asdict fails for attributes of type Mapping with keys of type Tuple

@attr.s
class A:
    a = attr.ib()
        
instance = A({(1, ): 1})
attr.asdict(instance)

produces:
TypeError: unhashable type: 'list'

when the retain_collection_types flag is set to false (the default). This can take a while to figure out, since the error is not the most descriptive (and the classes are not toy examples like this one).
I just thought it would be worth mentioning. In any case, thanks for this awesome library.

@hynek

I stumbled upon the same issue and it is indeed not easy to figure out what is going wrong. I'm trying to think how this could be fixed. @hynek Could you comment on the following? Thanks!

Would it be acceptable to change the default value for retain_collection_types to True? This would reduce the likelihood of this issue to appear unexpectedly. It is not a full fix though.
Would it be ok to use tuple as default collection type, when retain_collection_types=False? I would expect this to completely resolve the issue.

We cannot change the defaults for attr.asdict due to backward compatibility but we can do that for the upcoming attrs.asdict.

@hynek

@hynek I understand. I've been thinking about related corner cases for asdict, which may also be addressed when making a newer version.

Frozen sets are not changed into a default collection type, while sets are:

>>> import attr
>>> @attr.s
... class A:
...     a = attr.ib()
... 
>>> instance = A(frozenset([1, 2]))
>>> attr.asdict(instance)
{'a': frozenset({1, 2})}
>>>
>>> instance = A(set([1, 2]))
>>> attr.asdict(instance)
{'a': [1, 2]}
>>>

This type of behavior can be made consistent in general by checking whether objects are instances of the classes defined in collections.abc.

(Before making a new version, it would also be useful to understand the motivation for the current behavior.)

I have just merged #704 that fixes the frozenset issue – sorry for the late reply!

I have created a new issue for the new defaults, so closing this one.

No problem. Thank you for fixing!

I think I still experience this issue on latest stable attrs == 21.2.0. Is this expected?

import attr

@attr.s
class Test:
    x = attr.ib()

t = Test(x={(1, 2): 3})
attr.asdict(t)

Traceback (most recent call last):
  File "/tmp/t.py", line 11, in <module>
    attr.asdict(t)
  File "/tmp/.venv/lib/python3.9/site-packages/attr/_funcs.py", line 86, in asdict
    rv[a.name] = df(
TypeError: unhashable type: 'list'

You have to pass retain_collection_types=True because we can't change the default value – that would be a very breaking change.

We intend to change it when you can import it from attrs. I hope I'll get to it later this year.

Add tuple_keys to asdict

Serialize keys as tuples in asdict (#888)

Merge #465 #466

Merge #465 #467

⬆️ Bump attrs from 21.2.0 to 21.3.0 (#18)

Merge #3746 #3747

Merge #465 #467

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`asdict` fails for attributes of type Mapping with keys of type `Tuple` #646

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asdict fails for attributes of type Mapping with keys of type Tuple #646

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tovrstra commented on Aug 27, 2020

hynek commented on Sep 1, 2020

tovrstra commented on Sep 1, 2020

hynek commented on Oct 19, 2020

hynek commented on Oct 19, 2020

tovrstra commented on Oct 19, 2020

ikalnytskyi commented on Jun 16, 2021

hynek commented on Jun 28, 2021

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`asdict` fails for attributes of type Mapping with keys of type `Tuple` #646