[Python-ideas] Default arguments in Python - the return
tjreedy at udel.edu
Sat May 9 01:34:42 CEST 2009
Pascal Chambon wrote:
> I'm surely not original in any way there, but I'd like to put back on
> the table the matter of "default argument values".
There have been two proposals:
1. Evaluate the expression once, store the result, and copy on each
- Nearly always not needed.
- Not always possible.
2. Store the expression and evaluate on each function call (your
- The result may be different for each function call, and might raise an
- This is the job of the suite !!!!!!!!!!!!!!!!
- Which is to say, run-time code belongs in the function body, not the
> And no one seemed to enjoy the possibilities of getting "potentially
> static variables" this way.
You did not search hard enough.
> Static variables are imo a rather bad idea,
So you want to take them away from everyone else. I think *that* is a
rather bad idea ;-). No one is forcing you to use them.
> On the other hand, when people write "def func(mylist=):", they
> basically DO want a fresh new list at each call,
Maybe, maybe not.
> be it given by the caller or the default argument system.
> So it's really a pity to need tricks like
> >/ def f(a, L=None):
> />/ if L is None:
> />/ L = 
Or don't supply a default arg if that is not what you really want.
Putting call time code in the function body is not a trick.
> /to get what we want (and if None was also a possible value ?
__none = object()
def(par = __none):
if par == __none: ...
as had been posted each time this question has been asked.
> I guess this will mean some overhead during function call,
I absolutely guarantee that this will. Functions calls are expensive.
Adding a function call for each default arg (and many functions have
more than one) multiplies the calling overhead.
> so this might become another issue.
Is and always has been.
Terry Jan Reedy
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