Derived class disambiguating fails, but only sometimes.

[scottee]

• cattrs version: 23.2.3
• Python version: 3.9
• Operating System: MacOS and Linux

Description

I have a set of classes that all inherit from one base class. I have one class that has a unique required attribute to differentiate it defined like this:

@define(kw_only=True, init=False)
class BadSubClass(BaseClass):
    # This "arg1" field is used in other derived classes, but is never required in any other class.
    # BTW, my converter has registered structure hooks for Union[str, List[str]], which forward to str_to_list.
    arg1: Union[List[str], str] = field(converter=str_to_list)  # Converter converts single str to a list.
    other_arg1: Union[List[str], str] = field(factory=list, converter=str_to_list)
    other_arg2: Optional[str] = None

I have a unit test which tries to structure json with this BadSubClass. About half the time I get an exception that the BaseClass deserializing got extra keys (the keys of BadSubClass). Well I say half the time, but now that I'm trying to recreate the error, it won't throw the error. (You might say I'm just crazy. You'd be right, but not for this reason.)

The question is, Has anyone seen this disambiguating problem, especially when it is transient like this?

What I Did

Test case looks like this:

def test_parse_bad_class():
    # Aargh: cattrs is non-deterministic in whether it can deserialize this class.
    # Should work all the time, but sometimes it throws an exception.
    json_obj = {
        "arg1": "foo",
        "other_arg1": "blah"
        "base_class_arg4": {  # This is a different non-sub-class
            "key1": "..."
        },
    }
    result = my_converter.structure(json_obj, BaseClass)
    assert isinstance(result, BadSubClass)

[Tinche]

Can you paste in the exact exception that sometimes flies out? Maybe it'll help.

[Tinche]

From your snippets I'm also going to assume you're using include_subclasses too? Otherwise that structure call would always just return BaseClass.

[scottee]

I'll add the exact exception when I can recreate it.

And yes, I am using include_subclasses.

[scottee]

Here is the exception that results periodically from this issues:

      | Exception Group Traceback (most recent call last):
      |   File "", line 5, in structure_mapping
      |   File "/opt/homebrew/Caskroom/mambaforge/base/lib/python3.10/site-packages/cattrs/strategies/_subclasses.py", line 125, in struct_hook
      |     return _base_hook(val, _cl)
      |   File "<cattrs generated structure blah.BaseClass>", line 91, in structure_BaseClass
      |     if errors: raise __c_cve('While structuring ' + 'BaseClass', errors, __cl)
      | cattrs.errors.ClassValidationError: While structuring BaseClass (1 sub-exception)
      | Structuring mapping value @ key 'arg1'
      +-+---------------- 1 ----------------
        | cattrs.errors.ForbiddenExtraKeysError: Extra fields in constructor for BaseClass: other_arg1, arg1, other_arg2
        +------------------------------------