Difference between 'is' and '=='

[Terry Reedy]

"Clemens Hepper" <ethrandil at gmx.net> wrote in message 
news:e08qgg$nt$1 at news2.open-news-network.org...

> It's strange: python seem to cache constants from 0 to 99:

The Python specification allows but does not require such behind-the-scenes 
implementation optimization hacks.  As released, CPython 2.4 caches -5 to 
99, I believe.  In 2.5, the upper limit was increased to 256.  The limits 
are in a pair of #define statements in the int object source file.  Anyone 
who compiles from source can adjust as desired (though the corresponding 
test will fail unless also adjusted ;-).

I think the visibility of this implementation detail from Python code is an 
example of a leaky abstraction.  For more, see
http://www.joelonsoftware.com/articles/LeakyAbstractions.html

Terry Jan Reedy