更新原始数组会自动从原始数组定义另一个数组？

Question

原始数组如：

arr = range(1,10)
print arr
# [1, 2, 3, 4, 5, 6, 7, 8, 9]

New将此原始数组提供给新变量x和y，

x = arr
y = arr
print arr, x, y
# [1, 2, 3, 4, 5, 6, 7, 8, 9] [1, 2, 3, 4, 5, 6, 7, 8, 9] [1, 2, 3, 4, 5, 6, 7, 8, 9]

更新原始数组，

arr.extend([10,11]) 
print arr, x, y  
# [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

或只更新x：

x.extend([12, 13])
print x, arr
# [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13] [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]

为什么？