Question

我期待使用新的typing.NamedTuple类，它允许使用通常的Python类语法创建命名元组类（包括添加文档字符串和方法，提供默认值，类型提示，等等。）

但是：底部的类产生以下错误消息：

AttributeError: Cannot overwrite NamedTuple attribute __new__

从这里我收集它所说的内容：覆盖__new__仍然是禁止的。这非常令人失望。

旧方式＆＃34;关于这个将是从一个命名的元组类继承，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')

我是否有其他替代方法可以在创建命名元组之前将zero，width和precision参数强加到int，但仍然使用相同的类创建语法？还是我坚持使用旧的方式？

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类，并在子类中进行参数修改：

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)

我并不太关心这种方法，但至少它比“旧方式”更具可读性（即使它仍然需要悬挂__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，与简单地做到相比，这些都非常笨重：

fspec = FormatSpec(*parse_format_spec(some_format_spec_string))

在创建实例之前修改参数以键入.NamedTuple

1 个答案: