[Python-Dev] redefining is
Shane Hathaway
shane at zope.com
Tue Mar 23 17:18:37 EST 2004
Tim Peters wrote:
> [Andrew Koenig]
>
>>... and I wouldn't mind it if there were a way of testing for
>>substitutability that were as easy to spell as "is" -- in fact, I
>>wouldn't mind if it were spelled "is" even though I realize it's
>>probably impractical to do so.
>
>
> It's definitely impractical to do so in the 2.3 line. Looking beyond that,
> I'm not sure we have real use cases for substitutability. It seemed to boil
> down to:
>
> x substitutable y =
> x is y if x is mutable, or a mutable object is reachable
> from x, or y is mutable, or a mutable object is
> reachable from y
> x == y otherwise (x and y are immutable, and only immutable
> objects are reachable from them)
As I understood it, 'b' and 'c' are supposed to be substitutable in the
following example, since 'b' and 'c' will have the same value even after
'a' changes:
a = []
b = (a,)
c = (a,)
Shane
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