Tail recursion to while iteration in 2 easy steps

[Dave Angel]

On 2/10/2013 21:24, Steven D'Aprano wrote:

> On Wed, 02 Oct 2013 18:17:06 -0400, Terry Reedy wrote:
>
>> CPython core developers have be very conservative about what
>> tranformations they put into the compiler. (1,2,3) can always be
>> compiled as a constant, and so it is. [1,2,3] might or might not be a
>> constant, depending on the context, and no attempt is made to analyze
>> that.
>
> The first sentence of this is correct. The next two don't quite make 
> sense to me, since I don't understand what you mean by "constant" in this 
> context. I *think* you might be referring to the LOAD_CONST byte-code, 
> which in Python 3.3 understands tuples like (1, 2, 3), but not lists. So 
> a literal (1, 2, 3) gets created at compile-time with a single LOAD_CONST 
> call:
>
> py> from dis import dis
> py> dis(compile("x = (1, 2, 3)", '', 'exec'))
>   1           0 LOAD_CONST               4 ((1, 2, 3))
>               3 STORE_NAME               0 (x)
>               6 LOAD_CONST               3 (None)
>               9 RETURN_VALUE
>
>
> while a literal [1, 2, 3] does not:
>
>

The difference is that a tuple can be reused, so it makes sense for the
comiler to produce it as a const.  (Much like the interning of small
integers)  The list, however, would always have to be copied from the
compile-time object.  So that object itself would be a phantom, used
only as the template with which the list is to be made.


-- 
DaveA