[Python-ideas] Default arguments in Python - the return - running out of ideas but...

[spir]

Le Fri, 15 May 2009 18:32:32 +0900,
"Stephen J. Turnbull" <stephen at xemacs.org> s'exprima ainsi:

> Chris Rebert writes:
> 
>  > +1 on throwing a ValueError for non-hash()-able (and thus probably
>  > mutable) default argument values. It's by no means perfect since
>  > objects are hash()-able by default using their ID, but it would at
>  > least help in the frequent "well-behaved mutable container object"
>  > cases.  
> 
> -1
> 
> nonet_default_options = { 'dryrun': False, 'verbose': False }
> net_default_options = { 'dryrun': True, 'verbose': True }
> 
> def command1(options=nonet_default_options):
>     pass
> 
> def command2(options=net_default_options):
>     pass
> 
> def command3(options=net_default_options):
>     pass

Yop. The main issue is not that a default be mutable -- and the example above is perfect to show this -- but that it is changed in the func body, this change affects the default back and propagates to later calls.
/This/ could be used as criteria for a warning (or error), but probably it's much more costly to detect (parameter name on left side of "=").

Denis
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