Should constants be introduced to Python?

[Chris Angelico]

On Fri, Nov 17, 2017 at 9:27 AM, Terry Reedy <tjreedy at udel.edu> wrote:
> CPython, at least, already has anonymous constant objects.  Number and
> string literals are turned into objects when parsed.  I presume that all
> implementations do this.  Some constant expressions are replaced by
> (constant) objects during compiling, instead of when running.  For instance
> "n = 10 + 3' will run as 'n = 13', and "for lang in ('C', 'Java', 'Python')
> will have the tuple pre-computed (whereas "for lang in ['C', 'Java',
> 'Python] will have to construct the list each time run).

Just to thoroughly confuse people, CPython can actually optimize one
into the other...

>>> def f():
...  for lang in ['C', 'Java', 'Python']:
...   print(lang)
...
>>> dis.dis(f)
  2           0 SETUP_LOOP              20 (to 22)
              2 LOAD_CONST               4 (('C', 'Java', 'Python'))
              4 GET_ITER
        >>    6 FOR_ITER                12 (to 20)
              8 STORE_FAST               0 (lang)

  3          10 LOAD_GLOBAL              0 (print)
             12 LOAD_FAST                0 (lang)
             14 CALL_FUNCTION            1
             16 POP_TOP
             18 JUMP_ABSOLUTE            6
        >>   20 POP_BLOCK
        >>   22 LOAD_CONST               0 (None)
             24 RETURN_VALUE

Since there's no way to get a reference to the underlying list during
iteration (which would allow you to mutate it), Python can happily
pretend that you used a tuple. Similarly with membership tests:

>>> dis.dis("x in [1,2,3,4,5]")
  1           0 LOAD_NAME                0 (x)
              2 LOAD_CONST               5 ((1, 2, 3, 4, 5))
              4 COMPARE_OP               6 (in)
              6 RETURN_VALUE
>>> dis.dis("x in {1,2,3,4,5}")
  1           0 LOAD_NAME                0 (x)
              2 LOAD_CONST               5 (frozenset({1, 2, 3, 4, 5}))
              4 COMPARE_OP               6 (in)
              6 RETURN_VALUE

However, outside of these special optimization cases, you're right
that a tuple is a constant and a list gets built at run time.

ChrisA