[Python-ideas] "While" suggestion

[Steven Bethard]

On Tue, Jul 29, 2008 at 3:19 PM, Terry Reedy <tjreedy at udel.edu> wrote:
> Roman Susi wrote:
>> Python already has dict/list literals + list comprehensions.
>
> Better to think that Python has generator expressions which can be used in
> list, set, and dict comprehensions (the latter two in 3.0 and maybe 2.6).

You probably don't want to think about it that way - a list/set/dict
comprehension does not actually create a generator. Instead, it
basically just inlines the equivalent for loop. Note that there's no
YIELD_VALUE opcode for the comprehensions:

>>> def dict_comp(x, y):
...     return {z:x for z in y}
...
>>> def dict_gen(x, y):
...     return dict((z, x) for z in y)
...
>>> dis.dis(dict_comp.__code__.co_consts[1])
  2           0 BUILD_MAP                0
              3 DUP_TOP
              4 STORE_FAST               1 (_[1])
              7 LOAD_FAST                0 (.0)
        >>   10 FOR_ITER                17 (to 30)
             13 STORE_FAST               2 (z)
             16 LOAD_FAST                1 (_[1])
             19 LOAD_DEREF               0 (x)
             22 ROT_TWO
             23 LOAD_FAST                2 (z)
             26 STORE_SUBSCR
             27 JUMP_ABSOLUTE           10
        >>   30 RETURN_VALUE
>>> dis.dis(dict_gen.__code__.co_consts[1])
  2           0 LOAD_FAST                0 (.0)
        >>    3 FOR_ITER                17 (to 23)
              6 STORE_FAST               1 (z)
              9 LOAD_FAST                1 (z)
             12 LOAD_DEREF               0 (x)
             15 BUILD_TUPLE              2
             18 YIELD_VALUE
             19 POP_TOP
             20 JUMP_ABSOLUTE            3
        >>   23 LOAD_CONST               0 (None)
             26 RETURN_VALUE

Steve
-- 
I'm not *in*-sane. Indeed, I am so far *out* of sane that you appear a
tiny blip on the distant coast of sanity.
 --- Bucky Katt, Get Fuzzy