def X(l=[]): weirdness. Python bug ?

[Andrew Lee]

Bart van Deenen wrote:
> Hi all.
> 
> I've stumbled onto a python behavior that I don't understand at all.
> 
> Python 2.5.2 (r252:60911, Jul 31 2008, 17:28:52) 
> 
> # function 
> def X(l=[]):
>    l.append(1)
>    print l
> 
> # first call of X
> X()
> [1]
> 
> #second call of X
> X()
> [1, 1]
> 
> Where does the list parameter 'l' live between the two successive calls of X(). 
> Why is it not recreated with an empty list?
> Is this correct behavior or is it a Python bug?
> Does anyone have any pointers to the language documentation where this behavior is described?
> 
> Thanks all
> 
> Bart van Deenen
> 

I happen to be reading about decorators at the moment:

from copy import deepcopy
def nodefault(myfunc):
     myfunc_defaults = myfunc.func_defaults
     def fresh(*args, **kwargs):
         myfunc.func_defaults = deepcopy(myfunc_defaults)
         return myfunc(*args, **kwargs)
     return fresh

@nodefault
def X(l=[]):
     l.append(1)
     print l

 >>> for i in range(1,6):
...     X()
...
[1]
[1]
[1]
[1]
[1]


Which is just a very fancy way of doing:
def X(l=[]):
     if l is None:
	l = []
     l.append(1)
     print l

* sound of two pennies *