英文文档：
sorted(iterable[, key][, reverse])
Return a new sorted list from the items in iterable.
Has two optional arguments which must be specified as keyword arguments.
key specifies a function of one argument that is used to extract a comparison key from each list element: key=str.lower. The default value is None (compare the elements directly).
reverse is a boolean value. If set to True, then the list elements are sorted as if each comparison were reversed.
Use functools.cmp_to_key() to convert an old-style cmp function to a key function.
The built-in sorted() function is guaranteed to be stable. A sort is stable if it guarantees not to change the relative order of elements that compare equal — this is helpful for sorting in multiple passes (for example, sort by department, then by salary grade).
说明：
1. 函数功能对一个可迭代对象进行排序，返回一个排序后列表。
>>> a = sorted('dcabegf')
>>> a # 返回结果是列表
['a', 'b', 'c', 'd', 'e', 'f', 'g']
2. 函数调用时可以提供一个可选的命名参数key，它是一个方法，默认值是None，用来指定具体排序的算法；函数对可迭代对象每个元素使用key算法后再排序，返回的任然是可迭代对象中的元素。
>>> a = ['a','b','d','c','B','A']
>>> a
['a', 'b', 'd', 'c', 'B', 'A'] >>> sorted(a) # 默认按字符ascii码排序
['A', 'B', 'a', 'b', 'c', 'd'] >>> sorted(a,key = str.lower) # 转换成小写后再排序，'a'和'A'值一样，'b'和'B'值一样
['a', 'A', 'b', 'B', 'c', 'd']
3. 函数调用时可以提供一个可选的命名参数reverse，它的默认值是False，用来排序结果是否倒转。
>>> a = sorted('dcabegf')
>>> a
['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> a = sorted('dcabegf',reverse = True) # 排序结果倒置
>>> a
['g', 'f', 'e', 'd', 'c', 'b', 'a']