Tail recursion to while iteration in 2 easy steps
Dave Angel
davea at davea.name
Wed Oct 2 21:39:24 EDT 2013
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
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