def qwe(lst, new = []):
    for el in lst:
        qwe(el) if isinstance(el, list) else new.append(el)
    return new
При таком раскладе получаю:
a = qwe([1, [2, 3, [4, 5]]])
print a
[1, 2, 3, 4, 5]

def qwe(lst):
    new = []
    for el in lst:
        qwe(el) if isinstance(el, list) else new.append(el)
    return new
a = qwe([1, [2, 3, [4, 5]]])
print a
[1]

def qwe(lst):
    global new
    new = []
    for el in lst:
        qwe(el) if isinstance(el, list) else new.append(el)
    return new
a = qwe([1, [2, 3, [4, 5]]])
print a
[4, 5]

>>> import inspect
>>> def some_func(a, b=[]):
...     b.append(a)
...     return b
... 
>>> inspect.getargspec(some_func)
ArgSpec(args=['a', 'b'], varargs=None, keywords=None, defaults=([],))

>>> some_func(1)
[1]
>>> some_func(2)
[1, 2]
>>> inspect.getargspec(some_func)
ArgSpec(args=['a', 'b'], varargs=None, keywords=None, defaults=([1, 2],))
>>>