Bizarre behavior with mutable default arguments

[Steven D'Aprano]

On Sat, 29 Dec 2007 09:50:53 -0800, bukzor wrote:

> I've found some bizzare behavior when using mutable values (lists,
> dicts, etc) as the default argument of a function.

This FAQ is so Frequently Asked that I sometimes wonder if Python should, 
by default, print a warning when it compiles a function with a list or 
dict as as default value.

There's precedence for such a thing: the sum() built-in (un)helpfully 
raises an exception if you try to use it on strings.

I say unhelpfully because the one time I wanted to use sum() on strings 
was specifically to demonstrate the difference between O(n**2) behaviour 
and O(n). I was quite put out that Python, which normally allows you to 
shoot yourself in the foot if you insist, was so unnecessarily protective 
in this case. Give me a warning, if you wish, but don't stop me.



-- 
Steven