I'd be really
happy if I knew what's really going on in here...

Regards,

Xavier Defrang
Perceval R&D Team
xavier at perceval.net

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