如何使python数据类可哈希化?

问题描述

说a我在python3中有一个数据类.我希望能够对这些对象进行哈希处理和排序.

Say a I have a dataclass in python3. I want to be able to hash and order these objects.

我只希望它们按ID排序/散列.

I only want them ordered/hashed on id.

我在文档中看到我可以只实现__hash__以及所有其他功能，但是我想让datacalses为我做这些工作，因为它们旨在解决这个问题.

I see in the docs that I can just implement __hash__ and all that but I'd like to get datacalsses to do the work for me because they are intended to handle this.

from dataclasses import dataclass, field

@dataclass(eq=True, order=True)
class Category:
    id: str = field(compare=True)
    name: str = field(default="set this in post_init", compare=False)

a = sorted(list(set([ Category(id='x'), Category(id='y')])))

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'Category'

推荐答案

来自文档:

[...]

如果eq和frozen都为true，则默认情况下dataclass()将 为您生成一个__hash__()方法.如果eq为true并且frozen 为false时，__hash__()将设置为None，将其标记为不可散列 (它是可变的，因为它是可变的).如果eq为false，则__hash__() 将保持不变，这意味着__hash__()方法 将使用超类(如果超类是对象，则意味着它 将回退到基于ID的哈希).

If eq and frozen are both true, by default dataclass() will generate a __hash__() method for you. If eq is true and frozen is false, __hash__() will be set to None, marking it unhashable (which it is, since it is mutable). If eq is false, __hash__() will be left untouched meaning the __hash__() method of the superclass will be used (if the superclass is object, this means it will fall back to id-based hashing).

由于您设置了eq=True并将frozen保留为默认值(False)，因此您的数据类不可散列.

Since you set eq=True and left frozen at the default (False), your dataclass is unhashable.

您有3个选择:

• 设置frozen=True(除了eq=True之外)，这将使您的类不可变且可哈希化.

• Set unsafe_hash=True，它将创建一个__hash__方法，但使您的类保持可变状态，因此，如果在存储在dict或set中的同时修改了您的类的实例，则可能会出现问题:

• Set frozen=True (in addition to eq=True), which will make your class immutable and hashable.

• Set unsafe_hash=True, which will create a __hash__ method but leave your class mutable, thus risking problems if an instance of your class is modified while stored in a dict or set:

cat = Category('foo', 'bar')
categories = {cat}
cat.id = 'baz'

print(cat in categories)  # False

手动实现__hash__方法.

Manually implement a __hash__ method.

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