All cattrs functionality is exposed through a cattrs.Converter object. Global cattrs functions, such as cattrs.unstructure(), use a single global converter. Changes done to this global converter, such as registering new structure and unstructure hooks, affect all code using the global functions.

Global Converter#

A global converter is provided for convenience as cattrs.global_converter. The following functions implicitly use this global converter:

Changes made to the global converter will affect the behavior of these functions.

Larger applications are strongly encouraged to create and customize a different, private instance of cattrs.Converter.

Converter Objects#

To create a private converter, simply instantiate a cattrs.Converter.

The core functionality of a converter is structuring and unstructuring data by composing provided and custom handling functions, called hooks.

Currently, a converter contains the following state:

  • a registry of unstructure hooks, backed by a singledispatch and a function_dispatch.

  • a registry of structure hooks, backed by a different singledispatch and function_dispatch.

  • a LRU cache of union disambiguation functions.

  • a reference to an unstructuring strategy (either AS_DICT or AS_TUPLE).

  • a dict_factory callable, used for creating dicts when dumping attrs classes using AS_DICT.

Converters may be cloned using the Converter.copy() method. The new copy may be changed through the copy arguments, but will retain all manually registered hooks from the original.

Fallback Hook Factories#

By default, when a converter cannot handle a type it will:

These behaviors can be customized by providing custom hook factories when creating the converter.

>>> from pickle import dumps

>>> class Unsupported:
...     """An artisinal (non-attrs) class, unsupported by default."""

>>> converter = Converter(unstructure_fallback_factory=lambda _: dumps)
>>> instance = Unsupported()
>>> converter.unstructure(instance)

This also enables converters to be chained.

>>> parent = Converter()

>>> child = Converter(
...     unstructure_fallback_factory=parent._unstructure_func.dispatch, 
...     structure_fallback_factory=parent._structure_func.dispatch,
... )


Converter._structure_func.dispatch and Converter._unstructure_func.dispatch are slated to become public APIs in a future release.

New in version 23.2.0.


The Converter is a converter variant that automatically generates, compiles and caches specialized structuring and unstructuring hooks for attrs classes, dataclasses and TypedDicts.

Converter differs from the cattrs.BaseConverter in the following ways:

  • structuring and unstructuring of attrs classes is slower the first time, but faster every subsequent time

  • structuring and unstructuring can be customized

  • support for attrs classes with PEP563 (postponed) annotations

  • support for generic attrs classes

  • support for easy overriding collection unstructuring

The Converter used to be called GenConverter, and that alias is still present for backwards compatibility reasons.


The BaseConverter is a simpler and slower Converter variant. It does no code generation, so it may be faster on first-use which can be useful in specific cases, like CLI applications where startup time is more important than throughput.