Quick n easy example of mutability mess

mutability_ex.py

class MutableClass():
    def __init__(self):
        self.thing = [] # mutable
        self.nonething = None

    def fn(self, new):
        self.thing.append(new) 
        self.nonething = new

m, m2 = MutableClass(), MutableClass()
m.fn('first')
m.fn('second')
# something missing...
# >>> ['first', 'second'] second