On 03/10/2013 02:39, Dave Angel wrote:
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.
The key point here is that the tuple is immutable, including its items.
--
https://mail.python.org/mailman/listinfo/python-list
