[Python-3000] default argument surprises

[Chris Monson]

On Tue, Aug 26, 2008 at 9:10 PM, Bruce Leban <bruce at leapyear.org> 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
>
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