On Tue, Aug 26, 2008 at 9:10 PM, Bruce Leban <[EMAIL PROTECTED]> wrote:
> There are two issues** here: > > (1) def foo(L=[]): > the [] is a single list, not a new list created every time the function is > called. > > (2) def foo(L=list()): > the list() is evaluated once when the function is declared. > > I think (1) is easy to explain; I find (2) confusing. (**Yes, I realize > that these can be considered the same issue, but I'm speaking here as a > person writing python programs, not designing the language.) On the other > hand, consider this code: > > bar = 1 > def foo(arg=bar): > print(arg) > bar = 2 > foo() > > I would be very surprised if foo() printed 2 rather than 1. The difference > to my (programmer's) mind is that in this case bar looks like a simple value > and list() looks like a function call. So it's not quite that simple. At > least the way it works now is completely consistent and the code: > > def foo(arg=None): > arg = bar if arg is None else arg > arg2 = list() if arg is None else arg > > is unambiguous if clumsy. > > Now thinking as a language designer, C# has a ?? operator where A??B is > shorthand for (B if A is None else A) except you only have to write A once > and A is only evaluated once. A Pythonesque version of this would be just > "else": > > def foo(arg=None, arg2=None): > arg = arg else bar > arg2 = arg2 else list() > > And I think that metaphor is easy to read. Chains of else operators can be > useful: > > x = f() else g() else h() else 0 > Not a bad idea. Looks like the time machine is at work again: x = f() or g() or h() or 0 :-) - C > > > --- Bruce > > _______________________________________________ > Python-3000 mailing list > Python-3000@python.org > http://mail.python.org/mailman/listinfo/python-3000 > Unsubscribe: > http://mail.python.org/mailman/options/python-3000/shiblon%40gmail.com > >
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