[Python-Dev] redefining is

[Barry Warsaw]

On Fri, 2004-03-19 at 14:25, Michael Chermside wrote:

> I hear you, but I disagree. There is a good reason why the default
> implementation of '==' compares by object identity.
> 
> I don't care much whether we use "if obj is None" rather than
> "if obj == None", since the PARTICULAR case of comparing with None
> is mostly just an idiom. 

I think it's more than that.  Python says there's only one None object,
and when I'm testing for, e.g. the return value from dict.get(), I'm
specifically testing that the object returned is that singleton object. 
By using identity test, I'm avoiding any possibility that someone
slipped me a subversive (or buggy) object that happens to claim it's
equal to None when it is not None.

When I use "if obj is None" I really, truly want an identity test.  I
don't think == is appropriate for that.

> But if you have two different identity
> objects and want to compare them, then I would prefer
> 
>     if currentCustomer == desiredCustomer:
> 
> to
> 
>     if currentCustomer is desiredCustomer:

It depends on what you want compared.  If you're happy with any old
arbitrary claims of equality that the objects can make about themselves,
then fine.  If you're asking, "do I have the exact object that I want"
then it's not.

> However, either one works, and I won't gripe about your using 'is'
> if you prefer it. I _WOULD_ gripe if you overrode __eq__ to raise
> an exception in order to force me to use 'is' not '==' on your
> objects (not that you ever suggested doing so).

Hey, we're all consenting adults here (you high school kids, look away!
:).

-Barry