Stop initializing an argument with an empty list

The user needs to be very careful not to append anything to the lists if no subscriber/method was given. Otherwise the appended stuff will be class-wide accessible.

```python
class A(object):

    def __init__(self, a=[]):
        self.a = a

instance_1 = A()
instance_1.a   # []
instance_1.a.append(1)

instance_2 = A()
instance_2.a  # [1]
```