Assume I created a @dataclass class Foo, and added a __post_init__ to perform type checking and processing.

When I attempt to yaml.load a !Foo object, __post_init__ is not called.

from dataclasses import dataclass, fieldsfrom ruamel.yaml import yaml_object, YAMLyaml = YAML()@yaml_object(yaml)
@dataclass
class Foo:foo: intbar: intdef __post_init__(self):raise Exceptionfor field in fields(self):value = getattr(self, field.name)typ = field.typeif not isinstance(value, typ):raise Exceptions = '''\
!Foo
foo: "foo"
bar: "bar"
'''
yaml.load(s)

How do I perform parameter checking when loading dataclasses via ruamel.yaml?

This behavior occurs in Python 3.7 as well as 3.6 with pip install dataclasses.

This is now supported in 0.17.34, where you can do

from dataclasses import dataclass
from ruamel.yaml import YAMLyaml = ruamel.yaml.YAML()@yaml.register_class
@dataclass
class Foo:....

The reason why __post_init__ is not called, is because ruamel.yaml (and the PyYAML code in its Constructors), was created long before dataclasses was created.

Of course code for making a call to __post_init_() could be added to ruamel.yaml's Python object constructors, preferably after a test if something was created using @dataclass, as otherwise a non Data-Class class, that happens to have such a method named __post_init_, will all of a sudden have that method called during loading.

If you have no such classes, you can add your own, smarter, constructor to the YAML() instance before first loading/dumping (at which moment the constructor is instantiated) using yaml.Constructor = MyConstructor. But adding a constructor is not as trivial as subclassing the RoundTripConstructor, because all supported node types need to be registered on such a new constructor type.

Most of the time I find it easier to just patch the appropriate method on the RoundTripConstructor:

from dataclasses import dataclass, fields
from ruamel.yaml import yaml_object, YAML, RoundTripConstructordef my_construct_yaml_object(self, node, cls):for data in self.org_construct_yaml_object(node, cls):yield data# not doing a try-except, in case `__post_init__` does catch the AttributeErrorpost_init = getattr(data, '__post_init__', None)if post_init:post_init()RoundTripConstructor.org_construct_yaml_object = RoundTripConstructor.construct_yaml_object
RoundTripConstructor.construct_yaml_object = my_construct_yaml_objectyaml = YAML()
yaml.preserve_quotes = True@yaml_object(yaml)
@dataclass
class Foo:foo: intbar: intdef __post_init__(self):for field in fields(self):value = getattr(self, field.name)typ = field.typeif not isinstance(value, typ):raise Exceptions = '''\
!Foo
foo: "foo"
bar: "bar"
'''
d = yaml.load(s)

throws an exception:

Traceback (most recent call last):File "try.py", line 36, in <module>d = yaml.load(s)File "/home/venv/tmp-46489abf428c4cd4/lib/python3.7/site-packages/ruamel/yaml/main.py", line 266, in loadreturn constructor.get_single_data()File "/home/venv/tmp-46489abf428c4cd4/lib/python3.7/site-packages/ruamel/yaml/constructor.py", line 105, in get_single_datareturn self.construct_document(node)File "/home/venv/tmp-46489abf428c4cd4/lib/python3.7/site-packages/ruamel/yaml/constructor.py", line 115, in construct_documentfor dummy in generator:File "try.py", line 10, in my_construct_yaml_objectpost_init()File "try.py", line 29, in __post_init__raise Exception
Exception

Please note that the double quotes in your YAML are superfluous, so if you want to preserve these on round-trip you need to do yaml.preserve_quotes = True