What is a clean, pythonic way to have multiple constructors in Python?

Question

I can\'t find a definitive answer for this. As far as I know, you can\'t have multiple __init__ functions in a Python class. So how do I solve this problem?

Answer 1

One should definitely prefer the solutions already posted, but since no one mentioned this solution yet, I think it is worth mentioning for completeness.

The @classmethod approach can be modified to provide an alternative constructor which does not invoke the default constructor (__init__). Instead, an instance is created using __new__.

This could be used if the type of initialization cannot be selected based on the type of the constructor argument, and the constructors do not share code.

Example:

class MyClass(set):

    def __init__(self, filename):
        self._value = load_from_file(filename)

    @classmethod
    def from_somewhere(cls, somename):
        obj = cls.__new__(cls)  # Does not call __init__
        super(MyClass, obj).__init__()  # Don't forget to call any polymorphic base class initializers
        obj._value = load_from_somewhere(somename)
        return obj