如何获取dataclass字段的索引

Question

假设我有一个简单的dataclass实例

import dataclasses as dc

@dc.dataclass
class DCItem:
    name: str
    unit_price: float

item = DCItem('test', 11)

现在我要确定实例属性item.unit_price.的位置(索引)我如何使它简单的使用和表演？我想过使用dc.asdict的get方法

@dc.dataclass
class DCItem:
    name: str
    unit_price: float

    def get_index(self, name):
        return list(dc.asdict(self)).index(name)

item.get_index('unit_price')  # 1

但这有两个缺点：

它不是很好的性能，至少在许多实例属性item.unit_price

• 中，它失去了的良好的自动完成功能。

是否有将数据块的特性与IntEnum和enum.auto()特性相结合的解决方案，而没有上述缺点？

Answer 1

如果类在运行时不被更改，则可以将类属性中的索引缓存为字典。

import dataclasses as dc

@dc.dataclass
class DCItem:
    name: str
    unit_price: float
    
    @classmethod
    def get_index(cls, name):
        if '_idx_mapping' not in cls.__dict__:
            flds = dc.fields(cls)
            cls._idx_mapping = {flds[idx].name: idx for idx in range(len(flds))}
        return cls._idx_mapping[name]


>>> item = DCItem('test', 11)
>>> item.get_index('unit_price')
1

在最坏的情况下，访问字典应该是快速的O(n)。

>>> from timeit import timeit
>>> timeit("item.get_index('unit_price')", "from __main__ import item")
0.21372696105390787

作为比较，您的解决方案非常慢，正如您提到的：

>>> timeit("item.get_index('unit_price')", "from __main__ import item")
4.260601775022224

注意到：我还没有用继承测试这个类。

编辑：解决第二点会使解决方案更加复杂。我使用Python描述符提出了以下内容。

import dataclasses as dc
from typing import Any
from collections import defaultdict


class IndexedField:
    def __init__(self, a_type: type, value: Any, index: int):
        self._validate_type(a_type, value)  # This line can be removed when type checking is not required.
        self._a_type = a_type
        self._value = value
        self._index = index

    @staticmethod
    def _validate_type(a_type: type, value: Any):
        if not isinstance(value, a_type):
            raise TypeError(f"value is of type {type(value)} but {a_type} is expected")

    @property
    def a_type(self):  # read-only
        return self._a_type

    @property
    def index(self):  # read-only
        return self._index

    @property
    def value(self):
        return self._value

    @value.setter
    def value(self, new_value):
        self._validate_type(self._a_type, new_value)  # This line can be removed when type checking is not required.
        self._value = new_value

    def __repr__(self):
        return (f'{self.__class__.__name__}'
                f'(a_type={self._a_type!r}, index={self._index!r}, value={self._value!r})')


class IndexedFieldDescriptor:
    _class_last_index = defaultdict(int)
    _class_indexes = defaultdict(dict)

    def __init__(self, a_type) -> None:
        self._name = None
        self._type = a_type

    def __get__(self, instance, owner):
        if instance is None:
            return self
        return instance.__dict__[self._name]

    def __set_name__(self, owner, name):
        self._name = name
        self._class_indexes[owner.__name__][name] = self._class_last_index[owner.__name__]
        self._class_last_index[owner.__name__] += 1

    def __set__(self, instance, value):
        index = self._class_indexes[instance.__class__.__name__][self._name]
        instance.__dict__[self._name] = IndexedField(self._type, value, index)


@dc.dataclass
class DCItem:
    name: IndexedField = IndexedFieldDescriptor(str)
    unit_price: IndexedField = IndexedFieldDescriptor(float)


item = DCItem('test', 11.0)
print(item)
print(f"* name field value: {item.name.value!r}, name field index: {item.name.index!r}, name field type: {item.name.a_type!r}")
print(f"* unit_price field value: {item.unit_price.value!r}, unit_price field index: {item.unit_price.index!r}, unit_price field type: {item.unit_price.a_type!r}")

from timeit import timeit
print(f'* Index access time: {timeit("item.name.index", "from __main__ import item")}')
print(f'* Value access time: {timeit("item.name.value", "from __main__ import item")}')

输出：

DCItem(name=IndexedField(a_type=<class 'str'>, index=0, value='test'), unit_price=IndexedField(a_type=<class 'float'>, index=1, value=11.0))
* name field value: 'test', name field index: 0, name field type: <class 'str'>
* unit_price field value: 11.0, unit_price field index: 1, unit_price field type: <class 'float'>
* Index access time: 0.2253845389932394
* Value access time: 0.2729280750500038