Customizing class un/structuring

This section deals with customizing the unstructuring and structuring processes in cattrs.

Using cattr.gen.GenConverter

The cattr.gen module contains a Converter subclass, the GenConverter. The GenConverter, upon first encountering an attrs class, will use the generation functions mentioned here to generate the specialized hooks for it, register the hooks and use them.

Manual un/structuring hooks

You can write your own structuring and unstructuring functions and register them for types using Converter.register_structure_hook and Converter.register_unstructure_hook. This approach is the most flexible but also requires the most amount of boilerplate.

Using cattr.gen generators

cattrs includes a module, cattr.gen, which allows for generating and compiling specialized functions for unstructuring attrs classes.

One reason for generating these functions in advance is that they can bypass a lot of cattrs machinery and be significantly faster than normal cattrs.

Another reason is that it’s possible to override behavior on a per-attribute basis.

Currently, the overrides only support generating dictionary un/structuring functions (as opposed to tuples), and support omit_if_default, forbid_extra_keys and rename.

omit_if_default

This override can be applied on a per-class or per-attribute basis. The generated unstructuring function will skip unstructuring values that are equal to their default or factory values.

>>> from cattr.gen import make_dict_unstructure_fn, override
>>>
>>> @attr.s
... class WithDefault:
...    a = attr.ib()
...    b = attr.ib(factory=dict)
>>>
>>> c = cattr.Converter()
>>> c.register_unstructure_hook(WithDefault, make_dict_unstructure_fn(WithDefault, c, b=override(omit_if_default=True)))
>>> c.unstructure(WithDefault(1))
{'a': 1}

Note that the per-attribute value overrides the per-class value. A side-effect of this is the ability to force the presence of a subset of fields. For example, consider a class with a DateTime field and a factory for it: skipping the unstructuring of the DateTime field would be inconsistent and based on the current time. So we apply the omit_if_default rule to the class, but not to the DateTime field.

>>> from pendulum import DateTime
>>> from cattr.gen import make_dict_unstructure_fn, override
>>>
>>> @attr.s
... class TestClass:
...     a: Optional[int] = attr.ib(default=None)
...     b: DateTime = attr.ib(factory=DateTime.utcnow)
>>>
>>> c = cattr.Converter()
>>> hook = make_dict_unstructure_fn(TestClass, c, omit_if_default=True, b=override(omit_if_default=False))
>>> c.register_unstructure_hook(TestClass, hook)
>>> c.unstructure(TestClass())
{'b': ...}

This override has no effect when generating structuring functions.

forbid_extra_keys

By default cattrs is lenient in accepting unstructured input. If extra keys are present in a dictionary, they will be ignored when generating a structured object. Sometimes it may be desirable to enforce a stricter contract, and to raise an error when unknown keys are present - in particular when fields have default values this may help with catching typos. forbid_extra_keys can also be enabled (or disabled) on a per-class basis when creating structure hooks with make_dict_structure_fn.

>>> from cattr.gen import make_dict_structure_fn
>>>
>>> @attr.s
... class TestClass:
...    number: int = attr.ib(default=1)
>>>
>>> c = cattr.GenConverter(forbid_extra_keys=True)
>>> c.structure({"nummber": 2}, TestClass)
Traceback (most recent call last):
...
Exception: Extra fields in constructor for TestClass: nummber
>>> hook = make_dict_structure_fn(TestClass, c, _cattr_forbid_extra_keys=False)
>>> c.register_structure_hook(TestClass, hook)
>>> c.structure({"nummber": 2}, TestClass)
TestClass(number=1)

This behavior can only be applied to classes or to the default for the GenConverter, and has no effect when generating unstructuring functions.

rename

Using the rename override makes cattrs simply use the provided name instead of the real attribute name. This is useful if an attribute name is a reserved keyword in Python.

>>> from pendulum import DateTime
>>> from cattr.gen import make_dict_unstructure_fn, make_dict_structure_fn, override
>>>
>>> @attr.s
... class ExampleClass:
...     klass: Optional[int] = attr.ib()
>>>
>>> c = cattr.Converter()
>>> unst_hook = make_dict_unstructure_fn(ExampleClass, c, klass=override(rename="class"))
>>> st_hook = make_dict_structure_fn(ExampleClass, c, klass=override(rename="class"))
>>> c.register_unstructure_hook(ExampleClass, unst_hook)
>>> c.register_structure_hook(ExampleClass, st_hook)
>>> c.unstructure(ExampleClass(1))
{'class': 1}
>>> c.structure({'class': 1}, ExampleClass)
ExampleClass(klass=1)