Python，子类化不可变类型 [英] Python, subclassing immutable types

问题描述

我有以下课程:

class MySet(set):

    def __init__(self, arg=None):
        if isinstance(arg, basestring):
            arg = arg.split()
        set.__init__(self, arg)

这可以按预期工作(使用字符串的单词而不是字母来初始化集合).但是，当我想对set的不可变版本执行相同操作时，__init__方法似乎被忽略了:

This works as expected (initialising the set with the words of the string rather than the letters). However when I want to do the same with the immutable version of set, the __init__ method seems to be ignored:

class MySet(frozenset):

    def __init__(self, arg=None):
        if isinstance(arg, basestring):
            arg = arg.split()
        frozenset.__init__(self, arg)

我可以用__new__达到类似的目的吗?

Can I achieve something similar with __new__ ?

推荐答案

是的，您需要覆盖__new__特殊方法:

Yes, you need to override __new__ special method:

class MySet(frozenset):

    def __new__(cls, *args):
        if args and isinstance (args[0], basestring):
            args = (args[0].split (),) + args[1:]
        return super (MySet, cls).__new__(cls, *args)

print MySet ('foo bar baz')

输出为:

MySet(['baz', 'foo', 'bar'])

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