Re: Python "is" behavior

[Jean-Paul Calderone]

On Fri, 20 Jun 2008 10:07:56 -0700 (PDT), George Sakkis <ge***********@ gmail.comwrote:

>On Jun 20, 12:45 pm, michalis.avr... @gmail.com wrote:

>On Jun 20, 9:42 am, George Sakkis <george.sak...@ gmail.comwrote:

On Jun 20, 12:31 pm, michalis.avr... @gmail.com wrote:

I am not certain why this is the case, but...

>a = 256
b = 256
a is b

True

>a = 257
b = 257
a is b

False

Can anyone explain this further? Why does it happen? 8-bit integer
differences?

No, implementation-dependent optimization (caching). For all we know,
the next python version may cache up to 1024 or it may turn off
caching completely; do not rely on it. More generally, do not use 'is'
when you really mean '=='.

George

Thank you George. I am very curious about some of these internal
Python things that I keep stumbling upon through friends. And thank
you for all the help!

As far it's plain curiosity it's ok, but it's a small implementation
detail you shouldn't rely on. There's nothing magic about 256, just
the size decided for 2.5. If you tried it on 2.4 you'd get:

Python 2.4.2 (#1, Mar 8 2006, 13:24:00)
[GCC 3.4.4 20050721 (Red Hat 3.4.4-2)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.

>>>a=99
b=99
a is b

True

>>>a=100
b=100
a is b

False

I was more surprised by the following:

Python 2.5.1 (r251:54863, May 8 2007, 14:46:30)
[GCC 3.4.6 20060404 (Red Hat 3.4.6-3)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.

>>>a= 123456; b=123456; a is b

True

For some reason, stacking multiple statements reuses the same object.

This is because using the ";" puts the statements into the same compilation
unit as each other. So secretly an integer object is created for 123456
and then a and b are both given a reference to it. This is a different
mechanism than the other case, where the builtin integer cache causes the
literal 100 to refer to the same object each time it is evaluated.

Jean-Paul