在此示例中，使用元类提供了一个不错的解决方案。元类的用途有限，但效果很好。
>>> class MetaInit(type):
def __call__(cls, *args, **kwargs):
if args or kwargs:
return super().__call__(*args, **kwargs)
return cls.__new__(cls)
>>> class String(Metaclass=MetaInit):
def __init__(self, string):
self.__string = tuple(string.split())
self.__simple = tuple(self.__simple())
def __simple(self):
letter = lambda s: ''.join(filter(lambda s: 'a' <= s <= 'z', s))
return filter(bool, map(letter, map(str.lower, self.__string)))
def __eq__(self, other):
assert isinstance(other, String)
return self.__simple == other.__simple
def __getitem__(self, key):
assert isinstance(key, slice)
string = String()
string.__string = self.__string[key]
string.__simple = self.__simple[key]
return string
def __iter__(self):
return iter(self.__string)
>>> String('Hello, world!')[1:]
<__main__.String object at 0x02E78830>
>>> _._String__string, _._String__simple
(('world!',), ('world',))
>>>
六年后，我的观点比我自己的方法更支持AlexMartelli的答案。在仍然考虑元类的情况下，以下答案显示了如何使用和不使用它们都可以解决问题：
#! /usr/bin/env python3
METHOD = 'Metaclass'
class NoInitMeta(type):
def new(cls):
return cls.__new__(cls)
class String(Metaclass=NoInitMeta if METHOD == 'Metaclass' else type):
def __init__(self, value):
self.__value = tuple(value.split())
self.__alpha = tuple(filter(None, (
''.join(c for c in word.casefold() if 'a' <= c <= 'z') for word in
self.__value)))
def __str__(self):
return ' '.join(self.__value)
def __eq__(self, other):
if not isinstance(other, type(self)):
return NotImplemented
return self.__alpha == other.__alpha
if METHOD == 'Metaclass':
def __getitem__(self, key):
if not isinstance(key, slice):
raise NotImplementedError
instance = type(self).new()
instance.__value = self.__value[key]
instance.__alpha = self.__alpha[key]
return instance
elif METHOD == 'classmethod':
def __getitem__(self, key):
if not isinstance(key, slice):
raise NotImplementedError
instance = self.new()
instance.__value = self.__value[key]
instance.__alpha = self.__alpha[key]
return instance
@classmethod
def new(cls):
return cls.__new__(cls)
elif METHOD == 'inline':
def __getitem__(self, key):
if not isinstance(key, slice):
raise NotImplementedError
cls = type(self)
instance = cls.__new__(cls)
instance.__value = self.__value[key]
instance.__alpha = self.__alpha[key]
return instance
else:
raise ValueError('METHOD did not have an appropriate value')
def __iter__(self):
return iter(self.__value)
def main():
x = String('Hello, world!')
y = x[1:]
print(y)
if __name__ == '__main__':
main()
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