numpy matrix class: default constructor attributes for inherited class

Anyway, the following works for me:

class MyMatrix(np.matrix):
   def __new__(cls):
   # note that we have to send cls to super 's __new__, even though we gave it to super already.
# I think this is because __new__ is technically a staticmethod even though it should be a classmethod
return super(MyMatrix, cls).__new__(cls, "1 2; 3 4")

mat = MyMatrix()

print mat
# outputs[[1 2][3 4]]

Addendum: you might want to consider using a factory function, rather than a subclass, for the behaviour you want. This would give you the following code, which is much shorter and clearer, and doesn't depend on the __new__-vs-__init__ implementation detail:

def mymatrix():
   return np.matrix('1 2; 3 4')

mat = mymatrix()
print mat
# outputs[[1 2][3 4]]

Suggestion : 2

Subclassing ndarray is complicated by the fact that new instances of ndarray classes can come about in three different ways. These are:,The last two are characteristics of ndarrays - in order to support things like array slicing. The complications of subclassing ndarray are due to the mechanisms numpy has to support these latter two routes of instance creation.,New instances of an ndarray subclass can also come about by a very similar mechanism to View casting, when numpy finds it needs to create a new instance from a template instance. The most obvious place this has to happen is when you are taking slices of subclassed arrays. For example:,Subclassing ndarray is relatively simple, but it has some complications compared to other Python objects. On this page we explain the machinery that allows you to subclass ndarray, and the implications for implementing a subclass.

>>> import numpy as np
>>> # create a completely useless ndarray subclass
>>> class C(np.ndarray): pass
>>> # create a standard ndarray
>>> arr = np.zeros((3,))
>>> # take a view of it, as our useless subclass
>>> c_arr = arr.view(C)
>>> type(c_arr)
<class '__main__.C'>

>>> v = c_arr[1:]
>>> type(v) # the view is of type 'C'
<class '__main__.C'>
   >>> v is c_arr # but it's a new instance
   False

>>> class C:
   >>>
   def __new__(cls, * args):
   >>>
   print('Cls in __new__:', cls) >>>
   print('Args in __new__:', args) >>>
   # The `object`
type __new__ method takes a single argument. >>>
   return object.__new__(cls) >>>
      def __init__(self, * args):
      >>>
      print('type(self) in __init__:', type(self)) >>>
      print('Args in __init__:', args)

>>> c = C('hello')
Cls in __new__: <class 'C'>
   Args in __new__: ('hello',)
   type(self) in __init__: <class 'C'>
      Args in __init__: ('hello',)

class D(C):
   def __new__(cls, * args):
   print('D cls is:', cls)
print('D args in __new__:', args)
return C.__new__(C, * args)

def __init__(self, * args):
   # we never get here
print('In D __init__')

>>> obj = D('hello')
D cls is: <class 'D'>
   D args in __new__: ('hello',)
   Cls in __new__: <class 'C'>
      Args in __new__: ('hello',)
      >>> type(obj)
      <class 'C'>

Suggestion : 3

Calling a Super Class Constructor in Python,Python has super function which allows us to access temporary object of the super class.,Constructors are generally used for instantiating an object. The task of constructors is to initialize(assign values) to the data members of the class when an object of the class is created. In Python, the __init__() method is called the constructor and is always called when an object is created.,Writing code in comment? Please use ide.geeksforgeeks.org, generate link and share the link here.

Syntax :

object = ClassName()

Output :

[1, 2, 3, 4, 5]

Suggestion : 4

We can pass any number of arguments at the time of creating the class object, depending upon the __init__() definition. It is mostly used to initialize the class attributes. Every class must have a constructor, even if it simply relies on the default constructor.,In C++ or Java, the constructor has the same name as its class, but it treats constructor differently in Python. It is used to create an object.,When we do not include the constructor in the class or forget to declare it, then that becomes the default constructor. It does not perform any task but initializes the objects. Consider the following example.,The non-parameterized constructor uses when we do not want to manipulate the value or the constructor that has only self as an argument. Consider the following example.

ID: 101
Name: John
ID: 102
Name: David

The number of students: 3

This is parametrized constructor
Hello John

101 Joseph

The Second Constructor

John
23
True
AttributeError: 'Student'
object has no attribute 'age'

Suggestion : 5

In C++ inheritance, we can derive a child class from the base class in different access modes. For example,,In this tutorial, we will learn to use public, protected and private inheritance in C++ with the help of examples.,This means that we have created a derived class from the base class in public mode. Alternatively, we can also derive classes in protected or private modes.,private inheritance makes the public and protected members of the base class private in the derived class.

In C++ inheritance, we can derive a child class from the base class in different access modes. For example,

class Base {
   ...........
};

class Derived: public Base {
   ...........
};

Notice the keyword public in the code

class Derived: public Base

Note: private members of the base class are inaccessible to the derived class.

class Base {
   public: int x;
   protected: int y;
   private: int z;
};

class PublicDerived: public Base {
   // x is public
   // y is protected
   // z is not accessible from PublicDerived
};

class ProtectedDerived: protected Base {
   // x is protected
   // y is protected
   // z is not accessible from ProtectedDerived
};

class PrivateDerived: private Base {
   // x is private
   // y is private
   // z is not accessible from PrivateDerived
};

Output

Private = 1
Protected = 2
Public = 3

Since private and protected members are not accessible from main(), we need to create public functions getPVT() and getProt() to access them:

// Error: member "Base::pvt" is inaccessible
cout << "Private = " << object1.pvt;

// Error: member "Base::prot" is inaccessible
cout << "Protected = " << object1.prot;

Suggestion : 6

Class Starts on 13th August,2022,Class Starts on 3rd September,2022,Class Starts on 24th September,2022

A constructor is basically a method that is automatically called when an object(instance) is created of that class. It is used to initialize an object’s data members.

public class Edureka {
   Edureka() {
      System.out.println("constructor is made");
   }
}

Example illustrating Default Constructor in Java:

public class Edureka {
   Edureka() {
      System.out.println("I am a constructor");
   }
   public static void main(String args[]) {
      Edureka obj = new Edureka();
   }
}

public class Edureka{
Edureka()
{ System.out.println("I am a constructor");}
public static void main(String args[]){
Edureka obj = new Edureka();
}
}

Output: I am a constructor

public class Edureka{
String studentName;
int studentAge;
//constructor
Edureka(String name, int age){
studentName = name;
studentAge = age;
}
void display(){
System.out.println(studentName+ " "+studentAge);
}
public static void main(String args[])
{
Edureka myObj = new Edureka("Manan" , 19);
myObj.display();
}
}

Output: Manan - 19

Example illustrating Passing Objects as Arguments:

public class Edureka {
   String studentName;
   Edureka(String name) {
      studentName = name;
   }
   Edureka(Edureka myObj) {
      this.studentName = myObj.studentName;
   }
   void display() {
      System.out.println("Student:" + studentName);
   }
   public static void main(String args[]) {
      Edureka obj1 = new Edureka("Manan");
      /* passing the object as an argument for the constructor 
       * this will invoke the copy constructor
       */
      Edureka obj2 = new Edureka(obj1);
      System.out.println("Printing obj1 - ");
      obj1.display();
      System.out.println("Printing obj2 - ");
      obj2.display();
   }
}