Python class vs instance attributes

example.py

>>> class Foo(object):
...    # Immutable objects are fine as class attrs.
...    mystr = 'a' 
...    myint = 1
...    mytpl = (1,)
...
...    # But mutable objects often surprise you.
...    mylst = []
...    mydct = {}
>>> foo = Foo()
>>> foo.__dict__
{}
>>> Foo.__dict__
dict_proxy({'__dict__': <attribute '__dict__' of 'Foo' objects>,
            '__doc__': None,
            '__module__': '__main__',
            '__weakref__': <attribute '__weakref__' of 'Foo' objects>,
            'mydct': {},
            'myint': 1,
            'mylst': [],
            'mystr': 'a',
            'mytpl': (1,)})

>>> # Here we simply make foo.mystr *point* to a new object 'b'.
>>> foo.mystr = 'b'
>>> foo.__dict__
{'mystr': 'b'}
>>> Foo.__dict__
dict_proxy({'__dict__': <attribute '__dict__' of 'Foo' objects>,
            '__doc__': None,
            '__module__': '__main__',
            '__weakref__': <attribute '__weakref__' of 'Foo' objects>,
            'mydct': {},
            'myint': 1,
            'mylst': [],
            'mystr': 'a',
            'mytpl': (1,)})

>>> # But here we are actually mutating the object that foo.mylst already points too.
>>> foo.mylst.append(2)
>>> foo.__dict__
{'mystr': 'b'}
>>> Foo.__dict__
dict_proxy({'__dict__': <attribute '__dict__' of 'Foo' objects>,
            '__doc__': None,
            '__module__': '__main__',
            '__weakref__': <attribute '__weakref__' of 'Foo' objects>,
            'mydct': {},
            'myint': 1,
            'mylst': [2],
            'mystr': 'a',
            'mytpl': (1,)})
>>> foo.mylst is Foo.mylst
True
>>> # But here we set it to a new object.
>>> foo.mylst = ['A new list!']
>>> foo.mylst is Foo.mylst
False
>>> foo.__dict__
{'mylst': ['A new list!'], 'mystr': 'b'}
>>> Foo.__dict__
dict_proxy({'__dict__': <attribute '__dict__' of 'Foo' objects>,
            '__doc__': None,
            '__module__': '__main__',
            '__weakref__': <attribute '__weakref__' of 'Foo' objects>,
            'mydct': {},
            'myint': 1,
            'mylst': [2],
            'mystr': 'a',
            'mytpl': (1,)})

>>> # So you need to set the instance attribute first. The best way to do that
>>> # is in the __init__.
>>> import copy
>>> class Bar(Foo)
...     def __init__(self, mylst=None, mydct=None, myset=None):
...         # If you really want to set a default on the class, make a copy of it.
...         if mylst is None:
...             self.mylst = copy.deepcopy(self.mylst)
...         if mydct is None:
...             self.mydct = copy.deepcopy(self.mydct)
...
...         # But the more common practice is to just put your default here
...         # rather than on the class (and certainly *not* in the function kwargs!)
...         if myset is None:
...             self.myset = set()