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python - 使用 ruamel.yaml 安全转储和加载 defaultdict

问题描述

我正在尝试（反）序列化具有ruamel.yamlcollections.defaultdict属性的类类（在我的情况下为 3.6+）。

这将是我想开始工作的一个最小示例：

from collections import defaultdict
import ruamel.yaml
from pathlib import Path

class Foo:
    def __init__(self):
        self.x = defaultdict()


YAML = ruamel.yaml.YAML(typ="safe")
YAML.register_class(Foo)
YAML.register_class(defaultdict)

fp =  Path("./test.yaml")
YAML.dump(Foo(), fp)
YAML.load(fp)

但这失败了：

AttributeError: 'collections.defaultdict' object has no attribute '__dict__'

任何不需要为每个“Foo-like”类编写自定义代码的想法？我希望我可以添加一个不同的代表defaultdict对象添加一个不同的表示器，但到目前为止我的尝试都是徒劳的。

完整追溯：

Traceback (most recent call last):
File "./tests/test_yaml.py", line 18, in <module>
    YAML.dump(Foo(), fp)
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\main.py", line 439, in dump
    return self.dump_all([data], stream, _kw, transform=transform)
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\main.py", line 453, in dump_all
    self._context_manager.dump(data)
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\main.py", line 801, in dump
    self._yaml.representer.represent(data)
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\representer.py", line 81, in represent
    node = self.represent_data(data)
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\representer.py", line 108, in represent_data
    node = self.yaml_representers[data_types[0]](self, data)
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\main.py", line 638, in t_y
    tag, data, cls, flow_style=representer.default_flow_style
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\representer.py", line 384, in represent_yaml_object
    return self.represent_mapping(tag, state, flow_style=flow_style)
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\representer.py", line 218, in represent_mapping
    node_value = self.represent_data(item_value)
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\representer.py", line 108, in represent_data
    node = self.yaml_representers[data_types[0]](self, data)
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\main.py", line 638, in t_y
    tag, data, cls, flow_style=representer.default_flow_style
File "C:\miniconda-windows\envs\ratio\lib\site-packages\ruamel\yaml\representer.py", line 383, in represent_yaml_object
    state = data.__dict__.copy()
AttributeError: 'collections.defaultdict' object has no attribute '__dict__'

标签： pythonyamldefaultdictruamel.yaml

解决方案

这是因为defaultdict它是内置类的子类dict，它没有__dict__属性供 YAML 编码器生成类属性名称。在这种情况下defaultdict应该被视为一个dict，但问题是该类的represent_data方法ruamel.yaml.representer.BaseRepresenter只查看对象本身的类来确定是否存在该对象的表示器：

data_types = type(data).__mro__
# ...skipped
if data_types[0] in self.yaml_representers:
    node = self.yaml_representers[data_types[0]](self, data)

相反，应该做的是检查其中的任何数据类型__mro__是否具有表示器，如果找到则使用它：

if any(data_type in self.yaml_representers for data_type in data_types):
    node = self.yaml_representers[next(data_type for data_type in data_types if data_type in self.yaml_representers)](self, data)

所以我们可以自己猴子修补这个方法：

def represent_data(self, data):
    # type: (Any) -> Any
    if self.ignore_aliases(data):
        self.alias_key = None
    else:
        self.alias_key = id(data)
    if self.alias_key is not None:
        if self.alias_key in self.represented_objects:
            node = self.represented_objects[self.alias_key]
            # if node is None:
            #     raise RepresenterError(
            #          "recursive objects are not allowed: %r" % data)
            return node
        # self.represented_objects[alias_key] = None
        self.object_keeper.append(data)
    data_types = type(data).__mro__
    if representer.PY2:
        # if type(data) is types.InstanceType:
        if isinstance(data, representer.types.InstanceType):
            data_types = representer.get_classobj_bases(data.__class__) + list(data_types)
    if any(data_type in self.yaml_representers for data_type in data_types):
        node = self.yaml_representers[next(data_type for data_type in data_types if data_type in self.yaml_representers)](self, data)
    else:
        for data_type in data_types:
            if data_type in self.yaml_multi_representers:
                node = self.yaml_multi_representers[data_type](self, data)
                break
        else:
            if None in self.yaml_multi_representers:
                node = self.yaml_multi_representers[None](self, data)
            elif None in self.yaml_representers:
                node = self.yaml_representers[None](self, data)
            else:
                node = representer.ScalarNode(None, representer.text_type(data))
    # if alias_key is not None:
    #     self.represented_objects[alias_key] = node
    return node
representer.BaseRepresenter.represent_data = represent_data

这样您的代码无需注册即可工作defaultdict：

class Foo:
    def __init__(self):
        self.x = defaultdict()

YAML = ruamel.yaml.YAML(typ="safe")
YAML.register_class(Foo)
# YAML.register_class(defaultdict)
fp =  Path("/temp/test.yaml")
YAML.dump(Foo(), fp)
YAML.load(fp)

编辑：一个更优雅的解决方案是简单地将SafeRepresenter.represent_dict方法添加为代表defaultdict：

from ruamel.yaml import representer
representer.SafeRepresenter.add_representer(defaultdict, representer.SafeRepresenter.represent_dict)

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