import random
import yaml

class Config:
    @classmethod
    def register(cls, key):
        return property(
            lambda self: self.yaml[key],
            lambda self, value: self.save(key, value),
        )

    def __init__(self, path):
        self.path = path

        with open(self.path) as file:
            self.yaml = yaml.load(file)

    def save(self, key, value):
        if key in self.yaml and self.yaml[key] == value:
            return

        self.yaml[key] = value
        with open(self.path, 'w') as file:
            file.write(yaml.dump(self.yaml, default_flow_style=False))

Config.val1 = Config.register('val1')
Config.val2 = Config.register('val2')
Config.val3 = Config.register('val3')

config = Config('config.yaml')

print('val1')
print('before:', config.val1)
config.val1 = random.random()
print('after:', config.val1)
print('--------------------')
print('val2')
print('before:', config.val2)
config.val2 = random.random()
print('after:', config.val2)
print('--------------------')
print('val3')
print('before:', config.val3)
config.val3 = random.random()
print('after:', config.val3)