reversed 序列逆置

内置函数（类）reversed，Python 官方文档描述如下：

help(reversed)

Help on class reversed in module builtins:

class reversed(object)
 |  reversed(sequence, /)
 |  
 |  Return a reverse iterator over the values of the given sequence.
 |  
 |  Methods defined here:
 |  
 |  __getattribute__(self, name, /)
 |      Return getattr(self, name).
 |  
 |  __iter__(self, /)
 |      Implement iter(self).
 |  
 |  __length_hint__(...)
 |      Private method returning an estimate of len(list(it)).
 |  
 |  __next__(self, /)
 |      Implement next(self).
 |  
 |  __reduce__(...)
 |      Return state information for pickling.
 |  
 |  __setstate__(...)
 |      Set state information for unpickling.
 |  
 |  ----------------------------------------------------------------------
 |  Static methods defined here:
 |  
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.

返回给定的序列逆置之后的迭代器。

type(reversed)

type

reversed('1234')

<reversed at 0x14b684fe460>

list(reversed('1234'))

['4', '3', '2', '1']

因为字典顺序会确保为插入顺序，字典和字典视图都是可逆的。3.8 新版可以返回一个逆序获取字典键的迭代器。

d = reversed({'a':1,'b':2,'c':3})
list(d)

['c', 'b', 'a']