Python list append causes strange result

Question

I have really strange problem. Here is the sample code:

class SomeClass(object):
    a = []
    b = []
    def __init__(self, *args, **kwargs):
        self.a =          


        

Answer 1

Because self.a and self.b are actually references to the same list object.

If you want to modify one without changing the other, try this

class SomeClass(object):
    # a = []
    # b = [] # these two class member not necessary in this code
    def __init__(self, *args, **kwargs):
        self.a = [(1,2), (3,4)]
        self.b = list(self.a) # copy to a new list
        self.a.append((5,6))
        print self.b

SomeClass()