class A:
    js = []
    
    def add (self, j):
        self.js.append(j)

    def clone (self):
        c = A()
        for j in self.js:
            c.add(j)
        return c

a = A()
b = a.clone()
b.add(3)

print a.js
print b.js

The above code should print "[]\n[3]", but instead it
prints "[3]\n[3]"!

It works as expected, if you change "js = []" to "def
__init__ (self):
        self.js = []"

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user_id=849994

This is not a bug. The "js" list exists in the class's
namespace and exists only once, and if you don't rebind the
name, "self.js" always refers to this. Therefore it is
shared between instances.