Instances sharing attributes

[midarq]

Hello,

I have the following problem:
Why is the instance B updating and how do i prevent that?

class myclass():

    def __init__(self, x=[]):
        self.x = x

    def append_to(self):
        self.x.append('foo')
A=myclass()
B=myclass()
A.append_to()
B.x

This returns ['foo'].

Thank you for your help