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

self.b = self.a isn't copying the list. It's assigning the reference to that list, so both attributes point at the same object. If you modify it via one reference, you'll see the change via the other one too.

You can use copy.copy(the_list) to get the proper copy. Or copy.deepcopy(the_list) if you need the references below also updated.