On Mon, 07 Apr 2014 20:16:05 -0400, Terry Reedy wrote:
> On 4/7/2014 11:26 AM, Paul Kölle wrote:
>
>> >>> c = (1,2,3)
>> >>> d = (1,2,3)
>> >>> c is d
>> False
>
> An implementation would be allowed to make that True, as it does for
> small ints and short strings that could be identifiers.
And indeed, that happens in at least one circumstance in Python 3.3:
py> a, b = [(1, 2, 3) for _ in range(2)]
py> a is b
True
But:
py> x = 3
py> a, b = [(1, 2, x) for _ in range(2)]
py> a is b
False
As Terry knows, but for the benefit of others who may not, the re-use of
objects leading to object identity ("a is b") is an implementation detail
which *cannot be relied on*. It can change without notice, and is not a
promise of the language.
> >>> a = 'one'
> >>> b = 'one'
> >>> a == b; a is b
> True
> True
In this case, it is a promise of the language that a will equal b: a and
b must be bound to strings with the same value. But an implementation
detail whether Python creates two strings, both with value "one", or just
a single string, and uses it for both a and b.
--
Steven D'Aprano
http://import-that.dreamwidth.org/
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