在创建实例之前将参数修改为 typing.NamedTuple

Question

我期待着使用有点新的 typing.NamedTuple class，它允许使用通常的 Python class 创建命名元组 classes ] 语法（包括添加文档字符串和方法、提供默认值、类型提示等的能力）。

但是：底部的 class 产生以下错误消息：

AttributeError: Cannot overwrite NamedTuple attribute __new__

从这里我收集了它所说的内容：覆盖 __new__ 仍然是不可以的。这很令人失望。

执行此操作的 "old way" 将继承命名元组 class，但这需要我认为是一些丑陋的样板代码：

from collections import namedtuple

class FormatSpec(namedtuple('FormatSpecBase', 'fill align sign alt zero '
                                              'width comma decimal precision type')):
    __slots__ = ()
    def __new__(cls, fill, align, sign, alt, zero,
                width, comma, decimal, precision, type):
        to_int=lambda x: int(x) if x is not None else x
        zero=to_int(zero)
        width=to_int(width)
        precision=to_int(precision)
        return super().__new__(cls, fill, align, sign, alt, zero,
                               width, comma, decimal, precision, type)

FormatSpec.__doc__=_FormatSpec.__doc__.replace('FormatSpecBase','FormatSpec')

在创建命名元组之前，是否有其他替代方法可以将下面的 zero、width 和 precision 参数转换为 int，但仍然使用相同的 class 创建语法？还是我坚持使用旧方法？

from typing import NamedTuple, Optional

class FormatSpec(NamedTuple):
    """Represents a string that conforms to the [Format Specification
    Mini-Language][1] in the string module.

    [1]: https://docs.python.org/3/library/string.html#formatspec
    """
    fill: Optional[str]
    align: Optional[str]
    sign: Optional[str]
    alt: Optional[str]
    zero: Optional[int]
    width: Optional[int]
    comma: Optional[str]
    decimal: Optional[str]
    precision: Optional[int]
    type: str
    def __new__(cls, fill, align, sign, alt, zero, width, comma, decimal, precision, type):
        to_int=lambda x: int(x) if x is not None else x
        zero=to_int(zero)
        width=to_int(width)
        precision=to_int(precision)
        return super().__new__(cls, fill, align, sign, alt, zero,
                               width, comma, decimal, precision, type)
    def join(self):
        return ''.join('{!s}'.format(s) for s in self if s is not None)
    def __format__(self, format_spec):
        try:
            return format(self.join(), format_spec)
        except (TypeError, ValueError):
            return super().__format__(format_spec)

Answer 1

一种方法是将其拆分为两个 classes，并在子 class:

中进行参数修改

from typing import NamedTuple, Optional

class FormatSpecBase(NamedTuple):
    """Represents a string that conforms to the [Format Specification
    Mini-Language][1] in the string module.

    [1]: https://docs.python.org/3/library/string.html#formatspec
    """
    fill: Optional[str]
    align: Optional[str]
    sign: Optional[str]
    alt: Optional[str]
    zero: Optional[int]
    width: Optional[int]
    comma: Optional[str]
    decimal: Optional[str]
    precision: Optional[int]
    type: str
    def join(self):
        return ''.join('{!s}'.format(s) for s in self if s is not None)
    def __format__(self, format_spec):
        try:
            return format(self.join(), format_spec)
        except (TypeError, ValueError):
            return super().__format__(format_spec)


class FormatSpec(FormatSpecBase):
    __slots__ = ()
    def __new__(cls, fill, align, sign, alt, zero, width, comma, decimal, precision, type):
        to_int=lambda x: int(x) if x is not None else x
        zero=to_int(zero)
        width=to_int(width)
        precision=to_int(precision)
        return super().__new__(cls, fill, align, sign, alt, zero,
                                    width, comma, decimal, precision, type)

我不太喜欢这种方法，但至少它比 "old way" 更具可读性（尽管它仍然需要那些悬而未决的 __slots__ 废话）。

另一种方式是工厂：

def MakeFormatSpec(cls, fill, align, sign, alt, zero,
                   width, comma, decimal, precision, type):
    to_int=lambda x: int(x) if x is not None else x
    zero=to_int(zero)
    width=to_int(width)
    precision=to_int(precision)
    return FormatSpec(fill, align, sign, alt, zero,
                      width, comma, decimal, precision, type)

fspec = MakeFormatSpec(*parse_format_spec(some_format_spec_string))

...或工厂方法：

    @classmethod
    def make(cls, fill, align, sign, alt, zero, width, comma, decimal, precision, type):
        to_int=lambda x: int(x) if x is not None else x
        zero=to_int(zero)
        width=to_int(width)
        precision=to_int(precision)
        return cls(fill, align, sign, alt, zero,
                   width, comma, decimal, precision, type)

fspec = FormatSpec.make(*parse_format_spec(some_format_spec_string))

然而，与简单地能够做到的相比，这些都非常笨拙：

fspec = FormatSpec(*parse_format_spec(some_format_spec_string))

在创建实例之前将参数修改为 typing.NamedTuple

Modify arguments to typing.NamedTuple prior to instance creation

python

type-hinting

namedtuple