[issue39442] from __future__ import annotations makes dataclasses.Field.type a string, not type

[Wojciech Łopata]

Wojciech Łopata <wlopata at gmail.com> added the comment:

> Isn't that the entire point of "from __future__ import annotations"?
I'm not complaining about Foo.__annotations__ storing strings instead of types. I'm complaining about dataclass.Field.type being a string instead of type. I don't think the former needs to imply the latter. I'm trying to access Field objects at runtime, when it should already be possible to resolve the types, as far as I understand.


> Also, please show the traceback when reporting errors so that I can see what's going on.

That's the error I get trying to use dataclasses_serialization module:

$ cat test.py 
from __future__ import annotations
from dataclasses import dataclass
from dataclasses_serialization.json import JSONSerializer

@dataclass
class Foo:
    x: int

JSONSerializer.deserialize(Foo, {'x': 42})
$ python3 test.py 
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/dataclasses_serialization/serializer_base.py", line 125, in dict_to_dataclass
    for fld, fld_type in zip(flds, fld_types)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/dataclasses_serialization/serializer_base.py", line 126, in <dictcomp>
    if fld.name in dct
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/toolz/functoolz.py", line 303, in __call__
    return self._partial(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/dataclasses_serialization/serializer_base.py", line 234, in deserialize
    if issubclass(cls, type_):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/dataclasses_serialization/serializer_base.py", line 72, in issubclass
    return original_issubclass(cls, classinfo)
TypeError: issubclass() arg 1 must be a class

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "test.py", line 9, in <module>
    JSONSerializer.deserialize(Foo, {'x': 42})
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/toolz/functoolz.py", line 303, in __call__
    return self._partial(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/dataclasses_serialization/serializer_base.py", line 238, in deserialize
    return self.deserialization_functions[dataclass](cls, serialized_obj)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/toolz/functoolz.py", line 303, in __call__
    return self._partial(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/dataclasses_serialization/serializer_base.py", line 131, in dict_to_dataclass
    cls
dataclasses_serialization.serializer_base.DeserializationError: Missing one or more required fields to deserialize {'x': 42} as <class '__main__.Foo'>

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39442>
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