Python reversed() function returns a reversed iterator from the specified sequence argument.

Python reversed（）函数从指定的sequence参数返回反向迭代器。

python reversed（） (Python reversed())

Python reversed() function syntax is:

Python reversed（）函数语法为：

reversed(seq)

• The input argument must be a sequence, for example tuple, list, string etc.输入参数必须是一个序列，例如tuple ， list ， string等。
• The returned object is of type reversed and it’s an iterator.返回的对象是reversed类型，它是一个迭代器。
• We can supply a custom object as reversed() function argument if it has __reversed__() method or supports the sequence protocol.如果自定义对象具有__reversed__()方法或支持序列协议，则可以提供它作为reversed（）函数参数。
• We need to implement __len__() method and the __getitem__() method with integer arguments starting at 0 to support sequence protocol.我们需要使用从0开始的整数参数来实现__len__()方法和__getitem__()方法，以支持序列协议。

带序列的reversed（） (reversed() with sequence)

Let’s look at reversed() function examples with standard sequence objects such as string, bytes, tuple, list etc.

让我们看一下带有标准序列对象（例如字符串， 字节 ，元组，列表等）的reversed（）函数示例。

def print_iterator(it):for x in it:print(x, end=' ')print('\n')# reversed string
r = reversed('abc')
print(type(r))
print(r)
print_iterator(r)# reversed list
r = reversed([1, 2, 3])
print_iterator(r)# reversed tuple
r = reversed((1, 2, 3))
print_iterator(r)# reversed bytes
r = reversed(bytes('abc', 'utf-8'))
print_iterator(r)# reversed bytearray
r = reversed(bytearray('abc', 'utf-8'))
print_iterator(r)

Output:

输出：

<class 'reversed'>
<reversed object at 0x109d1f208>
c b a 3 2 1 3 2 1 99 98 97 99 98 97

Notice that bytes and bytearray elements are converted to integer while printing on console.

请注意，在控制台上打印时，字节和字节数组元素将转换为整数。

具有具有__reversed__方法的对象的reversed（） (reversed() with object having __reversed__ method)

# object with __reversed__ method
class Data:name = ''def __init__(self, n):self.name = ndef __reversed__(self):return reversed(self.name)d = Data('ABC')r = reversed(d)print_iterator(r)

Output: C B A

输出： CBA

具有对象支持序列协议的reversed（） (reversed() with object supporting sequence protocol)

# object supporting sequence protocol i.e.
# implementing __len__() and __getitem__ method
class MyTupleWrapper:t = ()def __init__(self, tu):if not isinstance(tu, tuple):return ValueError('Only accepts tuple')self.t = tudef __len__(self):return len(self.t)def __getitem__(self, index):return self.t[index]mt = MyTupleWrapper((1, 2, 3, 4))
r = reversed(mt)
print_iterator(r)

Output: 4 3 2 1

输出： 4 3 2 1

Reference: Official Documentation

参考： 官方文档

翻译自: https://www.journaldev.com/23113/python-reversed-function