Q: List and Dicts as default args

Christian Tismer tismer at tismer.com
Mon Apr 3 11:43:39 CEST 2000


Stefan Migowsky wrote:
> 
> Hi,
> 
> I was just wondering how to handle lists and dictionaries in
> a simple way as default arguments to functions. Since following
> strange behaviour occured :

[Stefan, exploring the side effects of mutable default parameters]

> This behaviour only occurs with dictionaries and list. All other
> types are "well" behaved. I could try the following but that
> doesn't look so nice:
> 
> def f(Index,List = None, Dict = None):
>     if not List: List = []
>     List.append(1)
>     List.append(2)
>     print List
>     if not Dict: Dict = {}
>     Dict[Index] = Index
>     print Dict

The above is correct. When supplying default args, you need
to take care of possible side effects. Lists and dicts are
mutable objects, and the default assignment is done at
compile time.
You can use "real" values as defaults if:
- they are immutable, like ints, strings, tuples
- they are only used for lookup (not changed)
- you *want* to change state permanently

In all other cases, use something like you did above.

ciao - chris

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