Immutable/Frozen 具有固定键值类型的字典子类

Question

问题

类似于previous questions，我想创建一个frozen/immutable字典。具体来说，初始化后，用户在尝试使用 __delitem__ 和 __setitem__ 方法时应该得到一个 ValueError。

与前面的问题不同，我特别希望它是一个子class，其中初始化类型被限制为特定的键和值类型。

尝试的解决方案

我自己在 accomplishing this with collections.UserDict 上的尝试失败了：

class WorkflowParams(UserDict):
    def __init__(self, __dict: Mapping[str, str]) -> None:
        super().__init__(__dict=__dict)

    def __setitem__(self, key: str, item: str) -> None:
        raise AttributeError("WorkflowParams is immutable.")

    def __delitem__(self, key: str) -> None:
        raise AttributeError("WorkflowParams is immutable.")

尝试使用时：

workflow_parameters = WorkflowParams(
    {
        "s3-bucket": "my-big-bucket",
        "input-val": "1",
    }
)

它失败了

Traceback (most recent call last):
  File "examples/python_step/python_step.py", line 38, in <module>
    workflow_parameters = WorkflowParams(
  File "/home/sean/git/scargo/scargo/core.py", line 14, in __init__
    super().__init__(__dict=__dict)
  File "/home/sean/miniconda3/envs/scargo/lib/python3.8/collections/__init__.py", line 1001, in __init__
    self.update(kwargs)
  File "/home/sean/miniconda3/envs/scargo/lib/python3.8/_collections_abc.py", line 832, in update
    self[key] = other[key]
  File "/home/sean/git/scargo/scargo/core.py", line 17, in __setitem__
    raise AttributeError("WorkflowParams is immutable.")
AttributeError: WorkflowParams is immutable.

因为.

不合格的备选方案

因为我需要一个subclass，所以commonly suggested solution of using MappingProxyType不符合我的要求。

此外，我对推荐 subclassing dict 的答案表示怀疑，因为这似乎会导致一些 unintended behaviour.

Answer 1

这对我来说似乎工作得很好（用 Python 3.6 和 3.8 测试）：

from collections import UserDict
from typing import Mapping


class WorkflowParams(UserDict):
    def __init__(self, __dict: Mapping[str, str]) -> None:
        super().__init__()
        for key, value in __dict.items():
            super().__setitem__(key, value)

    def __setitem__(self, key: str, item: str) -> None:
        raise AttributeError("WorkflowParams is immutable.")

    def __delitem__(self, key: str) -> None:
        raise AttributeError("WorkflowParams is immutable.")


workflow_parameters = WorkflowParams(
    {
        "s3-bucket": "my-big-bucket",
        "input-val": "1",
    }
)

print(workflow_parameters)
# output: {'s3-bucket': 'my-big-bucket', 'input-val': '1'}

workflow_parameters['test'] = 'dummy'
# expected exception: AttributeError: WorkflowParams is immutable.

Answer 2

我会用 collections.abc 来完成它，它只是为了快速构建容器类，只需实施几件事就可以完成

>>> import collections
>>> class FrozenDict(collections.abc.Mapping):

    def __init__(self,/,*argv,**karg):
        self._data = dict(*argv,**karg)

    def __getitem__(self,key):
        return self._data[key]

    def __iter__(self):
        return iter(self._data)

    def __len__(self):
        return len(self._data)

    def __repr__(self):
        return f"{type(self).__name__}({self._data!r})"

    
>>> t=FrozenDict( {
        "s3-bucket": "my-big-bucket",
        "input-val": "1",
    })
>>> t
FrozenDict({'s3-bucket': 'my-big-bucket', 'input-val': '1'})
>>> t["test"]=23
Traceback (most recent call last):
  File "<pyshell#38>", line 1, in <module>
    t["test"]=23
TypeError: 'FrozenDict' object does not support item assignment
>>> del t["input-val"]
Traceback (most recent call last):
  File "<pyshell#39>", line 1, in <module>
    del t["input-val"]
TypeError: 'FrozenDict' object does not support item deletion
>>>

Immutable/Frozen 具有固定键值类型的字典子类

Immutable/Frozen Dictionary subclass with fixed key, value types

python

dictionary

immutability

data-structures

python-3.x

问题

尝试的解决方案

不合格的备选方案