Should Python raise a warning for mutable default arguments?

Steven D'Aprano steve at
Fri Aug 22 16:42:43 CEST 2008

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 

What do people think?


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