preferring [] or () in list of error codes?

samwyse samwyse at gmail.com
Tue Jun 9 03:57:34 CEST 2009


On Jun 8, 7:37 pm, Carl Banks <pavlovevide... at gmail.com> wrote:
> On Jun 8, 4:43 pm, Ben Finney <ben+pyt... at benfinney.id.au> wrote:
> > m... at pixar.com writes:
> > > Is there any reason to prefer one or the other of these statements?
>
> > >         if e.message.code in [25401,25402,25408]:
> > >         if e.message.code in (25401,25402,25408):
>
> If you want to go strictly by the book, I would say he ought to be
> using a set since his collection of numbers has no meaningful order
> nor does it make sense to list any item twice.

As the length of the list increases, the increased speeds of looking
something up makes using a set makes more sense.  But what's the best
way to express this?  Here are a few more comparisons (using Python
3.0)...

>>> S=lambda x:x in set((25401,25402,25408))
>>> dis(S)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_GLOBAL              0 (set)
              6 LOAD_CONST               3 ((25401, 25402, 25408))
              9 CALL_FUNCTION            1
             12 COMPARE_OP               6 (in)
             15 RETURN_VALUE
>>> S=lambda x:x in{25401,25402,25408}
>>> dis(S)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               0 (25401)
              6 LOAD_CONST               1 (25402)
              9 LOAD_CONST               2 (25408)
             12 BUILD_SET                3
             15 COMPARE_OP               6 (in)
             18 RETURN_VALUE
>>> S=lambda x:x in{(25401,25402,25408)}
>>> dis(S)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_CONST               3 ((25401, 25402, 25408))
              6 BUILD_SET                1
              9 COMPARE_OP               6 (in)
             12 RETURN_VALUE

I conclude that using constructors is generally a bad idea, since the
compiler doesn't know if you're calling the builtin or something with
an overloaded name.  I presume that the compiler will eventually
optimize the second example to match the last, but both of them use
the BUILD_SET opcode.  I expect that this can be expensive for long
lists, so I don't think that it's a good idea to use set constants
inside loops.  Instead it should be assigned to a global or class
variable.



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