Why don't optional mutable objects show up in vars(func)?

[Georg Brandl]

dannycolligan at gmail.com wrote:
> So I just got bitten by the "don't use a mutable object as an optional
> argument" gotcha.  I now realize that for this function:
> 
>>>> def func(x, y=[]):
> ...     y.append(x)
> ...     print y
> ...
> 
> y is initialized when the function is imported, not when the function
> is executed.  However, if this is the case, then why is y not showing
> up as an attribute of func?
> 
>>>> vars(func)
> {}
>>>> dir(func)
> ['__call__', '__class__', '__delattr__', '__dict__', '__doc__',
> '__get__', '__getattribute__', '__hash__', '__init__', '__module__',
> '__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
> '__setattr__', '__str__', 'func_closure', 'func_code', 'func_defaults',
> 'func_dict', 'func_doc', 'func_globals', 'func_name']
> 
> I'm using Python 2.4.3, if that is at all relevant.  Thanks in advance
> for any help.

y is not an attribute of func, it's a default parameter value and as such
stored in func_defaults:

 >>> def f(x=1):
...     pass
...
 >>> print f.func_defaults
(1,)
 >>>

Georg