在不定义__init__函数的情况下创建类的实例

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?

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.

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.