对象的YAML解析(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")

与下列YAML：

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

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

如果我手动实例化一个Settings对象(即不使用PyYAML)，它可以正常工作。

我做错了什么？

Answer 1

PyYAML中的Python对象是在两个步骤中构造的。首先调用__new__ (Constructor.make_python_instance()中)，然后设置属性(在Constructor.set_python_instance_state()中)。这两个步骤是必要的，因为YAML支持对对象的引用，如果该对象是(间接的)自引用，则不能一次构造该对象，因为它所依赖的参数(包括它本身)还不可用。

你可以用两种方法解决这个问题。您可以为__setstate__()定义Settings，并使用dict调用它，并从__init__()调用它：

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)

另一种更通用的(非PyYAML)解决方案是在第一次访问时创建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，那么您应该在第一次计算值时将其缓存在例如self._daily中：

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

Answer 2

一种可能是为构造函数编写一个Settings

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)

我必须使用一个修改过的yaml文件(我无法使它与注释!!python/object:v.Settings一起工作)：

!v.Settings
annual_volatility_target: 0.25