Question

such as

In [9]: dis.disassemble(compile("s = '123' + '456'", "<execfile>", "exec"))
  1           0 LOAD_CONST               3 ('123456')
              3 STORE_NAME               0 (s)
              6 LOAD_CONST               2 (None)
              9 RETURN_VALUE 

I want to know, when does python combine the constant string as the CONST.
If possible, please tell me which source code about this at cpython(whatever 2.x, 3.x).

Answer 1

It happens whenever the combined string is 20 characters or fewer.

The optimization occurs in the peephole optimizer. See line 219 in the fold_binops_on_constants() function in Python/peephole.c: http://hg.python.org/cpython/file/cd87afe18ff8/Python/peephole.c#l149