YAML 解析为对象 (PyYAML Python3)

Question

我有以下代码：

class Settings:
    def __init__(self, annual_volatility_target):
        self.annual_volatility_target = annual_volatility_target
        self.daily = annual_volatility_target/np.sqrt(252)

def yaml_load(name):
    with open('yaml/' + str(name) + '.yaml', 'r') as ymlfile:
        return yaml.load(ymlfile)

settings = yaml_load("settings")

使用以下 YAML：

!!python/object:v.Settings
annual_volatility_target: 0.25

问题是，当我加载settings 时，settings.daily 没有设置。 settings.annual_volatility_target 是，不管我在__init__ 中是否这样说。

如果我手动实例化 Settings 对象（即不使用 PyYAML），它可以正常工作。

我做错了什么？

Answer 1

一种可能性是为Settings 写一个constructor：

import yaml
import numpy as np

class Settings:
    def __init__(self, annual_volatility_target):
        self.annual_volatility_target = annual_volatility_target
        self.daily = annual_volatility_target/np.sqrt(252)

def yaml_load(name):
    with open(str(name) + '.yaml', 'r') as ymlfile:
        return yaml.load(ymlfile)

def settings_constructor(loader, node):
    fields = loader.construct_mapping(node)
    return Settings(fields['annual_volatility_target'])

yaml.add_constructor('!v.Settings', settings_constructor)

settings = yaml_load("settings")

print(settings.annual_volatility_target)
print(settings.daily)

我必须使用修改后的 yaml 文件（我无法使用注释 !!python/object:v.Settings 使其工作）：

!v.Settings
annual_volatility_target: 0.25

Answer 2

PyYAML 中的 Python 对象是通过两步过程构建的。首先调用__new__（在Constructor.make_python_instance() 中），然后设置属性（在Constructor.set_python_instance_state() 中）。之所以需要这两个步骤，是因为 YAML 支持对对象的引用，如果该对象是（间接）自引用的，则无法一次性构造它，因为它所依赖的参数（包括其自身）尚不可用。

您可以通过两种方式解决此问题。您可以为Settings 定义__setstate__()，这将使用dict 调用，也可以从__init__() 调用：

import yaml

yaml_str = """\
!!python/object:try.Settings
annual_volatility_target: 0.25
"""

class Settings:
    def __init__(self, annual_volatility_target):
        self.__setstate__({annual_volatility_target: annual_volatility_target})

    def __setstate__(self, kw):
        self.annual_volatility_target = kw.get('annual_volatility_target')
        self.daily = self.annual_volatility_target/np.sqrt(252)

    def __repr__(self):
        return "Setting({}, {})".format(self.annual_volatility_target, self.daily)

settings = yaml.load(yaml_str)

print(settings)

另一个更通用（非 PyYAML）的解决方案是在首次访问时创建 daily 值：

class Settings:
    def __init__(self, annual_volatility_target):
        self.annual_volatility_target = annual_volatility_target

    @property:
    def daily(self):
         return annual_volatility_target/np.sqrt(252)

如果您经常访问daily，那么您应该将其缓存在例如self._daily第一次计算值：

class Settings:
    def __init__(self, annual_volatility_target):
        self.annual_volatility_target = annual_volatility_target
        self._daily = None

    @property:
    def daily(self):
         if self._daily is None:  
             self._daily = annual_volatility_target/np.sqrt(252)
         return self._daily