Dataclasses.asdict - transformation of dict-fields - change type(obj) to dict directly

[cschroeer]

Hello all,

I refer to the current implementation of the public method asdict within dataclasses-module transforming the dataclass input to a dictionary.

Sometimes, a dataclass has itself a dictionary as field.
Therefore, the current implementation is used for transformation ( see. cpython/dataclasses.py at 0a7936a38f0bab1619ee9fe257880a51c9d839d5 · python/cpython · GitHub):

...
    elif isinstance(obj, dict):
        return type(obj)((_asdict_inner(k, dict_factory),
                          _asdict_inner(v, dict_factory))
                         for k, v in obj.items())
...

As it is already clear, that obj is a dict, I prefer to change this line to

...
    elif isinstance(obj, dict):
        return dict((_asdict_inner(k, dict_factory),
                          _asdict_inner(v, dict_factory))
                         for k, v in obj.items())
...

Otherwise, using dataclasses in combination with SqlAlchemy, the current implementation of asdict leads to an error, as already reported here:

TypeError _MKeyfuncMapped.__init__() when calling type() on mapped dict-field in a dataclass

What do you think?

Thanks and best,
Christoph

[DavidCEllis (David Ellis)]

It is not clear the object is a dict, this also catches subclasses of dict.

>>> class MyDict(dict): ...
...
>>> isinstance(MyDict(), dict)
True

This change would alter the behaviour of asdict in cases where subclasses of dict being returned as-is is relied upon.

Where asdict_altered has this change.

class MyDict(dict):
    """Assume this does something useful"""
    def __repr__(self):
        original_repr = super().__repr__()
        return f"MyDict({original_repr})"

@dataclass
class X:
    x: MyDict = field(default_factory=MyDict)


inst = X()
inst.x["Key"] = "value"

converted = asdict(inst, dict_factory=MyDict)
new_converted = asdict_altered(inst, dict_factory=MyDict)

print(f"{converted=}")
print(f"{new_converted=}")

Output:

converted=MyDict({'x': MyDict({'Key': 'value'})})
new_converted=MyDict({'x': {'Key': 'value'}})

In this case MyDict doesn’t do anything useful but if it did you’ve potentially broken code that relies on asdict returning the subclass.

[cschroeer]

Ok check. Thanks for clarification. Make sense.

However, adding an init-Method, the asdict-method leads to the following behavior.

Reproduce the error

from dataclasses import field, asdict, dataclass


class MyDict(dict):

    def __init__(self):
        pass

    """Assume this does something useful"""
    def __repr__(self):
        original_repr = super().__repr__()
        return f"MyDict({original_repr})"

@dataclass
class X:
    x: MyDict = field(default_factory=MyDict)


inst = X()
inst.x["Key"] = "value"

converted = asdict(inst, dict_factory=MyDict)

print(f"{converted=}")

Error

Traceback (most recent call last):
  File "/home/cschroeer/python-projects/dataclasses-asdict-core/mydict.py", line 22, in <module>
    converted = asdict(inst, dict_factory=MyDict)
  File "/usr/local/lib/python3.10/dataclasses.py", line 1238, in asdict
    return _asdict_inner(obj, dict_factory)
  File "/usr/local/lib/python3.10/dataclasses.py", line 1245, in _asdict_inner
    value = _asdict_inner(getattr(obj, f.name), dict_factory)
  File "/usr/local/lib/python3.10/dataclasses.py", line 1275, in _asdict_inner
    return type(obj)((_asdict_inner(k, dict_factory),
TypeError: MyDict.__init__() takes 1 positional argument but 2 were given

Is this also expected?

[DavidCEllis (David Ellis)]

Well, yes - you’ve changed the __init__ which is what’s being used by type(cls)(...). This usage requires the subclass to have the same signature as dict (or at least a compatible signature). This would also fail to function as the argument to dict_factory even with the change.

[cschroeer]

Thanks for clarification