Dataclass kw_only attribute incorrectly structured without detailed validation enabled

[moltob]

With cattrs version v24.1.2 the following test fails:

def test__cattrs_kw_only_propagation():
    import dataclasses

    import cattrs

    @dataclasses.dataclass
    class PartialKeywords:
        a1: str = 'Default'
        a2: str = dataclasses.field(kw_only=True)

    converter = cattrs.Converter(detailed_validation=False)

    assert converter.structure(
        {
            'a2': 'Value',
        },
        PartialKeywords,
    ) == PartialKeywords(
        a1='Default',
        a2='Value',
    )

What is the test showing?

A dataclass class is defined, with the first attribute possibly positional, and the second attribute with kw_only=True. The first attribute has a default value, so it is optional.

When structuring the passed dict, cattrs is expected to honor the kw_only attribute setting. It does not.

Suggested root cause

The _compat module has functions to support dataclasses by converting some features to the corresponding attrs concepts. The function adapted_fields() does this for class attribute descriptors by creating cattrs Attribute instances from dataclass fields.

The Attribute creation lacks the kw_only argument and therefore uses the default kw_only=False.

"Unfortunately" the defect hits only in one scenario:

• When using BaseConverter, this does not hit, as structure_attrs_fromdict always uses kw-based creation.
• When using Converter with detailed validation enabled, the generated instantiation script is also always using kw-based creation.
• Only when using Converter without detailed validation support, the generated instantion script depends on the kw_only setting of an attribute.

The latter distinction is part of Converter.make_dict_structure_n_from_attrs, where you find code like:

if a.kw_only:
    invocation_line = f"{a.alias}={invocation_line}"

Options to fix

As only the non-detailed-validation case is currently respecting kw_only at all, this could could be removed to even in that case always use kw-based class creation in the instantiation script.

If that code should be kept, adapted_fields can possibly be fixed easily by propagating the kw_only setting:

Attribute(
    attr.name,
    ...,

    # FIX:
    kw_only=attr.kw_only,
)
for attr in attrs
]

This patch fixes the test above.

[Tinche]

Mmm good catch. Let me see if I can release this as 24.1.3 soon.

[Tinche]

New version is out! Great job on the analysis.

[moltob]

Thank you for releasing this so quickly! This is a terrific library.