Mutable objects inside tuples - good or bad?

[Chris Angelico]

On Tue, Apr 8, 2014 at 1:26 AM, Paul Kölle <pkoelle at gmail.com> wrote:
> It seems a tuple's immutability is debatable, or is this another instance of
> the small-integer-reuse-implementation-detail-artifact?
>
> Python 2.6.6 (r266:84292, Dec 26 2010, 22:31:48)
> [GCC 4.4.5] on linux2
>
> Type "help", "copyright", "credits" or "license" for more information.
>>>> a = ([1,2],[3,4])
>>>> b = a
>>>> a is b
> True
>>>> a == b
> True
>>>> c = (1,2,3)
>>>> d = (1,2,3)
>>>> c is d
> False
>>>> c == d
> True

That's nothing to do with mutability or reuse. With a and b, you
assigned one to be the same as the other, so they are by definition
identical (and equal; tuples assume that identity implies equality,
even though that may not be true of their elements). With c and d, you
assigned separate tuples, so they're allowed to be separate objects.
I'm not sure if they're allowed to be constant-folded, but CPython
apparently isn't doing so. They are still equal, though; they contain
equal elements, ergo they are equal. (Note that (1, 2, 3) and (1.0,
2.0, 3.0) are equal, but they obviously can't be identical any more
than "1 is 1.0" can ever be True.)

ChrisA