当ruamel.yaml从字符串加载@dataclass时，不调用__post_init__ [英] When ruamel.yaml loads @dataclass from string, __post_init__ is not called

问题描述

假设我创建了一个@dataclass class Foo，并添加了一个__post_init__来执行类型检查和处理.

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

当我尝试yaml.load一个!Foo对象时，未调用__post_init__.

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

from dataclasses import dataclass, fields

from ruamel.yaml import yaml_object, YAML


yaml = YAML()


@yaml_object(yaml)
@dataclass
class Foo:
    foo: int
    bar: int

    def __post_init__(self):
        raise Exception
        for field in fields(self):
            value = getattr(self, field.name)
            typ = field.type
            if not isinstance(value, typ):
                raise Exception

s = '''\
!Foo
foo: "foo"
bar: "bar"
'''
yaml.load(s)

通过ruamel.yaml加载数据类时如何执行参数检查?

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

此行为在Python 3.7以及pip install dataclasses的3.6中发生.

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

推荐答案

之所以未调用__post_init__的原因是，ruamel.yaml(及其Constructor中的PyYAML代码)早于dataclasses已创建.

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.

当然，可以将调用__post_init_()的代码添加到ruamel.yaml的Python对象构造函数中，最好在测试是否使用@dataclass创建了某些东西之后，否则将使用非Data-Class类，碰巧有一个名为__post_init_的方法，突然在加载过程中会调用该方法.

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.

如果没有此类，则可以在使用yaml.Constructor = MyConstructor首次加载/转储(实例化构造函数的那一刻)之前，将自己的更聪明的构造函数添加到YAML()实例.但是添加构造函数并不像RoundTripConstructor的子类那么简单，因为所有支持的节点类型都需要在这种新的构造函数类型上注册.

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.

大多数时候，我发现在RoundTripConstructor上修补适当的方法会更容易:

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, RoundTripConstructor


def 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 AttributeError
    post_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_object

yaml = YAML()
yaml.preserve_quotes = True

@yaml_object(yaml)
@dataclass
class Foo:
    foo: int
    bar: int

    def __post_init__(self):
        for field in fields(self):
            value = getattr(self, field.name)
            typ = field.type
            if not isinstance(value, typ):
                raise Exception

s = '''\
!Foo
foo: "foo"
bar: "bar"
'''
d = yaml.load(s)

引发异常:

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 load
    return constructor.get_single_data()
  File "/home/venv/tmp-46489abf428c4cd4/lib/python3.7/site-packages/ruamel/yaml/constructor.py", line 105, in get_single_data
    return self.construct_document(node)
  File "/home/venv/tmp-46489abf428c4cd4/lib/python3.7/site-packages/ruamel/yaml/constructor.py", line 115, in construct_document
    for dummy in generator:
  File "try.py", line 10, in my_construct_yaml_object
    post_init()
  File "try.py", line 29, in __post_init__
    raise Exception
Exception

请注意，YAML中的双引号是多余的，因此，如果要在往返过程中保留双引号，则需要执行yaml.preserve_quotes = True

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

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