为什么我在Python3中收到此错误？

Question

>>>cast= ("1","2")
>>> cast.append("Gilliam")
Traceback (most recent call last):
  File "<pyshell#44>", line 1, in <module>
    cast.append("Gilliam")
AttributeError: 'tuple' object has no attribute 'append'

代码用 python 3 编写。