negative numbers are not equal...

[Dan Lenski]

On Fri, 15 Aug 2008 08:03:23 -0700, Carl Banks wrote:

> On Aug 14, 4:42 pm, Christian Heimes <li... at cheimes.de> wrote:
>> Integers
>> between -5 and +256 are singletons as are some other objects like
>> strings with one element or empty tuples.
> 
> Not quite.
> 
> Python 2.5.2 (r252:60911, May 28 2008, 08:35:32) [GCC 4.2.4 (Debian
> 4.2.4-1)] on linux2 Type "help", "copyright", "credits" or "license" for
> more information.
>>>> a = 'A'
>>>> b = "%c" % a
>>>> a
> 'A'
>>>> b
> 'A'
>>>> a is b
> False

Wow... wow wow.  How very odd.  That is one exception I did not expect, 
especially considering that string-formatting with a *literal* rather 
than a variable gives the opposite result.

>>>> a = 'A'
>>>> b = "%c" % 'A'
>>>> a
> 'A'
>>>> b
> 'A'
>>>> a is b
> True

>> You must not rely on the
>> optimization.
> 
> Good advice.

Indeed!  Corner cases like the above will bit you in the ass ;-)

Simple rule of thumb: use "is" when you really truly want to check if two 
symbols refer to the same object in memory.  If that's not what you 
*really* want to do, then don't use it!

Dan