[Tutor] loop performance in global namespace (python-2.6.1)

[Kent Johnson]

On Thu, Mar 12, 2009 at 11:41 AM, spir <denis.spir at free.fr> wrote:
> Le Thu, 12 Mar 2009 11:13:33 -0400,
> Kent Johnson <kent37 at tds.net> s'exprima ainsi:
>
>> Because local name lookup is faster than global name lookup. Local
>> variables are stored in an array in the stack frame and accessed by
>> index. Global names are stored in a dict and accessed with dict access
>> (dict.__getitem__()).
>
> ? I thought this was mainly because a name has first to be searched (unsuccessfully) locally before a global lookup is launched.

I'm pretty sure, local names are hard-coded to the stack frame.
Non-local names must follow the lookup sequence (containing scope),
(global scope), (builtins).

> Also, are locals really stored in an array? How does lookup then proceed? Is it a kind of (name,ref) sequence?

Yes, they are really stored in an array. A reference from GvR himself:
"The Python "compiler" optimizes most function bodies so that for
local variables, no dictionary lookup is necessary, but a simple array
indexing operation is sufficient."
http://www.python.org/doc/essays/list2str.html

Local variables are accessed using the LOAD_FAST and STORE_FAST
opcodes. From http://docs.python.org/library/dis.html:
LOAD_FAST(var_num)¶
    Pushes a reference to the local co_varnames[var_num] onto the stack.

STORE_FAST(var_num)¶
    Stores Top Of Stack into the local co_varnames[var_num].

Looking at the OP's code:

In [1]:    def f1():
   ...:            counter = 0
   ...:        while counter < limit:
   ...:                counter += 1

In [2]:

In [3]: import dis

In [4]: dis.dis(f1)
  3           0 LOAD_CONST               1 (0)
              3 STORE_FAST               0 (counter)

  4           6 SETUP_LOOP              28 (to 37)
        >>    9 LOAD_FAST                0 (counter)
             12 LOAD_GLOBAL              0 (limit)
             15 COMPARE_OP               0 (<)
             18 JUMP_IF_FALSE           14 (to 35)
             21 POP_TOP

  5          22 LOAD_FAST                0 (counter)
             25 LOAD_CONST               2 (1)
             28 INPLACE_ADD
             29 STORE_FAST               0 (counter)
             32 JUMP_ABSOLUTE            9
        >>   35 POP_TOP
             36 POP_BLOCK
        >>   37 LOAD_CONST               0 (None)
             40 RETURN_VALUE

Kent