On Sat, 26 Nov 2011 22:20:36 +0100, candide wrote:
> In which cases should we use the is() function ? The is() function
> compares identity of objects rather than values so I was wondering in
> which circumstances comparing identities of objects is really vital.
`is` is not a function. It is a keyword and an operator.
You should always use `is` when you intend to test for object identity,
and never use `is` when you do not intend to test for object identity.
For example:
TASK: check whether a number is equal to 42.
# WRONG don't do this
if x is 42: ...
# RIGHT use equality instead
if x == 42: ...
Object identity is the wrong solution here, because you cannot control
whether Python will re-use the same object for every instance of 42, or
different objects each time.
TASK: check whether an object is a specific sentinel value, and no other
value, even if it happens to compare equal to the sentinel. The most
common sentinel is the singleton None.
# WRONG don't do this
if x == None: ...
# RIGHT use is instead
if x is None: ...
Use of equality is inappropriate, because it tests whether the object
compares equal to None. Although there are no built-ins that compare
equal to None, there could be any number of custom objects that do, and
so your code contains a bug: you intend to branch *only* on None, but
might branch on some other object by mistake.
> Examining well reputated Python source code, I realize that is()
> function is mainly used in the following set form :
>
> spam is None
>
> But how much "spam is None" is different from "spam == None" ?
Even if you can guarantee that your code base does not contain any object
which compares equal to None except for None itself (and how would you do
that? a full audit of every line of code in every library you use?), the
use of `is` should be preferred because it signals your intention much
better.
If your intention is to accept arbitrary objects which compare equal to
None, than by all means use == for your comparison. But normally the
intention is to accept None, and nothing else.
> is() function makes comparaison of (abstract representation of) adresses
> of objects in memory. Comparing addresses of objects is a low level
> feature performed by low level langages such as C but seldom needed in
> high level languages like Python, isn'it ?
That is correct. You probably should rarely use `is`. Apart from testing
for None, use of `is` should be rare.
--
Steven
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