Multiple constructors: the Pythonic way? [duplicate]

Question

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?

Answer 1

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.