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


Question

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

Suppose I have an class called Cheese with the number_of_holes property. How can I have two ways of creating cheese-objects...

  1. one that takes a number of holes like this: parmesan = Cheese(num_holes = 15)
  2. and one that takes no arguments and just randomizes the number_of_holes property: gouda = Cheese()

I can think of only one way to do this, but that seems kinda clunky:

class Cheese():
    def __init__(self, num_holes = 0):
        if (num_holes == 0):
            # randomize number_of_holes
        else:
            number_of_holes = num_holes

What do you say? Is there another way?

1
638
5/21/2018 6:53:16 AM

Accepted Answer

Actually None is much better for "magic" values:

class Cheese():
    def __init__(self, num_holes = None):
        if num_holes is None:
            ...

Now if you want complete freedom of adding more parameters:

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

To better explain the concept of *args and **kwargs (you can actually change these names):

def f(*args, **kwargs):
   print 'args: ', args, ' kwargs: ', kwargs

>>> f('a')
args:  ('a',)  kwargs:  {}
>>> f(ar='a')
args:  ()  kwargs:  {'ar': 'a'}
>>> f(1,2,param=3)
args:  (1, 2)  kwargs:  {'param': 3}

http://docs.python.org/reference/expressions.html#calls

756
3/7/2016 12:41:58 AM

Using num_holes=None as the default is fine if you are going to have just __init__.

If you want multiple, independent "constructors", you can provide these as class methods. These are usually called factory methods. In this case you could have the default for num_holes be 0.

class Cheese(object):
    def __init__(self, num_holes=0):
        "defaults to a solid cheese"
        self.number_of_holes = num_holes

    @classmethod
    def random(cls):
        return cls(randint(0, 100))

    @classmethod
    def slightly_holey(cls):
        return cls(randint(0, 33))

    @classmethod
    def very_holey(cls):
        return cls(randint(66, 100))

Now create object like this:

gouda = Cheese()
emmentaler = Cheese.random()
leerdammer = Cheese.slightly_holey()

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