Derived class disambiguating fails, but only sometimes.

Start by reproducing the supplied test_parse_bad_class case with my_converter, BaseClass, and BadSubClass, checking whether structure sometimes selects BaseClass and reports the subclass keys as extra. Trace the disambiguation behavior involved in structuring the shown json_obj, then make the selection deterministic and verify the result is always a BadSubClass.

• 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)