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attrs — Classes without boilerplate

Version: 24.2.0 Type: Pure Python SPM target: Bundled in JsonSchema (no standalone target) Auto-included by: JsonSchema, Referencing, Cattrs Total Python modules: 6 (attrs/) + 13 (attr/ sibling package)

attrs is the modern "decorator → class" generator: declare fields with @define/field() and you get __init__, __repr__, __eq__, __hash__, slots, type-validation, immutability, and converters for free. The attrs package is the user-facing modern API; the older attr namespace ships alongside it (re-exported) for backwards compatibility — both come from the same wheel.

It's bundled because referencing (used by jsonschema) imports it for its frozen dataclass machinery.

Modules

attrs/ — modern API

Module What it does
attrs.__init__ Re-exports the public API: define, field, frozen, mutable, Factory, Attribute, Converter, NOTHING, evolve, fields, fields_dict, validate, has, make_class, assoc, asdict, astuple, cmp_using, resolve_types, inspect
attrs.converters Stock converters: optional, default_if_none, pipe, to_bool
attrs.exceptions FrozenInstanceError, FrozenAttributeError, AttrsAttributeNotFoundError, NotAnAttrsClassError, etc.
attrs.filters Predicates for asdict() / astuple(): include, exclude
attrs.setters Setter pipelines: frozen, validate, convert, pipe, NO_OP
attrs.validators Stock validators: instance_of, in_, lt/le/gt/ge, min_len/max_len, matches_re, deep_iterable, deep_mapping, optional, or_, and_

attr/ — classic API (re-exports)

The attr.* names that all old code uses (attr.s, attr.ib, attr.attrs, attr.attrib). Same submodule layout as attrs/ plus _make, _funcs, _cmp, _compat, _config, _next_gen, _version_info. New code should import attrs (the plural form) instead.

iOS-specific patches

None — attrs is pure Python with no platform-specific code paths.

Standalone example

from attrs import define, field, frozen, validators

@define
class Point:
    x: float = field(validator=validators.instance_of((int, float)))
    y: float = field(validator=validators.instance_of((int, float)))

@frozen
class Vector:
    """Immutable — assignment raises FrozenInstanceError."""
    dx: float
    dy: float

p = Point(1.5, 2.0)
print(p)          # Point(x=1.5, y=2.0)
print(p == Point(1.5, 2.0))  # True

v = Vector(3.0, 4.0)
# v.dx = 5.0  →  FrozenInstanceError

For a "tell me what fields this class has" workflow:

from attrs import fields
for f in fields(Point):
    print(f.name, f.type, f.default)

See also