Result of ``a is b''
peter at engcorp.com
Tue Mar 16 22:07:31 CET 2004
Axel Boldt wrote:
> David MacQuigg <dmq at gain.com> wrote
>>>>>x = 'akdijfkdienlskfi'
>>>>>y = 'akdijfkdienlskfi'
>>>>>x is y
>>>>>x = 'a b'
>>>>>y = 'a b'
>>>>>x is y
> Wow. So it seems that the action of "is" on immutables is unspecified
> and implementation dependent, thus useless to the programmer.
Absolutely not! "is" does what it does, and the definition of what it
does is very explicit. It checks *identity*, not equality. In both
examples above, the code is semantically meaningless because you are
checking identity on two things which you just created at the command
line, out of thin air. They may or may not be identical, but whether
they are or not has no importance.
I'm not being very clear, but what I mean is that "is" should be used
when *identity* is important to you. For example, you have a particular
object and you want to check whether a method call returns that
particular object, perhaps as a "sentinel" value. "is" is what you want
here, because you are checking whether *that specific object* is being
returned, not whether the returned item equals that object in value.
There are very good (and correct) times to use "is", but the trivial
examples above aren't them...
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