Creating an instance of a class without defining the __init__ function

Question

I am relativly new to python and I was wondering if you could create an instance of a class without defining the init explicity. Could I call it something else?

First example - with the init method:

class dog:
def __init__(self,name):
 self.name=name
 print('My name is',name)
Bob = dog('Bob')

Second example - without the init method:

class dog:
def init_instance(self,name):
 self.name = name
 print('My name is',name)
Bob = dog('Bob')

In the first example the code works but in the second example I get:

TypeError: object() takes no parameters

So based on this I assume that one has to explicitly call the init method. BUT I have seen code where the init method has not been used, how come?

• FujiApple: Check out __new__()
• xssChauhan: When creating an object from class the parameters you pass are passed to the __init__ function declared in the class. Since you have not declared it, python will fall back to the default implementation which does not accept any arguments. Bob = dog() and then Bob.init_instance("Bob") will work.
• Ry-: Well, Python isn’t going to know that it should call something named init_instance instead of __init__, but you can make init_instance a classmethod and call dog.init_instance('Bob') (changing it to also create an instance instead of just initializing one).
• Anake: Maybe if you said why you are trying to do this, you would get better help.
• John: @Anake I am not trying to do anything, just trying to understand why I explicity need to call __init__(). Why I cant name it someting else.

Answer 1

Every class has an __init__ method. If it doesn't explicitly define one, then it will inherit one from its parent class. In your 2nd example, the class inherits __init__ and a bunch of other methods (and other non-method attributes) from the base object class. We can see that via the dir function:

class Dog:
 def init_instance(self,name):
 self.name = name
 print('My name is',name)
print(dir(Dog))

output

['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'init_instance']

__init__ gets called automatically after the instance is constructed (via the __new__ method), so we might as well use it if we need to initialize our instance. But we can call your init_instance explicitly:

bob = Dog()
bob.init_instance('Bob')
print(bob.name) 

output

My name is Bob
Bob

If you give you class an initializer that isn't named __init__ then it won't get called automatically. How should Python know that that method is an initializer? Although it's customary to make __init__ the first method in the class definition, that's by no means mandatory, and some people like to put __init__ last.

You said: "I have seen code where the init method has not been used, how come?" Well, some classes simply don't need their instances to be initialized: their instance attributes are set via various other methods, or by direct assignment in code outside the class definition, eg bob.color = 'brown'. Or they inherit a perfectly usable __init__ from a parent class.

• John: This was a good explanation, finally. So I want to summarize how it works when creating an object: __new__() creates the instance __init__() initalize the instance, i.e. preparing my object to have e.g. names, colors etc. But we can make our own initializer and call it instead.
• PM 2Ring: @John Correct. In fact, with large classes (eg for a GUI), it's common to break up the initialization into separate logical stages, with a separate method for each stage, and have __init__ call those methods.

Answer 2

init is nothing else then a method to initially prepare the state of your object. In other languages they have similar concepts as Constructors and it's not necessarily needed.

• PM 2Ring: Sometimes __init__ is called the constructor, but technically speaking, that's not correct. The constructor in Python is __new__, and __init__ is the initializer.
• user1767754: Technically both are used to construct objects, but talking about priorities, __new__ is the one that is first called.
• PM 2Ring: Well sure, because __init__ (usually) constructs the instance attributes. OTOH, a lot of terminology used to describe the mechanics of other languages doesn't really apply well to Python because Python doesn't have declarations, it just defines things.
• user1767754: That's true, I just thought if the OP is coming from a different programming background Constructor might be the most close thing to understand of what init does.