Python __reverse__魔术方法

Question

我正在尝试这段代码,期望它在反转后返回一个文本字符串,[::-1]但它不起作用.

class meh:
  def __init__(self, bla):
    self.bla = bla
  def __reversed__(self):
    return "Reversing!!!! %s" % self.bla[::-1]

a = meh("Reversed successfully!!!!")
print a[::-1]

如何正确调用该__reverse__功能？示例:__gt__调用a > 25,调用__reverse__什么？

Answer 1

[::-1]是一片.object.__reversed__()而是由reversed()函数使用,并且仅适用于序列(同时提供a __len__和__getitem__方法的对象).

如果您不提供__reversed__,该函数使用这些__len__和__getitem__方法反向访问索引.__reversed__必须自己返回一个迭代器:

class Reversable(object):
    def __init__(self, seq):
        self.seq = seq

    def __len__(self):
        return len(self.seq)

    def __getitem__(self, item):
        return self.seq[item]

    def __reversed__(self):
        for elem in 'Reversing: ' + self.seq[::-1]:
            yield elem

演示:

>>> class Reversable(object):
...     def __init__(self, seq):
...         self.seq = seq
...     def __len__(self):
...         return len(self.seq)
...     def __getitem__(self, item):
...         return self.seq[item]
...     def __reversed__(self):
...         for elem in 'Reversing: ' + self.seq[::-1]:
...             yield elem
... 
>>> r = Reversable('Foo bar baz!')
>>> list(r)
['F', 'o', 'o', ' ', 'b', 'a', 'r', ' ', 'b', 'a', 'z', '!']
>>> list(reversed(r))
['R', 'e', 'v', 'e', 'r', 's', 'i', 'n', 'g', ':', ' ', '!', 'z', 'a', 'b', ' ', 'r', 'a', 'b', ' ', 'o', 'o', 'F']