Multiple constructors in python, using inheritance

Question

I have a class AbstractDataHandle, whith his init method, and a class Classifier. I would like to have two constructors in Classifier, Java like. One inheri

Answer 1

You can't have two constructors in one class.

Constructors have to be named __init__. And, unlike Java, Python doesn't allow overloading functions or methods by the type of their arguments. So, if you had two constructors, they would both be the same function.

There are a few ways around this.

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Use @classmethods as alternate constructors:

class Breakfast(object):
    @classmethod
    def from_eggs(cls, eggs):
        obj = cls()
        obj.spam, obj.eggs = 5, eggs
        return obj

    @classmethod
    def from_spam_and_eggs(cls, spam, eggs):
        obj = cls()
        obj.spam, obj.eggs = spam, eggs
        return obj

A simple example from the standard library is datetime.datetime, which can be constructed with now, fromtimestamp, or a few other alternate constructors, besides the default.

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Use default-valued, keyword-only, and/or variable-argument parameters to make a single constructor that can be called different ways:

class Breakfast(object):
    def __init__(self, eggs=0, spam=5):
        self.spam, self.eggs = spam, eggs

int is an example of this: You can create it from a string and a base, or from a single argument that knows how to convert itself to an integer.

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Create subclasses that each have different constructors:

class Breakfast(object):
    pass

class HealthyBreakfast(object):
    def __init__(self, spam):
        self.spam, self.eggs = spam, 0

class NormalBreakfast(object):
    def __init__(self, spam, eggs):
        self.spam, self.eggs = spam, eggs

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In any of these cases, you can factor out commonalities into a single "base" initializer. For example:

class Breakfast(object):
    def __init__(self, eggs, spam):
        self.spam, self.eggs = spam, eggs

class HealthyBreakfast(object):
    def __init__(self, spam):
        super(HealthyBreakfast, self).__init__(0, spam)

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Of course in no case is it possible to have breakfast without spam.