A class is a user-defined data type. Its a collection of objects, where objects can represent just about anything from integers to real world objects like a ball. Structure of Classes in python are very similar to that of c++ or java.
For example:
class world:
def hello(self):
print("Hello World!")
obj=world()
obj.hello()Output:
Hello World!
As we can see above, we create a class using the keyword class and the name of the class. This is followed by an indented block of statements which forms the body of the class. The body can contain methods and variables pertaining to the class. In the above example we have created a class "world" which defines a method "hello" to print the statement "Hello World!". We create an object of this class, which can access the data members and methods of the class and call the methods of the class by using the "." operator just like in c++.
1. The class methods in python need to have an extra parameter called self. This does not
have a value, when a method is called.
2. If we have a method with no parameters, in the class we must have one argument that is
the self argument. See hello() method in the above example.
3. This is similar to this pointer in c++ or this reference in Java.
4. When we call a method MyClass.method(arg1,arg2) this is automatically converted
by python into MyClass.method(object,arg1,arg2) where object is the self parameter.
The init method in python is similar to constructors in java or c++. That means it is run as soon as an object of the class is instantiated. This method is useful to initialize your objects in the class, before proceed further method calls.
For example:
class name:
def __init__(self,str):
self.str=str
def greet(self):
print('Hi, my name is ',self.str)
p=name('Vinitra')
p.greet()Output:
Hi, my name is Vinitra
Inheritance is one of the most powerful concepts of object oriented programming. It is the mechanism of deriving features of one class from another class facilitating code reusability. It constitutes defining a new class which is similar or identical to a class that already exists. The new class is hence called derived class or child class and the one from which it is inherited is called the base class or the parent class.
For example:
class side:
def __init__(self,l,b):
self.l=l
self.b=b
class area(side):
def __init__(self,l,b):
side.__init__(self,l,b)
def Area(self):
print('Area of the rectange is ',self.l*self.b)
a=area(4,5)
a.Area()Output:
Area of the rectangle is 20
super() method is used to access data members and functions of parent class.
The example given in the example for inheritance can also be executed by the
super() method.
For example:
class side:
def __init__(self,l,b):
self.l=l
self.b=b
class area(side):
def __init__(self,l,b):
super(area,self).__init__(l,b)
def Area(self):
print('Area of the rectange is ',self.l*self.b)
a=area(4,5)
a.Area()Output:
Area of the rectangle is 20
Python operators work for built-in classes. But the same operators can be made to behave in a user-defined way. For example, the + operator adds the primitive variables of primitive data types. But by operator overloading you can make the + operator to add two complex numbers! This feature that allows operators to have to different functions according to context is called Operator Overloading. See the example below to see how to overload operators. Example:
class Point:
def __init__(self,x,y):
self.x=x
self.y=y
def __str__(self):
return "({0},{1})",format(self.x,self.y)
def __add__(self,other):
x=self.x+other.x
y=self.y+other.y
print()
p1=Point(2,4)
p2=Point(-1,5)
print(p1+p2)Output:
(1,9)
What happens is that, when you write p1+p2, python will call p1.add(p2) which in turn is Point.add(p1,p2). Also in the above example, str is special function like the init method i.e it gets called whenever the class is instantiated. This method is used to control the output format
Special functions are those methods which get executed as soon as the class is instantiated. For example: the init method. There are many useful special functions in python. Given below are some of them.
- iter: This method returns the iterator object and is implicitly called at the start of loops.
- next: This method returns the next value and is implicity called at each loop increment.
Example:
class Counter:
def __init__(self, low, high):
self.current = low
self.high = high
def __iter__(self):
return self
def __next__(self): # Python 2: def next(self)
if self.current > self.high:
raise StopIteration
else:
self.current += 1
return self.current - 1
for c in Counter(3, 8):
print(c)Output:
3
4
5
6
7
8
- str & repr: Both the functions are similar, i.e both are used to "represent" an object in a string. repr gives you an official representation while __str gives informal representation. str() is used for creating output for end user while repr() is mainly used for debugging and development. Main goal of repr() is to be unambiguous and that of str() is to be readable. For example if we suspect a float has a small rounding error, repr will show us while str may not. For example:
x=1
print(repr(x))
print(str(x))
y='string'
print(repr(y))
print(str(y))Output:
1
1
'string'
string
In the above example, the return of repr() and str() are identical for int x, but not for string y. Therefore the default implementation of repr for a str object can be called as an argument to eval and the return value would be a valid str object. Look at another example here:
class Complex:
def __init__(self, real, imag):
self.real = real
self.imag = imag
# For call to repr(). Prints object information
def __repr__(self):
return '%s + i%s' % (self.real, self.imag)
# For call to str(). Prints readable form
def __str__(self):
return '%s + i%s' % (self.real, self.imag)
t = Complex(10, 20)
print str(t)
print(t)
print repr(t)Output:
10 + i20
10 + i20
Rational(10,20)
You can redefine str and repr to suit you needs as done in the above example.
- dir(): This method takes only 1 object as a parameter and returns a list of attributes of the object. For example:
class num:
def __init__(self):
return [1,2,3]
n=num()
print(dir(n))Output:
[1,2,3]