conventions/requirements for 'is' vs '==', 'not vs '!=', etc

[Duncan Booth]

John Salerno <johnjsal at NOSPAMgmail.com> wrote:

>>>> a = 'this is longer'
>>>> b = 'this is longer'
>>>> a == b
> True
>>>> a is b
> False
>>>> 
> 
> In the above example, Python has created only one string called
> 'hello' and both x and y reference it. However, 'this is longer' is
> two completely different objects. 

That is true when run interactively, but the behaviour changes again if you 
run it as a script:

C:\Temp>type t.py
a = 'this is longer'
b = 'this is longer'
print a is b

C:\Temp>t
True

In short, two equal strings may or may not be identical and any code which 
makes assumptions based on observed behaviour is broken. If you want to be 
able to test identity on strings safely then use the 'intern()' builtin to 
get repeatable behaviour.

-- 
Duncan Booth http://kupuguy.blogspot.com