How To Use Python Classmethod Decorator To Implement Class Method And Single Instance Pattern

In Python, the classmethod() function is used to define a class method. A class method is a method that is bound to the class and not the instance of the class, and can be called on the class itself rather than on an instance of the class. This article will tell you how to define a class method using the @classmethod decorator in python, and how to use class method to implement a single instance class.

1. How To Define Class Method In Python.

1. To define a class method using the classmethod() function, we need to use the @classmethod decorator before the method definition.
2. The method should take the class as its first parameter (usually named cls) instead of the instance of the class (usually named self).
3. Here is the syntax for defining a class method using the classmethod() function:

   class MyClass:
       @classmethod
       def my_class_method(cls, arg1, arg2, ...):
           # Method code here

4. In the above syntax, my_class_method is the name of the class method, cls is the first parameter that refers to the class itself, and arg1, arg2, etc. are the parameters that the method takes.

2. How To Use Python Class Method Examples.

2.1 How to use python classmethod decorator to define a class method.

1. The classmethod decorator in Python is a built-in decorator that define a function as a class method.
2. Class methods are methods that are bound to the class and not the instance of the class.
3. Class methods can be called on the class itself rather than on an instance of the class.
4. Here is an example.

   class MyClass:
       count = 0
       
       def __init__(self):
           MyClass.count += 1
           
       @classmethod
       def get_count(cls):
           return cls.count
           
   obj1 = MyClass()
   obj2 = MyClass()
   print(MyClass.get_count())  # Output: 2

5. In the above example, we define a class MyClass with a class variable count and a constructor that increments the count variable each time a new instance of the class is created.
6. We then define a class method get_count() using the @classmethod decorator.
7. The get_count() method returns the value of the count variable.
8. We can call the get_count() method on the class itself, rather than on an instance of the class.
9. The classmethod decorator takes a single argument, which is the function that we want to convert to a class method.
10. The function should take the class as its first parameter (usually named cls) instead of the instance of the class (usually named self).

2.2 How to create a class method to initialize an instance from a string.

1. Below is the example source code.

   class MyClass:
       def __init__(self, x, y):
           self.x = x
           self.y = y
       
       @classmethod
       def from_string(cls, string):
           x, y = map(int, string.split(','))
           return cls(x, y)
           
   obj = MyClass.from_string('5,10')
   print(obj.x)  # Output: 5
   print(obj.y)  # Output: 10

2. In this example, we define a class MyClass with a constructor that takes two parameters x and y.
3. We then define a class method from_string() using the @classmethod decorator.
4. The from_string() method takes a string argument in the format “x,y” and converts it to integers using the map() function.
5. It then creates and returns a new instance of the class with the given x and y values.
6. We can call the from_string() method on the class itself, rather than on an instance of the class.

2.3 How to use a class method to create a singleton instance of a class.

1. Below is the example source code.

   class MySingleton:
       instance = None
       
       def __init__(self):
           if MySingleton.instance is None:
               MySingleton.instance = self
           else:
               raise Exception("An instance of MySingleton already exists!")
       
       @classmethod
       def get_instance(cls):
           if cls.instance is None:
               cls.instance = MySingleton()
           return cls.instance
           
   obj1 = MySingleton.get_instance()
   obj2 = MySingleton.get_instance()
   print(obj1 is obj2)  # Output: True

2. In this example, we define a class MySingleton that can have only one instance.
3. We define a class variable instance to keep track of the singleton instance.
4. We then define a constructor that checks if an instance of the class already exists and raises an exception if it does.
5. We define a class method get_instance() using the @classmethod decorator.
6. The get_instance() method checks if the singleton instance already exists and returns it if it does.
7. If the singleton instance does not exist, it creates a new instance of the class and returns it.
8. We can call the get_instance() method on the class itself, rather than on an instance of the class.