Searching for patterns on the screen

[Paul McGuire]

"John Machin" <sjmachin at lexicon.net> wrote in message 
news:1158333401.286945.147870 at b28g2000cwb.googlegroups.com...
>
> Paul McGuire wrote:
>> "Jerry Hill" <malaclypse2 at gmail.com> wrote in message
>> news:mailman.125.1158327884.10491.python-list at python.org...
>> > Hello all,
>> As far as working just in Python, you could remove the tuple unpacking
>> inside removeColor, and shorten it to just:
>>
>> def removeColor(rgb):
>>     return rgb==(0,0,0)
>>
>> or even better:
>>
>> BLACK = (0,0,0)
>> def removeColor(rgb):
>>     return rgb==BLACK
>>
>> By defining BLACK once and just referring to it by name, you also avoid
>> dynamically constructing the (0,0,0) tuple in every call.
>
> Bzzzzzt. It's not dynamically constructed. Its a constant (in 2.4 at
> least)
>
> | >>> BLACK=(0,0,0)
> | >>> def func1(rgb):
> | ...    return rgb==BLACK
> | ...
> | >>> def func2(rgb):
> | ...    return rgb==(0,0,0)
> | ...
> | >>> import dis
> | >>> dis.dis(func1)
> |   2           0 LOAD_FAST                0 (rgb)
> |               3 LOAD_GLOBAL              1 (BLACK)
> |               6 COMPARE_OP               2 (==)
> |               9 RETURN_VALUE
> | >>> dis.dis(func2)
> |   2           0 LOAD_FAST                0 (rgb)
> |               3 LOAD_CONST               2 ((0, 0, 0))
> |               6 COMPARE_OP               2 (==)
> |               9 RETURN_VALUE
> | >>>
>
>
> C:\junk>python -mtimeit -s"BLACK=(0,0,0);rgb=(1,1,1)" "rgb==BLACK"
> 1000000 loops, best of 3: 0.129 usec per loop
>
> C:\junk>python -mtimeit -s"rgb=(1,1,1)" "rgb==(0,0,0)"
> 1000000 loops, best of 3: 0.127 usec per loop
>

Hunh!  It sure seems like I recently had to do something like this, and the 
value was rebuilt every time.  Perhaps I was building a "constant" list 
instead of a tuple...

>>> def fn(lst):
...    return lst == [1,1,1]
...
>>> import dis
>>> dis.dis(fn)
  2           0 LOAD_FAST                0 (lst)
              3 LOAD_CONST               1 (1)
              6 LOAD_CONST               1 (1)
              9 LOAD_CONST               1 (1)
             12 BUILD_LIST               3
             15 COMPARE_OP               2 (==)
             18 RETURN_VALUE
>>>

So tuples can be inlined as constants, but lists will be built on the fly.

-- Paul