default arg trap

gistfile1.py

class X:
    def __init__(self, x={}):
        self.var = x

    def set(self, val):
        self.var['key'] = val

class Y(X):
    def __init__(self,x=None):
        self.var = x if x is not None else {}
        
if __name__ == '__main__':
    x1 = X()
    x1.set('test')
    print x1.var # {'key': 'test'}
    
    x2 = X()
    print x2.var # {'key': 'test'}
    
    y1 = Y()
    y1.set('test')
    print y1.var # {'key': 'test'}
    
    y2 = Y()
    print y2.var # {}