python

Question

a = b = ['a', 'b'] 
b = ['x', 'y'] # reassigns the name b
print a # still the original list

a = b = ['a', 'b']
b[:] = ['x', 'y'] # changes the existing list bound to a 
print a # a changed too 

为什么a与b有关？ python使用什么样的机制来处理可变数据类型的链式赋值？