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Composable complex class support for attrs.

Project Description


cattrs is an experimental open source Python 3 library providing composable complex class conversion support for attrs classes. Other kinds of classes are supported by manually registering converters.

Python has a rich set of powerful, easy to use, built-in data types like dictionaries, lists and tuples. These data types are also the lingua franca of most data serialization libraries, for formats like json, msgpack, yaml or toml.

Data types like this, and mappings like dict s in particular, represent unstructured data. Your data is, in all likelihood, structured: not all combinations of field names are values are valid inputs to your programs. In Python, structured data is better represented with classes and enumerations. attrs is an excellent library for declaratively describing the structure of your data, and validating it.

When you’re handed unstructured data, cattrs helps to convert this data into structured data. When you have to convert your structured data into data types other libraries can handle, cattrs turns your classes and enumerations into dictionaries, integers and strings.

A taste:

>>> from enum import unique, Enum
>>> from typing import List, Sequence, Union
>>> from cattr import loads, dumps
>>> import attr
>>> from attr.validators import instance_of, optional
>>> @unique
... class CatBreed(Enum):
...     SIAMESE = "siamese"
...     MAINE_COON = "maine_coon"
...     SACRED_BIRMAN = "birman"
>>> @attr.s
... class Cat:
...     breed = attr.ib(validator=instance_of(CatBreed))
...     names = attr.ib(validator=instance_of(Sequence[str]))
>>> @attr.s
... class DogMicrochip:
...     chip_id = attr.ib()
...     time_chipped = attr.ib(validator=instance_of(float))
>>> @attr.s
... class Dog:
...     cuteness = attr.ib(validator=instance_of(int))
...     chip = attr.ib(validator=optional(instance_of(DogMicrochip)))
>>> p = dumps([Dog(cuteness=1, chip=DogMicrochip(chip_id=1, time_chipped=10.0)),
...            Cat(breed=CatBreed.MAINE_COON, names=('Fluffly', 'Fluffer'))])
>>> print(p)
[{'chip': {'chip_id': 1, 'time_chipped': 10.0}, 'cuteness': 1}, {'names': ('Fluffly', 'Fluffer'), 'breed': 'maine_coon'}]
>>> print(loads(p, List[Union[Dog, Cat]]))
[Dog(cuteness=1, chip=DogMicrochip(chip_id=1, time_chipped=10.0)), Cat(breed=<CatBreed.MAINE_COON: 'maine_coon'>, names=['Fluffly', 'Fluffer'])]

dumps and loads were chosen for their similarity to the functionality of modules like marshal, pickle and json. Consider unstructured data a low-level representation that needs to be converted to structured data to be handled, and use loads. When you’re done, dumps the data to its unstructured form and pass it along to another library or module.


When using Python 3.5 earlier or equal to 3.5.3 or Python 3.6.0, please use the bundled cattr.typing module instead of Python’s standard typing module. These versions of typing are incompatible with cattrs. If your Python version is a later one, please use Python’s typing instead.


  • Converts structured data into unstructured data, recursively:
    • attrs classes are converted into dictionaries, in a way similar to attrs.asdict.
    • Enumeration instances are converted to their values.
    • Other types are let through without conversion. This includes types such as integers, dictionaries, lists and instances of non-attrs classes.
    • Custom converters for any type can be registered using register_dumps_hook.
  • Converts unstructured data into structured data, recursively, according to your specification given as a type. The following types are supported:
    • typing.Optional[T].
    • typing.List[T], typing.MutableSequence[T], typing.Sequence[T] (converts to a list).
    • typing.Tuple (both variants, Tuple[T, ...] and Tuple[X, Y, Z]).
    • typing.MutableSet[T], typing.Set[T] (converts to a set).
    • typing.FrozenSet[T] (converts to a frozenset).
    • typing.Dict[K, V], typing.MutableMapping[K, V], typing.Mapping[K, V] (converts to a dict).
    • attrs classes with simple attributes and the usual __init__.
      • Simple attributes are attributes that can be assigned unstructured data, like numbers, strings, and collections of unstructured data.
    • All attrs classes with the usual __init__, if their complex attributes have type metadata.
    • typing.Union s of supported attrs classes, given that all of the classes have a unique required field.
    • typing.Union s of anything, given that you provide a disambiguation function for it.
    • Custom converters for any type can be registered using register_loads_hook.


Major credits to Hynek Schlawack for creating attrs and its predecessor, characteristic.

cattrs is tested with Hypothesis, by David R. MacIver.

cattrs is benchmarked using perf, by Victor Stinner.

This package was created with Cookiecutter and the audreyr/cookiecutter-pypackage project template.



  • Python 3.4 is no longer supported.
  • Introduced cattr.typing for use with Python versions 3.5.2 and 3.6.0.
  • Minor changes to work with newer versions of typing.
    • Bare Optionals are not supported any more (use Optional[Any]).
  • Attempting to load unrecognized classes will result in a ValueError, and a helpful message to register a loads hook.
  • Loading attrs classes is now documented.
  • The global converter is now documented.
  • cattr.loads_attrs_fromtuple and cattr.loads_attrs_fromdict are now exposed.

0.2.0 (2016-10-02)

  • Tests and documentation.

0.1.0 (2016-08-13)

  • First release on PyPI.
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