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

This is pretty clean way i guess and tricky

class A(object):
    def __init__(self,e,f,g):

        self.__dict__.update({k: v for k,v in locals().items() if k!='self'})
    def bc(self):
        print(self.f)


k=A(e=5,f=6,g=12)
k.bc() # >>>6

Answer 2

I'd use inheritance. Especially if there are going to be more differences than number of holes. Especially if Gouda will need to have different set of members then Parmesan.

class Gouda(Cheese):
    def __init__(self):
        super(Gouda).__init__(num_holes=10)


class Parmesan(Cheese):
    def __init__(self):
        super(Parmesan).__init__(num_holes=15) 

Answer 3

Since my initial answer was criticised on the basis that my special-purpose constructors did not call the (unique) default constructor, I post here a modified version that honours the wishes that all constructors shall call the default one:

class Cheese:
    def __init__(self, *args, _initialiser="_default_init", **kwargs):
        """A multi-initialiser.
        """
        getattr(self, _initialiser)(*args, **kwargs)

    def _default_init(self, ...):
        """A user-friendly smart or general-purpose initialiser.
        """
        ...

    def _init_parmesan(self, ...):
        """A special initialiser for Parmesan cheese.
        """
        ...

    def _init_gouda(self, ...):
        """A special initialiser for Gouda cheese.
        """
        ...

    @classmethod
    def make_parmesan(cls, *args, **kwargs):
        return cls(*args, **kwargs, _initialiser="_init_parmesan")

    @classmethod
    def make_gouda(cls, *args, **kwargs):
        return cls(*args, **kwargs, _initialiser="_init_gouda")

Answer 4

All of these answers are excellent if you want to use optional parameters, but another Pythonic possibility is to use a classmethod to generate a factory-style pseudo-constructor:

def __init__(self, num_holes):

  # do stuff with the number

@classmethod
def fromRandom(cls):

  return cls( # some-random-number )

Answer 5

class Cheese:
    def __init__(self, *args, **kwargs):
        """A user-friendly initialiser for the general-purpose constructor.
        """
        ...

    def _init_parmesan(self, *args, **kwargs):
        """A special initialiser for Parmesan cheese.
        """
        ...

    def _init_gauda(self, *args, **kwargs):
        """A special initialiser for Gauda cheese.
        """
        ...

    @classmethod
    def make_parmesan(cls, *args, **kwargs):
        new = cls.__new__(cls)
        new._init_parmesan(*args, **kwargs)
        return new

    @classmethod
    def make_gauda(cls, *args, **kwargs):
        new = cls.__new__(cls)
        new._init_gauda(*args, **kwargs)
        return new

Answer 6

Those are good ideas for your implementation, but if you are presenting a cheese making interface to a user. They don't care how many holes the cheese has or what internals go into making cheese. The user of your code just wants "gouda" or "parmesean" right?

So why not do this:

# cheese_user.py
from cheeses import make_gouda, make_parmesean

gouda = make_gouda()
paremesean = make_parmesean()

And then you can use any of the methods above to actually implement the functions:

# cheeses.py
class Cheese(object):
    def __init__(self, *args, **kwargs):
        #args -- tuple of anonymous arguments
        #kwargs -- dictionary of named arguments
        self.num_holes = kwargs.get('num_holes',random_holes())

def make_gouda():
    return Cheese()

def make_paremesean():
    return Cheese(num_holes=15)

This is a good encapsulation technique, and I think it is more Pythonic. To me this way of doing things fits more in line more with duck typing. You are simply asking for a gouda object and you don't really care what class it is.