Class without init python

Class initialization without __init__ method

In your 2nd example, the class inherits and a bunch of other methods (and other non-method attributes) from the base class. 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 .

Class initialization without __init__ method

While going through scapy source code (https://github.com/jwiegley/scapy), I came across the fact none of the Ether , IP , TCP , UDP or any other protocol classes contain any __init__ method, nor they have any class methods with @classmothod annotation. All of this classes inherit the Packet class, which by the way contains __init__ method.

Code structure is like this :

class Ether(Packet): # class methods class IP(Packet, IPTools): # class methods # Other protocol classes 

So, I am wondering how instances of this classes are created when we create a packet like this :

I could understand the «/» notation. The Packet class has overridden the __rdiv__() method, and as all these classes are subclasses of Packet, __rdiv__() of their parent Packet instance is called.

But, how the instances of these classes are being created is still unclear to me.

Also, IP class can be created like this

As, IP doesn’t have any __init__ method, how this initialization is possible?

For reference, __init__ of Packet class looks like this :

def __init__(self, _pkt="", post_transform=None, _internal=0, _underlayer=None, **fields): # initialization code 

Any help will be appreciated.

if the class doesnt contain a own __init__() it will just take the one from the superclass. a overwritten __init__() is only requiered, when adding changes. maybe this will help you understand

Python — Setting default/empty attributes for user, All class attributes should be included in the __init__ (initialiser) method. This is to ensure readability and aid debugging. The first issue is that …

Inheriting variables from a python class without using init or the class name of the parent

I am using kivy just to give you a bit of context.

but what I want is to be able to call class variables from another class. without init the classes

class FIRST-CLASS(Screen): variables def functions(): pass class SECOND-CLASS(Screen,CLASSIFIERS): *i would say FIRST-CLASS.variable but i cant. i need a way to say parent.variable. 

Here is my preferred method:

You can use global to make the variable global.

class Parent(): # You need to put the global before the declaration of the variable global var var = 'EXAMPLE' class Secondary(): var2 = var 

Python — How to create a non-instantiable class?, Our x is now an instance of Foo, without any attributes that is, and it can use any methods defined in Foo class. If you want, you can create your …

Creating an instance of a class without defining the __init__ function

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:

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?

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)) 

['__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) 

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.

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.

Initializing python dataclass object without passing, I want to initialize python dataclass object even if no instance variables are passed into it and we have not added default values to the param …

Is this Python example still initializing an object (without __init__)

I am still learning the concepts of OOP so please forgive me if this is a poor question:

If __init__ is needed to instantiate an object in Python, why does the following work with no call to __init__ ?:

class Person: #Note the lack of __init__(self): def run(self): print('I am a person') man = Person() man.run() #Prints 'I am a person') 

From python documentation:

The instantiation operation (“calling” a class object) creates an empty object. Many classes like to create objects with instances customized to a specific initial state. Therefore a class may define a special method named init (), like this.

So __init__ isn’t needed, it is the mechanism python provides to allow you to set an initial state of your object when it is initialized.

There is no need to create an __init__ function if you do not need to do any initialization other than that provided by the parent class. Note that __init__ functions of any parent class do get called automatically:

In [1]: class Parent(object): . def __init__(self): . print 'Inside __init__ of parent' In [2]: class Person(Parent): . def run(self): . print 'I am a person' In [3]: p = Person() Inside __init__ of parent In [4]: p.run() I am a person 

In modern Python, any class implicitly derives from the object class, which supposedly takes care of basic things like allocating memory space, even if you do not add any other properties.

__init__ is a Constructor , this is simple put a function that is called when you create an object. It is normally used to set up the object so that it is ready to be used. If no __init__ exist the object will still be created. It simply means that the object doesn’t get anything done when constructed.

Python — Is there a way to instantiate a class without, 1. If you can change the code of class, rewrite it so __init__ will not require any data. 2. If you can see the code of class, pass some default values. …

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Can you have a Python class without __ init __?

Your code is perfectly fine. You don’t have to have an __init__ method.

Do you need __ init __?

No, it isn’t necessary. For example. In fact you can even define a class in this manner. __init__ allows us to initialize this state information or data while creating an instance of the class.

How do you create a new object in Python?

A Class is like an object constructor, or a “blueprint” for creating objects.

1. Create a Class. To create a class, use the keyword class :
2. Create Object. Now we can use the class named MyClass to create objects:
3. The self Parameter.
4. Modify Object Properties.
5. Delete Object Properties.
6. Delete Objects.

Can you create an object without a constructor?

Yes, deserializing an object does not invoke its constructor. Deserialization involves creating objects without invoking a constructor.

Does an object need a constructor?

Yes you need at least one constructor to create an object. But the thing is that if you do not provide an explicit constructor, Java supplies a default constructor for you. Obviously, there job is to create objects. No object can be created without going through the constructor.

Can we call constructor from method?

No, you cannot call a constructor from a method. The only place from which you can invoke constructors using “this()” or, “super()” is the first line of another constructor. If you try to invoke constructors explicitly elsewhere, a compile time error will be generated.

Which constructor is called first in C++?

First, the base constructor is called, then the base-class members are initialized in the order in which they appear in the class declaration, and then the derived constructor is called.

Can constructor be virtual in C++?

Constructor can not be virtual, because when constructor of a class is executed there is no vtable in the memory, means no virtual pointer defined yet. Hence the constructor should always be non-virtual.

Can copy constructor be virtual in C++?

In C++ programming language, copy Constructor is used to creating an object copied from another. Copy constructor uses the virtual clone method whereas the virtual create method is used by the default constructors for creating a virtual constructor.

Can we make constructor as final?

No, a constructor can’t be made final. A final method cannot be overridden by any subclasses. In other words, constructors cannot be inherited in Java therefore, there is no need to write final before constructors.

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