如何根据输入参数以不同的方式初始化 NamedTuple child class？

Question

我正在构建一个可以接受不同初始化方式的 typing.NamedTuple class（see typing.NamedTuple docs here, or the older collections.namedtuples docs 它继承自）。

为什么在这种情况下使用 NamedTuple？我希望它是不可变的和可自动散列的，这样它就可以成为字典键，而且我不必编写散列函数。

我知道我需要使用 __new__ 而不是 __init__，因为 NamedTuples 是不可变的（例如，. I've searched and there are some tidbits out there (e.g. answers to this question on setting up a custom hash for a namedtuple），但我无法让一切正常，我收到有关无法覆盖 __new__.

的错误

这是我当前的代码：

from typing import NamedTuple

class TicTacToe(NamedTuple):
    """A tic-tac-toe board, each character is ' ', 'x', 'o'"""
    row1: str = '   '
    row2: str = '   '
    row3: str = '   '

    def __new__(cls, *args, **kwargs):
        print(f'Enter __new__ with {cls}, {args}, {kwargs}')
        if len(args) == 1 and args[0] == 0:
            new_args = ('   ', '   ', '   ')
        else:
            new_args = args
        self = super().__new__(cls, *new_args, *kwargs)
        return self

if __name__ == '__main__':
    a = TicTacToe(('xo ', 'x x', 'o o'))
    print(a)
    b = TicTacToe(0)
    print(b)

但我收到以下错误：

Traceback (most recent call last):
  File "c:/Code/lightcc/OpenPegs/test_namedtuple.py", line 4, in <module>
    class TicTacToe(NamedTuple):
  File "C:\Dev\Python37\lib\typing.py", line 1384, 
in __new__
    raise AttributeError("Cannot overwrite NamedTuple attribute " + key)
AttributeError: Cannot overwrite NamedTuple attribute __new__

我是否无法为继承自 NamedTuple 的子 class 创建单独的 __new__ 函数？从消息中可以看出，它正试图直接覆盖 NamedTuple 的 __new__，而不是 TicTacToe class.

这是怎么回事？

Answer 1

您可以通过定义一个 classmethod. In the sample code below, I've simply named it make(). It's analogous to the class method named _make()^✶ that collections.namedtype 子 class 来避免定义 __new__()。

^✶这是向任何 class.

提供“替代构造函数”的常用方法

请注意，我还更改了对函数的第一次调用，以便将参数正确传递给 make() 方法。

from typing import NamedTuple

class TicTacToe(NamedTuple):
    """A tic-tac-toe board, each character is ' ', 'x', 'o'."""
    row1: str = '   '
    row2: str = '   '
    row3: str = '   '

    @classmethod
    def make(cls, *args, **kwargs):
        print(f'Enter make() with {cls}, {args}, {kwargs}')
        if len(args) == 1 and args[0] == 0:
            new_args = ('   ', '   ', '   ')
        else:
            new_args = args
        self = cls(*new_args, *kwargs)
        return self

if __name__ == '__main__':
#    a = TicTacToe.make(('xo ', 'x x', 'o o'))
    a = TicTacToe.make('xo ', 'x x', 'o o')
    print(a)
    b = TicTacToe.make(0)
    print(b)

输出：

Enter make() with <class '__main__.TicTacToe'>, ('xo ', 'x x', 'o o'), {}
TicTacToe(row1='xo ', row2='x x', row3='o o')
Enter make() with <class '__main__.TicTacToe'>, (0,), {}
TicTacToe(row1='   ', row2='   ', row3='   ')

更新

无法重载 NamedTuple subclass' __new__() 方法的另一种解决方法是将派生的 class 拆分为两个 classes，一个public和一个private，所以前者不再是NamedTuple.

的直接subclass

这样做的一个好处是，不再需要使用像上面 make() 这样的特殊用途 class 方法来创建实例。

我的意思是：

from typing import NamedTuple

class _BaseBoard(NamedTuple):
    """Private base class for tic-tac-toe board."""
    row1: str = '   '
    row2: str = '   '
    row3: str = '   '


class TicTacToe(_BaseBoard):
    """A tic-tac-toe board, each character is ' ', 'x', 'o'."""
    __slots__ = ()  # Prevent creation of a __dict__.

    @classmethod
    def __new__(cls, *args, **kwargs):
        print(f'Enter __new__() with {cls}, {args}, {kwargs}')
        if len(args) == 1 and args[0] == 0:
            new_args = ('   ', '   ', '   ')
        else:
            new_args = args
        self = super().__new__(*new_args, *kwargs)
        return self


if __name__ == '__main__':

    a = TicTacToe('xo ', 'x x', 'o o')
    print(a)
    assert getattr(a, '__dict__', None) is None  # Verify not being created.
    b = TicTacToe(0)
    print(b)

请注意，此方法是应用 Andrew Koenig 的 fundamental theorem of software engineering 的示例，即：“我们可以通过引入额外的间接级别来解决任何问题。”

如何根据输入参数以不同的方式初始化 NamedTuple child class？

How to initialize a NamedTuple child class different ways based on input arguments?

python

inheritance

constructor

immutability

namedtuple

更新