多个构造函数:Pythonic 方式?

问题描述

我有一个保存数据的容器类.创建容器时，有不同的方法来传递数据.

1. 传递一个包含数据的文件
2. 直接通过参数传递数据
3. 不传递数据；只需创建一个空容器

在 Java 中，我将创建三个构造函数.如果在 Python 中可行的话，它会是这样的:

类容器:def __init__(self):self.timestamp = 0自我数据 = []self.metadata = {}def __init__(自我，文件):f = 文件.open()self.timestamp = f.get_timestamp()self.data = f.get_data()self.metadata = f.get_metadata()def __init__(自我，时间戳，数据，元数据):self.timestamp = 时间戳self.data = 数据self.metadata = 元数据

在 Python 中，我看到了三个明显的解决方案，但没有一个是漂亮的:

A:使用关键字参数:

def __init__(self, **kwargs):如果 kwargs 中的文件":...kwargs 中的 elif 'timestamp' 和 kwargs 中的 'data' 和 kwargs 中的 'metadata':...别的:...创建空容器

B:使用默认参数:

def __init__(self, file=None, timestamp=None, data=None, metadata=None):如果文件:...elif 时间戳以及数据和元数据:...别的:...创建空容器

C:只提供构造函数来创建空容器.提供用不同来源的数据填充容器的方法.

def __init__(self):self.timestamp = 0自我数据 = []self.metadata = {}def add_data_from_file(文件):...def add_data(时间戳，数据，元数据):...

解决方案 A 和 B 基本相同.我不喜欢执行 if/else，尤其是因为我必须检查是否提供了此方法所需的所有参数.如果要通过第四种方法扩展代码以添加数据，A 比 B 更灵活.

解决方案 C 似乎是最好的，但用户必须知道他需要哪种方法.例如:如果他不知道args是什么，他就不能做c = Container(args).

什么是最 Pythonic 的解决方案?

解决方案

Python 中不能有多个同名的方法.函数重载 - 与 Java 不同 - 不受支持.

使用默认参数或 **kwargs 和 *args 参数.

您可以使用 @staticmethod 或 @classmethod 装饰器创建静态方法或类方法，以返回您的类的实例，或添加其他构造函数.

我建议你这样做:

F 类:def __init__(self, timestamp=0, data=None, metadata=None):self.timestamp = 时间戳self.data = list() if data is None else dataself.metadata = dict() if metadata is None else metadata@classmethoddef from_file(cls, 路径):_file = cls.get_file(路径)时间戳 = _file.get_timestamp()数据 = _file.get_data()元数据 = _file.get_metadata()返回 cls(时间戳、数据、元数据)@classmethoddef from_metadata(cls，时间戳，数据，元数据):返回 cls(时间戳、数据、元数据)@静态方法def get_file(路径):# ...经过

<块引用>

⚠ 在 python 中，从不将可变类型作为默认值.⚠请参阅此处.

I have a container class that holds data. When the container is created, there are different methods to pass data.

1. Pass a file which contains the data
2. Pass the data directly via arguments
3. Don't pass data; just create an empty container

In Java, I would create three constructors. Here's how it would look like if it were possible in Python:

class Container:

    def __init__(self):
        self.timestamp = 0
        self.data = []
        self.metadata = {}

    def __init__(self, file):
        f = file.open()
        self.timestamp = f.get_timestamp()
        self.data = f.get_data()
        self.metadata = f.get_metadata()

    def __init__(self, timestamp, data, metadata):
        self.timestamp = timestamp
        self.data = data
        self.metadata = metadata

In Python, I see three obvious solutions, but none of them is pretty:

A: Using keyword arguments:

def __init__(self, **kwargs):
    if 'file' in kwargs:
        ...
    elif 'timestamp' in kwargs and 'data' in kwargs and 'metadata' in kwargs:
        ...
    else:
        ... create empty container

B: Using default arguments:

def __init__(self, file=None, timestamp=None, data=None, metadata=None):
    if file:
        ...
    elif timestamp and data and metadata:
        ...
    else:
        ... create empty container

C: Only provide constructor to create empty containers. Provide methods to fill containers with data from different sources.

def __init__(self):
    self.timestamp = 0
    self.data = []
    self.metadata = {}

def add_data_from_file(file):
    ...

def add_data(timestamp, data, metadata):
    ...

Solutions A and B are basically the same. I don't like doing the if/else, especially since I have to check if all arguments required for this method were provided. A is a bit more flexible than B if the code is ever to be extended by a fourth method to add data.

Solution C seems to be the nicest, but the user has to know which method he requires. For example: he cant do c = Container(args) if he doesn't know what args is.

Whats the most Pythonic solution?

解决方案

You can't have multiple methods with same name in Python. Function overloading - unlike in Java - isn't supported.

Use default parameters or **kwargs and *args arguments.

You can make static methods or class methods with the @staticmethod or @classmethod decorator to return an instance of your class, or to add other constructors.

I advise you to do:

class F:

    def __init__(self, timestamp=0, data=None, metadata=None):
        self.timestamp = timestamp
        self.data = list() if data is None else data
        self.metadata = dict() if metadata is None else metadata

    @classmethod
    def from_file(cls, path):
       _file = cls.get_file(path)
       timestamp = _file.get_timestamp()
       data = _file.get_data()
       metadata = _file.get_metadata()       
       return cls(timestamp, data, metadata)

    @classmethod
    def from_metadata(cls, timestamp, data, metadata):
        return cls(timestamp, data, metadata)

    @staticmethod
    def get_file(path):
        # ...
        pass

⚠ Never have mutable types as defaults in python. ⚠ See here.