cattrs
Composable complex class support for attrs and dataclasses.
Decision gist · record as of 2026-08-14
Yes. cattrs is actively maintained, permissively licensed, has low install friction, and solves a common problem—bridging unstructured and typed data—without requiring you to pollute your models with serialization logic. It's a good fit if you work with attrs or dataclasses and need flexible, recursive (un)structuring with validation.AI-flagged interpretation of the facts on this page — verify before relying
Before you install
- Requires Python 3.10 or later.
- Low friction installation with three lightweight runtime dependencies (attrs, exceptiongroup, typing-extensions).
- Actively maintained as of 2026-02-18 with recent commits and 1049 repository stars.
License · maintenance · safety
MIT (permissive) — MIT license permits unrestricted commercial and private use with minimal obligations—only attribution required.
last release 2026-02-18 (177 days) · last repo commit 2026-08-09 · 1,049 stars
0 known vulnerabilities (OSV.dev, 2026-08-14) · 73,830,482 downloads/mo, #455 on PyPI
Alternatives
Verify before relying
from attrs import define
from cattrs import structure, unstructure
@define
class C:
a: int
b: list[str]
instance = structure({'a': 1, 'b': ['x', 'y']}, C)
unstructure(instance)- Performance characteristics compared to alternative serialization libraries under real-world workloads.
- Extent of support for complex union disambiguation beyond the documented unique-field strategy.
What it is and what it does
cattrs is a data (un)structuring library that bridges unstructured data (dictionaries, JSON) and typed Python objects. It converts dictionaries into attrs classes or dataclasses recursively, validating types along the way, and can reverse the process to serialize objects back to dictionaries. The library keeps serialization rules separate from your data models, so you can define multiple conversion strategies for the same class without modifying it.
It handles nested structures, optional fields, collections (lists, sets, tuples, dicts), TypedDict, and custom types through registered hooks. The package includes pre-configured converters for JSON, msgpack, CBOR, BSON, YAML, TOML, and other serialization formats. Runtime dependencies are minimal: attrs (which it's designed around), exceptiongroup (for grouped validation errors), and typing-extensions (for type hint compatibility).
Use it for
- Deserialize API responses or configuration files into typed dataclasses with automatic validation.
- Convert database query results into attrs classes while enforcing field types and constraints.
- Build data pipelines that transform unstructured input into strongly-typed intermediate objects.
- Serialize domain models to multiple formats (JSON, YAML, msgpack) without adding format-specific code to the model.
- Validate and normalize user input from web forms or CLI arguments into typed objects.
Worth the install?
AI-flagged interpretation of the facts on this page. Verify before relying on it.
Yes.
cattrs is actively maintained, permissively licensed, has low install friction, and solves a common problem—bridging unstructured and typed data—without requiring you to pollute your models with serialization logic. It's a good fit if you work with attrs or dataclasses and need flexible, recursive (un)structuring with validation.
Install
cattrs on PyPI
Before you install
Low friction installation with three lightweight runtime dependencies (attrs, exceptiongroup, typing-extensions). Actively maintained as of 2026-02-18 with recent commits and 1049 repository stars.
Requires Python 3.10 or later.
License in practice
MIT license permits unrestricted commercial and private use with minimal obligations—only attribution required.
Quickstart
from attrs import define
from cattrs import structure, unstructure
@define
class C:
a: int
b: list[str]
instance = structure({'a': 1, 'b': ['x', 'y']}, C)
unstructure(instance)Verify before relying
- Performance characteristics compared to alternative serialization libraries under real-world workloads.
- Extent of support for complex union disambiguation beyond the documented unique-field strategy.
Package facts
| License | MIT permissive |
| Python support | Supports the current Python release >=3.10 |
| Install friction | Low. Pure-Python wheel |
| Runtime dependencies | 3 packagesattrsexceptiongrouptyping-extensions |
| Maintenance | Actively maintained 177 days since the last release |
| Last repo commit | |
| First released | |
| Downloads | 73,830,482 / month, #455 on PyPI 30-day window, as of 2026-08-14 |
| Known vulnerabilities | None known OSV.dev, checked 2026-08-14 |
| Classifiers | Development Status :: 5 - Production/StableIntended Audience :: DevelopersLicense :: OSI Approved :: MIT LicenseProgramming Language :: Python :: 3.10Programming Language :: Python :: 3.11Programming Language :: Python :: 3.12Programming Language :: Python :: 3.13Programming Language :: Python :: 3.14Programming Language :: Python :: Implementation :: CPythonProgramming Language :: Python :: Implementation :: PyPyTyping :: Typed |
Evidence: cattrs-26.1.0-py3-none-any.whl
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