On Aug 22, 10:42 am, Steven D'Aprano <[EMAIL PROTECTED]
cybersource.com.au> wrote:
> Sometimes it seems that barely a day goes by without some newbie, or not-
> so-newbie, getting confused by the behaviour of functions with mutable
> default arguments. No sooner does one thread finally, and painfully, fade
> away than another one starts up.
>
> I suggest that Python should raise warnings.RuntimeWarning (or similar?)
> when a function is defined with a default argument consisting of a list,
> dict or set. (This is not meant as an exhaustive list of all possible
> mutable types, but as the most common ones that I expect will trip up
> newbies.) The warning should refer to the relevant FAQ or section in the
> docs.
>
> What do people think?
-1
There's nothing questionable about using a mutable default argument,
as long as you don't mutate it. Python shouldn't raise a warning just
because something *might* be due to a misunderstanding.
I usefully use mutable default arguments all the time. Most commonly
in situations like this:
def f(data,substitutions = {}):
...
name = data['name']
obj.name = substitutions.get(name,name)
...
-0 on adding a warning that's disbaled by default.
Carl Banks
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