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

[radugrosu]

@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.

[tovrstra]

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.

[hynek]

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

[tovrstra]

@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.)

[hynek]

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

[hynek]

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

[tovrstra]

No problem. Thank you for fixing!

[ikalnytskyi]

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'

[hynek]

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.