Sample

Decide whether attrs still buys anything over dataclasses on modern Python, and what changes in each direction

sha256:db459e2a5619e5bc96877e0fc6aefd6229be3342cc28685624eacd9e170bf4b7

Case

Goal

Decide whether attrs still buys anything over dataclasses on modern Python, and what changes in each direction MIGRATION

Packages

attrs 26.1.0

Environment

python 3.12

Created

2026-08-14T12:07:55Z

Contract

• assert neither library enforces an annotation: a dataclass stores a str in an int field and so does an @define class with no validator
• assert an attrs validator runs at __init__, with instance_of raising TypeError and gt raising ValueError, and that ge, lt and le raise ValueError too
• assert the converter runs before the validator, so converter=int and instance_of(int) coexist and "9000" is stored as 9000
• assert @define runs converter and validator again on assignment, where the dataclass keeps whatever was assigned
• assert __post_init__ is not the equivalent hook: it runs at __init__ and again through dataclasses.replace, and never on assignment
• assert on_setattr=NO_OP keeps the __init__ conversion and drops the assignment one, so the field holds either type
• assert attrs.validators.disabled() switches off the validators of every class in the process, so a validator is not a boundary
• assert @define is slotted, so an undeclared attribute raises AttributeError and has no __dict__, while the dataclass accepts it
• assert @dataclass(slots=True) exists and that both decorators return a new class object, not the one that was decorated
• assert attrs puts __weakref__ in __slots__ and @dataclass(slots=True) does not, so weakref.ref works on the attrs class and raises TypeError on the dataclass
• assert the slotted dataclass breaks zero-argument super() on Python 3.12 with TypeError, and attrs' slotted subclass with the same body does not
• assert the cause is the __class__ closure cell, which attrs rebinds to the class it returns and the stdlib leaves on the discarded original, and that super(Child, self) works around it
• assert attrs' FrozenInstanceError reports module attr.exceptions and is a different class from dataclasses.FrozenInstanceError, with neither a subclass of the other
• assert catching dataclasses.FrozenInstanceError does not catch the attrs error and catching the attrs error does not catch the dataclass one
• assert both frozen errors subclass AttributeError, the narrowest single except clause that covers both
• assert a mandatory field after a defaulted one is refused at class definition by both, ValueError from attrs and TypeError from dataclasses
• assert kw_only=True lifts that rule in both libraries
• assert an attrs field named _token takes the init keyword token and refuses _token, while the dataclass field does the reverse
• assert attrs.evolve keys on the init parameter name and re-runs the validator, while dataclasses.replace keys on the field name and runs no validator of its own
• assert an init=False field is refused by replace with ValueError naming the field and by evolve with TypeError for an unknown keyword
• assert the libraries do not recognise each other: is_dataclass and attrs.has are False, dataclasses.fields raises TypeError and attrs.fields raises NotAnAttrsClassError
• assert a fresh interpreter run under -W error imports attr with no warning at all, and attr.__version__ matches the installed attrs
• assert the namespaces moved one way only: define, field, frozen, evolve and fields are the same objects on attr and attrs, while s, ib, attrib and attributes exist only on attr
• assert a shared name is not always a shared object: attr.asdict is not attrs.asdict and only the old one takes retain_collection_types, and attr.exceptions is not attrs.exceptions
• assert an @attr.s class is unslotted and converts at __init__ only, leaving object.__setattr__ in place where @define installs its own

Files

• csx.json
• requirements.txt
• src/__init__.py
• src/models.py
• test/contract.py

Origin Seeder

anonymous

Verification receipts

• python 3.12 · CONTAINER_RUN · compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS · pypi@1 · 2026-08-14 · ed25519:a2ec939a4c60e243
• · CONTAINER_RUN · compile:SKIPPED · contract:FAIL · load:SKIPPED · resolve:PASS · pypi@1 · 2026-08-14 · ed25519:7da3f610facce8bf