Should Python raise a warning for mutable default arguments?

Diez B. Roggisch deets at
Fri Aug 22 21:14:57 CEST 2008

Emile van Sebille schrieb:
> Steven D'Aprano 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
> People that have worked through the tutorial, something everyone should 
> do when they're starting out, will find this explicitly discussed.  See
> People that just skim the surface get stung -- sorry.

But obviously enough, it's not emphazized enough. Even if the 
interpreter isn't touched, at least the docs should be.


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