Calvin Spealman wrote:
There are a few problems here. Deep copy of the originally created default argument can be expensive For immutables, yes, it would make an unnecessary copy, though it seems less likely to have complex immutable objects than complex mutable objects as defaults.
and would not work in any useful way with non-literals as defaults, such as function calls or subscript lookups or even simple attributes. Good point, I hadn't considered that.
If any solution is possible, it would require a way to differentiate between mutable and immutable objects, and evaluate the immutables for every call. That doesn't seem to consider the other cases you just mentioned, though I see merit in your idea. How about there be some way to specify that a default argument value be re-evaluated at every call it's required for, while all other arguments have existing semantics?
Hypothetical syntax: def foo(a, b=4, c=<Bar([q, w, e], 7)>): #b's default value is evaluated exactly once, at definition-time #c's default value is evaluated every time foo() is called and no value for c is given Where the <>s indicate these special semantics.
Any solution would be an unjust addition of expense to defaults. That sounds a bit like premature optimization to me.
- Chris Rebert