On 01/09/2010 14:25, Lie Ryan wrote:
On 09/01/10 17:06, Stefan Behnel wrote:
MRAB, 31.08.2010 23:53:
On 31/08/2010 21:18, Terry Reedy wrote:
On 8/31/2010 12:33 PM, Aleksey wrote:
On Aug 30, 10:38 pm, Tobias Weber wrote:
Hi,
whenever I type an "object literal" I'm unsure what optimisation
will do
to it.
Optimizations are generally implentation dependent. CPython currently
creates numbers, strings, and tuple literals just once. Mutable literals
must be created each time as they may be bound and saved.
def m(arg):
if arg& set([1,2,3]):
set() is a function call, not a literal. When m is called, who knows
what 'set' will be bound to? In Py3, at least, you could write {1,2,3},
which is much faster as it avoids creating and deleting a list. On my
machine, .35 versus .88 usec. Even then, it must be calculated each time
because sets are mutable and could be returned to the calling code.
There's still the possibility of some optimisation. If the resulting
set is never stored anywhere (bound to a name, for example) then it
could be created once. When the expression is evaluated there could be
a check so see whether 'set' is bound to the built-in class, and, if it
is, then just use the pre-created set.
What if the set is mutated by the function? That will modify the global
cache of the set; one way to prevent mutation is to use frozenset, but
from the back of my mind, I think there was a discussion that rejects
set literals producing a frozen set instead of regular set.
[snip]
I was talking about a use case like the example code, where the set is
created, checked, and then discarded.
--
http://mail.python.org/mailman/listinfo/python-list
