元组也是一种线性结构，和list类似，但是它是不可变的
创建元组(tuple), 和List类似，但是由于不可变的特性，所以相比列表，它少了很多方法，注意如果只有一个元素时，一定要在结尾加上,,否则类型就不是元组了.

In [62]: t = ()

In [63]: type(t)
Out[63]: tuple

In [64]: t = (1)

In [65]: type(t)
Out[65]: int

In [66]: t = (1,)

In [67]: type(t)
Out[67]: tuple

In [68]: t
Out[68]: (1,)

In [69]: t[0]
Out[69]: 1

In [70]: t[0] = 2
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-70-29b3302c4f70> in <module>()
----> 1 t[0] = 2

TypeError: 'tuple' object does not support item assignment

In [71]: t2 = ('a')

In [72]: type(t2)
Out[72]: str

In [73]: t2
Out[73]: 'a'