eryksun wrote:
> On Wed, Oct 24, 2012 at 3:24 PM, Alan Gauld <alan.ga...@btinternet.com> wrote:
> > On 24/10/12 18:49, eryksun wrote:
> >
> >> Using "in ['25', '26']" also checks a compiled tuple constant:
> >>
> >> >>> compile("x in ['25', '26']", '', 'eval').co_consts
> >> ('25', '26', ('25', '26'))
> >>
> >> 3.x adds frozenset support:
> >>
> >> >>> compile("x in {'25', '26'}", '', 'eval').co_consts
> >> ('25', '26', frozenset({'25', '26'}))
> >
> >
> > I confess I don't know what that means!
> > And if I don't I doubt if the OP will either.
> > Can you explain the above in English please?
>
> Sorry, I was showing that the compiler (at least for CPython) special
> cases "in" and "not in" comparisons when the right-hand operand is a
> list literal of constants. Instead of going to the trouble of building
> a list every time the code runs, it uses a tuple that's stored in the
> code object's co_consts attribute.
>
> In Python 3, this optimization was extended to set literals constants.
> I guess I should show that in more detail by disassembling a function
> instead of manually compiling code.
>
> First in Python 2.7.3:
>
> >>> def func(x):
> ... return x in {1, 2, 3}
>
> >>> dis.dis(func)
> 2 0 LOAD_FAST 0 (x)
> 3 LOAD_CONST 1 (1)
> 6 LOAD_CONST 2 (2)
> 9 LOAD_CONST 3 (3)
> 12 BUILD_SET 3
> 15 COMPARE_OP 6 (in)
> 18 RETURN_VALUE
>
> Each time the function is called it has to build a set (BUILD_SET)
> from the constants 1, 2, and 3. Compare that to Python 3.2.3:
>
> >>> dis.dis(func)
> 2 0 LOAD_FAST 0 (x)
> 3 LOAD_CONST 4 (frozenset({1, 2, 3}))
> 6 COMPARE_OP 6 (in)
> 9 RETURN_VALUE
>
> Here the compiler stored a frozenset in the code object. So it only
> has to load the frozenset on the stack instead of building a new set
> each time the function is called.
>
> compile:
> http://docs.python.org/py3k/library/functions.html#compile
>
> frozenset:
> http://docs.python.org/py3k/library/functions.html#func-frozenset
>
> code objects:
> http://docs.python.org/py3k/reference/datamodel.html#index-51
>
> The optimization is in peephole.c, PyCode_Optimize/tuple_of_constants:
>
> http://hg.python.org/cpython/file/3d0686d90f55/Python/peephole.c#l469
> http://hg.python.org/cpython/file/3d0686d90f55/Python/peephole.c#l25
Thank you for the very detailed (and referenced) clarification.
I understood the gist of what you were trying to say in your
initial post, but this certainly helps fill in the details.
Do you happen to know offhand if there is a difference between
`in <list>` vs. `in <tuple>` vs. `in <set>`?
Ramit Prasad
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