为何修改HpoExperiment实例exp1时exp2也被改动？如何隔离？

问题描述 #

运行以下Python代码时，修改HpoExperiment类的实例exp1的trial_number后，实例exp2的该属性值也随之改变。需要让每个实例的状态相互独立，请问问题原因是什么？如何实现状态隔离？

class TrialDetails:
    trial_number: int
    def __init__(self, trial_number=-1):
        self.trial_number = trial_number
    def set_trial_number(self, trial_number):
        self.trial_number = trial_number
class HpoExperiment:
    experiment_name: str
    total_trials: int
    trialDetails: object
    def __init__(self, experiment_name, total_trials, trialDetails = TrialDetails()):
        self.experiment_name = experiment_name
        self.total_trials = total_trials
        self.trialDetails = trialDetails
    def set_trial(self, trial_number):
        self.trialDetails.set_trial_number(trial_number)
    def display(self):
        print ("exp name = ", self.experiment_name)
        print ("total trials = ", self.total_trials)
        print ("trial number = ", self.trialDetails.trial_number)

def main():
    exp1 = HpoExperiment("A", 5)
    exp2 = HpoExperiment("B", 3)
    print("Experiment Details")
    exp1.display()
    exp2.display()
    print("Update Experiment 1")
    exp1.set_trial(22)
    print("Experiment Details")
    exp1.display()
    exp2.display()

if __name__ == '__main__':
    main()

问题原因 #

这是Python中可变默认参数的常见陷阱：

• 函数的默认参数是在函数定义阶段创建的，而非每次调用函数时重新生成
• HpoExperiment.__init__方法里的trialDetails = TrialDetails()，会在类定义时生成一个TrialDetails实例，所有没有显式传入trialDetails参数的HpoExperiment实例，都会共享这同一个TrialDetails对象
• 修改exp1的trial_number时，实际是修改了这个共享对象的属性，exp2引用的是同一个对象，自然属性值也会跟着变化

实现实例状态隔离的方法 #

方法1：修改默认参数为None，在初始化时创建新实例 #

把__init__的默认参数改为None，然后在方法内部判断，若没有传入自定义的trialDetails，就新建一个独立的TrialDetails实例：

class HpoExperiment:
    experiment_name: str
    total_trials: int
    trialDetails: object
    def __init__(self, experiment_name, total_trials, trialDetails=None):
        self.experiment_name = experiment_name
        self.total_trials = total_trials
        # 为每个实例创建独立的TrialDetails对象
        self.trialDetails = trialDetails if trialDetails is not None else TrialDetails()
    def set_trial(self, trial_number):
        self.trialDetails.set_trial_number(trial_number)
    def display(self):
        print ("exp name = ", self.experiment_name)
        print ("total trials = ", self.total_trials)
        print ("trial number = ", self.trialDetails.trial_number)

方法2：显式传入独立的TrialDetails实例 #

在创建HpoExperiment实例时，手动为每个实例传入新的TrialDetails对象，确保每个实例拥有专属的状态：

def main():
    # 每个实例都传入独立的TrialDetails对象
    exp1 = HpoExperiment("A", 5, TrialDetails())
    exp2 = HpoExperiment("B", 3, TrialDetails())
    print("Experiment Details")
    exp1.display()
    exp2.display()
    print("Update Experiment 1")
    exp1.set_trial(22)
    print("Experiment Details")
    exp1.display()
    exp2.display()

内容的提问来源于stack exchange，提问作者Saad Khan