asdict fails with frozen dataclass keys, using raw dict form

[qdbp]

BPO	43141
Nosy	@terryjreedy, @ericvsmith, @qdbp, @GalaxySnail

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-02-06.05:46:54.621>
labels = ['type-bug', 'library', '3.9', '3.10']
title = '`asdict` fails with frozen dataclass keys, using raw dict form'
updated_at = <Date 2022-03-21.23:17:26.040>
user = 'https://github.com/qdbp'

bugs.python.org fields:

activity = <Date 2022-03-21.23:17:26.040>
actor = 'GalaxySnail'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-02-06.05:46:54.621>
creator = 'enaumov'
dependencies = []
files = []
hgrepos = []
issue_num = 43141
keywords = []
message_count = 2.0
messages = ['386545', '386880']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'eric.smith', 'enaumov', 'GalaxySnail']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43141'
versions = ['Python 3.9', 'Python 3.10']

[qdbp]

A simple example is documented here: pydantic/pydantic#2233 but it doesn't look like it was actually filed as a bug against CPython... Copy paste of the minimal reproducing example:

from dataclasses import asdict, dataclass

@dataclass(eq=True, frozen=True)
class A:
  a: str

@dataclass(eq=True, frozen=True)
class B:
  b: dict[A, str]

asdict(B({A("x"):"y"}))

The offending code are these lines in dataclasses.py:

elif isinstance(obj, dict):
    return type(obj)((_asdict_inner(k, dict_factory),
                      _asdict_inner(v, dict_factory))

The _asdict_inner(k, dict_factory) [correctly] turns dataclasses into dicts... which are not hashable. I assume there is a good reason the _asdict_inner function is called on the key, but its failure in this case looks like a (rather annoying!) error.

[terryjreedy]

On freshly compiled master on Win10, the example code gives "TypeError: unhashable type: 'dict'".

[nj-vs-vh]

Is there a plan to fix or somehow mitigate this? I stumbled upon this error in the wild and spent some time figuring out the cause.

From my point of view, it doesn't really make sense to call _asdict_inner on keys, because dicts are not hashable by design. So, maybe just remove it?

elif isinstance(obj, dict):
    return type(obj)((k, _asdict_inner(v, dict_factory))

[terryjreedy]

@ericvsmith dataclass issue