Hướng dẫn __call__ trong python

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• Difference between __init__() VS __call__()
• What is __ call __ function in Python?
• What are __ methods called in Python?
• What is the __ str __ method used for?
• What does __ class __ mean in Python?

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Python has a set of built-in methods and __call__ is one of them. The __call__ method enables Python programmers to write classes where the instances behave like functions and can be called like a function. When the instance is called as a function; if this method is defined, x(arg1, arg2, ...) is a shorthand for x.__call__(arg1, arg2, ...).

object() is shorthand for object.__call__()

Example 1:

class Example:

    def __init__(self):

        print("Instance Created")

    def __call__(self):

        print("Instance is called via special method")

e = Example()

e()

Output :

Instance Created
Instance is called via special method

Example 2:

class Product:

    def __init__(self):

        print("Instance Created")

    def __call__(self, a, b):

        print(a * b)

ans = Product()

ans(10, 20)

Output :

Instance Created
200

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Dunder or magic methods in Python are the methods having two prefixes and suffix underscores in the method name. Dunder here means “Double Under (Underscores)”. These are commonly used for operator overloading. Few examples for magic methods are: __init__, __add__, __len__, __repr__ etc. In this article, we are going to see the difference between two such methods.

Note: For more information, refer to Dunder or magic methods in Python

__init__()

This Python method is similar to a constructor in any other programming language. A constructor is a definition with the same name as the class and is invoked automatically when the object of that class is defined. A constructor initializes all the required entities of the program to make it more reliable.
Similar to this definition __init__() works as a Python constructor it is invoked automatically when an object of the class is defined. It initializes the required members with default values provided. It can also be invoked with the values provided during the time of declaration of the object of the class.

EXAMPLE:

class A:

    def __init__(self, x):

        print("inside __init__()")

        self.y = x

    def __str__(self):

        print("inside __str__()")

        print("value of y:", str(self.y))

a = A(3)

a.__str__()

b = A(10)

b.__str__()

OUTPUT:

inside __init__()
inside __str__()
('value of y:', '3')
inside __init__()
inside __str__()
('value of y:', '10')

__call__()

Before getting into application of __call__() we need to understand what a callable object is.
A callable object is one which can be called like a function.
In Python, __call__() is used to resolve the code associated with a callable object. Any object can be converted to a callable object just by writing it in a function call format. An object of that kind invokes the __call__() method and executes the code associated with it. This doesn’t make the object not to work like a normal one. The object can be used as a normal is used.
One thing to keep in mind is the object is itself used as a function, so syntax should be right.

EXAMPLE:

class A:

    def __init__(self, x):

        print("inside __init__()")

        self.y = x

    def __str__(self):

        print("inside __str__()")

        print("value of y:", str(self.y))

    def __call__(self):

        res = 0

        print("inside __call__()")

        print("adding 2 to the value of y")

        res = self.y + 2

        return res

a = A(3)

a.__str__()

r = a()

print(r)

b = A(10)

b.__str__()

r = b()

print(r)

OUTPUT:

inside __init__()
inside __str__()
('value of y:', '3')
inside __call__()
adding 2 to the value of y
5
inside __init__()
inside __str__()
('value of y:', '10')
inside __call__()
adding 2 to the value of y
12

Difference between __init__() VS __call__()