On 1/24/07, Jan Kanis <jan.kanis@phil.uu.nl> wrote:
I don't like new syntax for something like this, but I think the default argument values can be fixed with semantic changes (which should not break the most common current uses):
What I think should happen is compile a function like this
def popo(x=[]): x.append(666) print x
as if it had read
def popo(x=__default_argument_marker__): if x == __default_argument_marker__: x = [] x.append(666) print x
How is this different from the x=None idiom of today? def f(inlist=None): if inlist is None: inlist=[] The if (either 2 lines or against PEP8) is a bit ugly, but Calvin pointed out that you can now write it as def f(inlist=None): inlist = inlist if (inlist is not None) else [] I see below that you give it slightly different semantics, but I'm not entirely sure how to tell when those different semantics should apply (always? when the variable name is marked with __*__? When a specific non-None singleton appears?), or why you would ever want them.
When you use variables as default value instead of literals, I think most of the time you intend to have the function do something to the same object the variable is bound to, instead of the function creating it's own copy every time it's called. This behaviour still works with these semantics:
a = [] def foo(x=[[],a]): x[0].append(123) x[1].append(123) print x foo() [[123], [123]] foo() [[123], [123, 123]] foo() [[123], [123, 123, 123]]
So you're saying that x[1] should be persistent because it (also) has a name (as 'a'), but x[0] should be recreated fresh on each call because it doesn't? -jJ