Constructor chaining in python

Question

I have two constructors in my class:

def __init__(self):
  self(8)

def __init__(self, size):
  self.buffer = [1] * size

Where I want the first constructor to call the second with a default size. Is this achievable in python?

You don't have two constructors, the second overwrites the first. — u0b34a0f6ae, Dec 04 '11 at 16:43

score 10 · Accepted Answer · edited May 23 '17 at 12:07

10

You cannot define multiple initializers in Python (as pointed in the comments, __init__ is not really a constructor), but you can define default values, for instance:

def __init__(self, size=8):
  self.buffer = [1] * size

In the above code, a buffer of size 8 is created by default, but if a size parameter is specified, the parameter will be used instead.

For instance, let's suppose that the initializer is inside a class called Example. This call will create a new instance of the class with a buffer of size 8 (the default):

e = Example()

Whereas this call will create a new instance with a buffer of size 10:

e = Example(10)

Alternatively, you could also call the constructor like this:

e = Example(size=10)

edited May 23 '17 at 12:07

Community

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answered Dec 04 '11 at 16:38

Óscar López

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`__init__` is not a constructor, but rather an initializer. See [`__init__` as a constructor?](http://stackoverflow.com/questions/6578487/init-as-a-constructor). – Nathan Dec 04 '11 at 17:35
Thanks for pointing that out, @Nathan. I've edited my answer accordingly. – Óscar López Dec 04 '11 at 17:41

score 5 · Answer 2 · answered Dec 04 '11 at 16:38

5

No, you cannot overload methods in Python. In this case, you could just use a default value for the size parameter instead:

def __init__(self, size=8):
  self.buffer = [1] * size

answered Dec 04 '11 at 16:38

omz

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score 1 · Answer 3 · answered Dec 04 '11 at 16:53

probably not in this way. Python classes use an internal dictionary to store its method and properties, a second method with same name overrides the first one. You could assign a default value to your extra parameter to do this,

def __init__(self, size = 8):
  self.buffer = [1] * size

score 0 · Answer 4 · answered Jul 27 '22 at 15:03

What you suggest is possible in languages that allow constructor overloading. This is not possible in python. For simple cases, the suggested default parameter is the best way ( __init__(self,size=8)). If you need more complex alternative constructors (imagine reading and parsing parameters from a string or so), you would use classmethods in python:

class thingWithBuffer:
    def __init__(self, buffer_size):
        self.buffer = buffersize
    
    @classmethod
        def create_with_default_buffer(cls):
            return cls(buffer_size=8)

pynative has a nice overview of how to use classmethods

Constructor chaining in python

4 Answers4