Re: Does the "is" operator only matter for mutable object?

On Fri, Mar 4, 2011 at 10:13 PM, Steven D'Aprano <[email protected]> wrote:
> The operators is and is not compare whether two objects are really the same
> object; this usually only matters for mutable objects like lists, or for
> testing against singletons like None.

I think having text about mutability here is confusing.  Consider:

    >>> x = []
    >>> a = x,
    >>> b = x,

Here, a and b are different immutable objects, though they'll always
be equal.  "is" and "==" will produce different results; whether that
matters is context-dependent.  "==" will always return the *same*
result as the list in x is mutated.

Further:

    >>> c = [],
    >>> d = [],

c and d are different objects, and their equality will vary as the
contained lists are mutated.

a, b, c and d are all immutable.

So let's not condition the explanation with vague hints about when
testing identity makes sense.  I expect it will lead to more confusion
than it will avoid.


  -Fred

--
Fred L. Drake, Jr.    <fdrake at acm.org>
"A storm broke loose in my mind."  --Albert Einstein
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