classmethod as constructor and inheritance

问题

The problem is quite simple. If a class B inherit a class A and wants to override a ´classmethod´ that is used as a constructor (I guess you call that a "factory method"). The problem is that B.classmethod will want to reuse A.classmethod, but then it will have to create an instance of the class A, while it subclasses the class A - since, as a classmethod, it has no self. And then, it doesn't seem the right way to design that.

I did the example trivial, I do more complicate stuff by reading numpy arrays, etc. But I guess there is no loss of information here.

class A:
    def __init__(self, a):
        self.el1 = a

    @classmethod
    def from_csv(cls, csv_file):
        a = read_csv(csv_file) 
        return cls(a)

    @classmethod
    def from_hdf5 ...

class B(A):
    def __init__(self, a, b)
        A.(self, a)
        self.el2 = b

    @classmethod
    def from_csv(cls, csv_file):
        A_ = A.from_csv(csv_file) #instance of A created in B(A)
        b = [a_*2 for a_ in A.el]
        return cls(A.el, b) 

Is there a pythonic way to deal with that?

回答1:

One easy solution would be to have class B's __init__ method have a default value for its b parameter. This would let the cls(a) call made by A.from_csv work when it is inherited. If the default is used, the __init__ method could calculate a value to store from a (as you do in B.from_csv now).

class B(A):
    def __init__(self, a, b=None):
        super().__init__(a)   # use super(B, self).__init__(a) if you're in Python 2
        self.el2 = b if b is not None else [i*2 for i in a]

    # don't override from_csv, B.from_csv will already return a B instance!

回答2:

After doing some different trials. My conclusion is that you should override a classmethod without reusing the code inside. So the best way I found, for my particular problem, is to make the classmethod as simply as possible and put the code I want to reuse in another method, static in my case, since the classmethod is a constructor.

来源：https://stackoverflow.com/questions/20925137/classmethod-as-constructor-and-inheritance