On 10.02.13 13:37, Steven D'Aprano wrote:
> Unfortunately, Python has a minor design flaw. One of the most common
> use-cases for sets is for membership testing of literal sets:
>
> def example(arg):
> if arg in {'spam', 'ham', 'eggs', 'cheese'}:
> ...
>
> Unfortunately, set literals create *mutable* sets, not frozensets. That
> means that the compiler wastes time and memory creating am over-allocated
> mutable set object. If set literals created immutable frozensets, there
> would be some nice opportunities for the compiler to optimize this
> use-case.
CPython 3.2 optimizes this special case and creates a constant frozenset.
$ echo "arg in {'spam', 'ham', 'eggs', 'cheese'}" | python3.2 -m dis -
1 0 LOAD_NAME 0 (arg)
3 LOAD_CONST 5 (frozenset({'cheese', 'eggs', 'ham',
'spam'}))
6 COMPARE_OP 6 (in)
9 POP_TOP
10 LOAD_CONST 4 (None)
13 RETURN_VALUE
--
http://mail.python.org/mailman/listinfo/python-list
