Python bug with [] as default value in functions?

[Duncan Booth]

[posted and mailed]

Dinu Gherman <gherman at darwin.in-berlin.de> wrote in 
<3AD5989D.E22E2CBF at darwin.in-berlin.de>:

> I noticed a very strange behaviour: I've got a recursive func-
> tion like this:
> 
>   foo(spam, eggs=[]):
>     ...
>     return foo(spam, eggs=eggs)
>     # the following is the same:
>     # return foo(spam, eggs)
> 
> where the default argument for eggs, the empty list, is *empty* 
> for the top-level call *only* when it is explicitly provided 
> with the function call like this:
> 
>   bar = foo(mySpam, eggs=[])
> 
This is documented behaviour. See the FAQ at 
http://www.python.org/doc/FAQ.html#6.25 for a description.

In general if you want a mutable object to be used as a default argument 
(and the function needs to modify the object), use None instead and create 
the object inside the function.