使用 ruamel.yaml 安全转储和加载 defaultdict

Question

我正在尝试（反）序列化 class 具有 collections.defaultdict 属性且在 Python 中具有 ruamel.yaml 的元素（在我的例子中是 3.6+）。

这将是我想要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" class 编写自定义代码的想法？我希望我可以为 defaultdict 个对象添加一个不同的代表，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__'

Answer 1

这是因为 defaultdict 是内置 class dict 的子 class，它没有 __dict__ 属性用于生成 class 属性名称的 YAML 编码器。在这种情况下 defaultdict 应该被视为 dict，但问题是 ruamel.yaml.representer.BaseRepresenter class 的 represent_data 方法只查看 class 对象本身来确定是否有对象的代表：

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

它应该做的是检查 __mro__ 中的任何数据类型是否有代表，如果找到就使用它：

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)

所以我们可以自己修改这个方法：

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

这样您的代码就可以在不注册的情况下运行 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添加 SafeRepresenter.represent_dict 方法作为 defaultdict:

的代表

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

Answer 2

现在有一个包 ruamel.yaml.pytypes 支持转储 defaultdict 个实例。请注意，如果您提供一个函数作为参数（对于 default_factory），您将需要指定 typ='unsafe'，否则您的工厂函数将无法表示。

在您的 virtualenv 中安装 ruamel.yaml.pytypes 和 ruamel.yaml 后，您可以：

yaml = ruamel.yaml.YAML(typ=['unsafe', 'pytypes'])
yaml.default_flow_style = False
buf = ruamel.yaml.compat.StringIO()

def factory():
    import datetime
    return datetime.datetime.now()

data = defaultdict(factory)

x = data[4]
data[2] = 42
yaml.dump(data, buf)
print(buf.getvalue(), end='')
d = yaml.load(buf.getvalue())
assert data == d
assert data.default_factory == d.default_factory

将打印以上内容（您的日期时间会有所不同）。

!defaultdict
- !!python/name:__main__.factory 
- 2: 42
  4: 2019-08-19 13:06:05.129019

（断言不会抛出异常）

查看编辑历史以了解 "manual" 种实现类似结果的方法。

使用 ruamel.yaml 安全转储和加载 defaultdict

Safe dumping and loading of defaultdict with ruamel.yaml

python

yaml

defaultdict

ruamel.yaml