Customizing class un/structuring¶
This section deals with customizing the unstructuring and structuring processes in cattrs.
Using cattr.GenConverter
¶
The cattr
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
,
rename
and omit
.
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
>>>
>>> @define
... class WithDefault:
... a: int
... b: dict = 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.
Note
The parameter to make_dict_unstructure_function is named _cattrs_omit_if_default
instead of just omit_if_default
to avoid potential collisions with an override for a field named omit_if_default
.
>>> from pendulum import DateTime
>>> from cattr.gen import make_dict_unstructure_fn, override
>>>
>>> @define
... class TestClass:
... a: Optional[int] = None
... b: DateTime = Factory(DateTime.utcnow)
>>>
>>> c = cattr.Converter()
>>> hook = make_dict_unstructure_fn(TestClass, c, _cattrs_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
>>>
>>> @define
... class TestClass:
... number: int = 1
>>>
>>> c = cattr.GenConverter(forbid_extra_keys=True)
>>> c.structure({"nummber": 2}, TestClass)
Traceback (most recent call last):
...
ForbiddenExtraKeyError: Extra fields in constructor for TestClass: nummber
>>> hook = make_dict_structure_fn(TestClass, c, _cattrs_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
>>>
>>> @define
... class ExampleClass:
... klass: Optional[int]
>>>
>>> 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)
omit
¶
This override can only be applied to individual attributes. Using the omit
override will simply skip the attribute completely when generating a structuring
or unstructuring function.
>>> from cattr.gen import make_dict_unstructure_fn, override
>>>
>>> @define
... class ExampleClass:
... an_int: int
>>>
>>> c = cattr.Converter()
>>> unst_hook = make_dict_unstructure_fn(ExampleClass, c, an_int=override(omit=True))
>>> c.register_unstructure_hook(ExampleClass, unst_hook)
>>> c.unstructure(ExampleClass(1))
{}