On Thu, Mar 12, 2009 at 11:41 AM, spir <denis.s...@free.fr> wrote: > Le Thu, 12 Mar 2009 11:13:33 -0400, > Kent Johnson <ken...@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 _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
