具体的にこれですが


class A :
L = []
def __init__(self): pass

class X :
L = []
def __init__(self): self.L = []

a = A()
a.L.append(7)
b = A()
print(b.L) # 新規生成なのに値が入ってる

a = X()
a.L.append(7)
b = X()
print(b.L) # 正常
print(a.L) # 正常