Should Python raise a warning for mutable default arguments?

Diez B. Roggisch deets at
Fri Aug 22 16:49:46 CEST 2008

Steven D'Aprano schrieb:
> 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.

I just suggested a documentation enhancement in one of the plethora of 
threads... so I'm certainly +1 for any enhancement in this area.


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