Python：是否有可能有一个作用于类切片的类方法？

Question

我正在 Python 中创建一个容器类，它要么继承自 list，要么只实现所有标准列表方法（我真的不在乎哪个）。

如何创建一个只作用于从切片返回的项目的方法？我已经能够创建一个作用于整个容器的方法（见下文），但我似乎无法弄清楚如何只作用于切片。

我正在使用 python 2.7.6 并在我的所有代码中使用 from __future__ import print_function, division。

示例代码：

from __future__ import print_function, division
import itertools

class MyContainerClass(list):
    """ For now, I'm inheriting from list. Is this my problem? """
    def __init__(self, data_array):
        list.__init__(self, data_array)

    def __getitem__(self, *args):

        arg = args[0]

        # specific indices   MyContainerClass[0, 3, 5] => indexes 0, 3, and 5
        if isinstance(arg, (list, tuple)) and not isinstance(arg[0], bool):
            return [list.__getitem__(self, _i) for _i in arg]

        # standard slice notation
        elif isinstance(arg, slice):
            return list.__getitem__(self, arg)

        # Using an array mask   MyContainerClass[[True, False, False,...,True]] => index 0 and -1]
        elif isinstance(arg[0], bool):  # or isinstance(arg, np.ndarray):
            return list(itertools.compress(self, arg))

        else:
            # I'll eventually replace this with and exception raise.
            return 'error'

    def my_method(self):
        """
        Will act on entire list, but I want it to act on only what's
        returned by the slice (which may be the entire list in some cases).
        """
        return "a, ".join([str(_i) for _i in self])

这是我想要的用法示例：

>>> data = MyContainerClass([1, 2, 3, 4, 5, 6, 7])
>>> data[5:]
[6, 7]
>>> data.my_method()           # This works as expected
"1a, 2a, 3a, 4a, 5a, 6a, 7"
>>> data[0:3].my_method()      # Doesn't work
"1a, 2a, 3"                    # but should return this

---

---

看起来现在一切正常。谢谢一群人！这是我想出的：

from __future__ import print_function, division
import itertools

class MyContainerClass(list):
    """
    """
    def __init__(self, array):
        if isinstance(array, int):
            list.__init__(self, [array])
        else:
            list.__init__(self, array)

    def __getitem__(self, arg):
        # Standard Slice notation
        if isinstance(arg, slice):
            retval = super(MyContainerClass, self).__getitem__(arg)

        # specific indices
        elif isinstance(arg, (list, tuple)) and not isinstance(arg[0], bool):
            retval = [list.__getitem__(self, _i) for _i in arg]

        # a single specific index
        elif isinstance(arg, int):
            retval = list.__getitem__(self, arg)

        # an array mask of T/F values
        elif isinstance(arg[0], bool):      # or isinstance(arg, np.ndarray):
            retval = list(itertools.compress(self, arg))

        # raise an error on unknown
        else:
            raise SyntaxError("Unknown notation for list slice or index")

        retval = type(self)(retval)
        return retval

    def __getslice__(self, i, j):
        # Python 2 built-in types only
        return self.__getitem__(slice(i, j))

    def my_method(self):
        return "a, ".join([str(_i) for _i in self])

并且行为类似于：

>>> data = MyContainerClass([1, 2, 3, 4, 5, 6, 7])
>>> mask = [True, True, False, True, False, False, False]
>>> print(data)
[1, 2, 3, 4, 5, 6, 7]
>>> print(type(data[5:]))
<class '__main__.MyContainerClass'>
>>> print(data.my_method())
1a, 2a, 3a, 4a, 5a, 6a, 7
>>> print(data[0:5].my_method())
1a, 2a, 3a, 4a, 5
>>> print(data[1, 5, 2].my_method())
2a, 6a, 3
>>> print(data[mask].my_method())
1a, 2a, 4
>>> print(data[2].my_method())
3

Answer 1

您必须确保您的 __getitem__ 再次返回您的类型：

class MyContainerClass(list):
    """ For now, I'm inheriting from list. Is this my problem? """
    def __init__(self, data_array):
        list.__init__(self, data_array)

    def my_method(self):
        """
        Will act on entire list, but I want it to act on only what's
        returned by the slice (which may be the entire list in some cases).
        """
        return "a, ".join([str(_i) for _i in self])

    def __getitem__(self, index):
        retval = super(MyContainerClass, self).__getitem__(index)
        if isinstance(index, slice):
            retval = type(self)(retval)
        return retval

    def __getslice__(self, i, j):
        # Python 2 built-in types only
        return self.__getitem__(slice(i, j))

仅在 Python 2 中需要额外的 __getslice__ method，并且仅当您从已经实现 __getslice__ 的类型继承时才需要。 list就是这样的类型。

演示：

>>> data = MyContainerClass([1, 2, 3, 4, 5, 6, 7])
>>> data[:5]
[1, 2, 3, 4, 5]
>>> type(data[:5])
<class '__main__.MyContainerClass'>
>>> data.my_method()
'1a, 2a, 3a, 4a, 5a, 6a, 7'
>>> data[:3].my_method()
'1a, 2a, 3'