Python递归错误：在__new__方法中调用派生类classmethod

递归错误的核心是循环调用链：

1. 调用File("data.csv")触发File.__new__，将cls切换为CSVFile
2. 执行CSVFile.create_instance(filename)，其中的cls(filename)即CSVFile(filename)
3. 由于CSVFile继承自File，创建实例时会优先调用父类File.__new__，再次回到第一步逻辑，形成无限递归

方案一：修改派生类的create_instance方法，绕过递归触发点 #

让create_instance直接通过object.__new__创建实例，手动调用初始化方法，避免触发File.__new__：

class File:
    def __new__(cls, filename):
        extension = filename.split(".")[-1].lower()

        if extension == "csv":
            cls = CSVFile
        elif extension == "json":
            cls = JSONFile
        else:
            raise ValueError(f"Unsupported file extension: {extension}")

        instance = cls.create_instance(filename)
        return instance

class CSVFile(File):
    def __init__(self, filename):
        self.filename = filename

    @classmethod
    def create_instance(cls, filename):
        print("CSVFile, create_instance")
        instance = object.__new__(cls)  # 直接调用基类创建实例，绕过File.__new__
        instance.__init__(filename)
        return instance

class JSONFile(File):
    def __init__(self, filename):
        self.filename = filename

    @classmethod
    def create_instance(cls, filename):
        print("JSONFile, create_instance")
        instance = object.__new__(cls)
        instance.__init__(filename)
        return instance

# Example usage
file_instance = File("data.csv")
print(f"File instance: {file_instance.filename}")

方案二：优化File.__new__的逻辑，仅作为工厂类触发分支判断 #

在File.__new__开头增加判断，仅当当前类是File本身时才执行扩展名分支逻辑；如果是派生类直接调用，直接返回实例，打破递归链：

class File:
    def __new__(cls, filename):
        # 仅当调用File()时才执行工厂逻辑，派生类调用直接创建实例
        if cls is not File:
            return super().__new__(cls)
        
        extension = filename.split(".")[-1].lower()

        if extension == "csv":
            cls = CSVFile
        elif extension == "json":
            cls = JSONFile
        else:
            raise ValueError(f"Unsupported file extension: {extension}")

        instance = cls.create_instance(filename)
        return instance

class CSVFile(File):
    def __init__(self, filename):
        self.filename = filename

    @classmethod
    def create_instance(cls, filename):
        print("CSVFile, create_instance")
        return cls(filename)

class JSONFile(File):
    def __init__(self, filename):
        self.filename = filename

    @classmethod
    def create_instance(cls, filename):
        print("JSONFile, create_instance")
        return cls(filename)

# Example usage
file_instance = File("data.csv")
print(f"File instance: {file_instance.filename}")

方案二更推荐，它既保留了create_instance方法的原有逻辑，又明确了File类作为工厂的角色，同时让派生类的实例化流程更符合Python的正常机制。

内容的提问来源于stack exchange，提问作者gromajus