Poromenos' Stuff

Your function is wrong, it is not changing the object pointed to by arg, it is assigning "arg" to a new object, leaving original unchanged.

Try this:

>>> l = ["original value"]
>>> def func(arg):
... arg.append("value added by func")
...
>>> print l
['original value']
>>> func(l)
>>> print l
['original value', 'value added by func']
>>>