1. the list is a class which provides methods like sort(), append(), ...
list.sort(*, key=None, reverse=None)
key specifies a function of one argument that is used to extract a comparison key from each list element (for example, key=str.lower). The key corresponding to each item in the list is calculated once and then used for the entire sorting process. The default value of None means that list items are sorted directly without calculating a separate key value.
reverse is a boolean value. If set to True, then the list elements are sorted as if each comparison were reversed.
2. sorted() is a built-in function. sorted(iterable, *, key=None, reverse=False)
1. Why use lambda expression?
See the following code:
def key(x):
return x[1]
a = [(1,2),(3,1),(5,10),(11,-3)]
a.sort(key = key) # a = [(11,-3),(3,1),(1,2), (5,10)]
a = [(1,2),(3,1),(5,10),(11,-3)]
a.sort( key = lambda x : x[1] )
#explanation:
原文：https://www.cnblogs.com/sarah-zhang/p/12207997.html