Singleton Method – Python Design Patterns

Prerequisite: Singleton Design pattern | Introduction

What is Singleton Method in Python

Singleton Method is a type of Creational Design pattern and is one of the simplest design patterns available to us. It is a way to provide one and only one object of a particular type. It involves only one class to create methods and specify the objects. 
Singleton Design Pattern can be understood by a very simple example of Database connectivity. When each object creates a unique Database Connection to the Database, it will highly affect the cost and expenses of the project. So, it is always better to make a single connection rather than making extra irrelevant connections which can be easily done by Singleton Design Pattern.

singleton-pattern

Definition: The singleton pattern is a design pattern that restricts the instantiation of a class to one object.

Now let’s have a look at the different implementations of the Singleton Design pattern. 

Method 1: Monostate/Borg Singleton Design pattern

Singleton behavior can be implemented by Borg’s pattern but instead of having only one instance of the class, there are multiple instances that share the same state. Here we don’t focus on the sharing of the instance identity instead we focus on the sharing state. 

# Singleton Borg pattern
class Borg:

    # state shared by each instance
    __shared_state = dict()

    # constructor method
    def __init__(self):

        self.__dict__ = self.__shared_state
        self.state = 'GeeksforGeeks'

    def __str__(self):

        return self.state


# main method
if __name__ == "__main__":

    person1 = Borg()    # object of class Borg
    person2 = Borg()    # object of class Borg
    person3 = Borg()    # object of class Borg

    person1.state = 'DataStructures'  # person1 changed the state
    person2.state = 'Algorithms'     # person2 changed the state

    print(person1)    # output --> Algorithms
    print(person2)    # output --> Algorithms

    person3.state = 'Geeks'  # person3 changed the
    # the shared state

    print(person1)    # output --> Geeks
    print(person2)    # output --> Geeks
    print(person3)    # output --> Geeks

Output: 

Algorithms
Algorithms
Geeks
Geeks
Geeks

Singleton-Design-pattern

Double Checked Locking Singleton Design pattern

It is easy to notice that once an object is created, the synchronization of the threading is no longer useful because now the object will never be equal to None and any sequence of operations will lead to consistent results. 
So, when the object will be equal to None, then only we will acquire the Lock on the getInstance method.

# Double Checked Locking singleton pattern
import threading


class SingletonDoubleChecked(object):

    # resources shared by each and every
    # instance

    __singleton_lock = threading.Lock()
    __singleton_instance = None

    # define the classmethod
    @classmethod
    def instance(cls):

        # check for the singleton instance
        if not cls.__singleton_instance:
            with cls.__singleton_lock:
                if not cls.__singleton_instance:
                    cls.__singleton_instance = cls()

        # return the singleton instance
        return cls.__singleton_instance


# main method
if __name__ == '__main__':

    # create class X
    class X(SingletonDoubleChecked):
        pass

    # create class Y
    class Y(SingletonDoubleChecked):
        pass

    A1, A2 = X.instance(), X.instance()
    B1, B2 = Y.instance(), Y.instance()

    assert A1 is not B1
    assert A1 is A2
    assert B1 is B2

    print('A1 : ', A1)
    print('A2 : ', A2)
    print('B1 : ', B1)
    print('B2 : ', B2)

Output:  

A1 :  __main__.X object at 0x02EA2590
A2 :  __main__.X object at 0x02EA2590
B1 :  __main__.Y object at 0x02EA25B0
B2 :  __main__.Y object at 0x02EA25B0

Creating a singleton in Python

In the classic implementation of the Singleton Design pattern, we simply use the static method for creating the getInstance method which has the ability to return the shared resource. We also make use of the so-called Virtual private Constructor to raise the exception against it although it is not much required.

# classic implementation of Singleton Design pattern
class Singleton:

    __shared_instance = 'GeeksforGeeks'

    @staticmethod
    def getInstance():
        """Static Access Method"""
        if Singleton.__shared_instance == 'GeeksforGeeks':
            Singleton()
        return Singleton.__shared_instance

    def __init__(self):
        """virtual private constructor"""
        if Singleton.__shared_instance != 'GeeksforGeeks':
            raise Exception("This class is a singleton class !")
        else:
            Singleton.__shared_instance = self


# main method
if __name__ == "__main__":

    # create object of Singleton Class
    obj = Singleton()
    print(obj)

    # pick the instance of the class
    obj = Singleton.getInstance()
    print(obj)

Output:  

 __main__.Singleton object at 0x014FFE90
 __main__.Singleton object at 0x014FFE90

Class diagram

Class Diagram of Singleton Design Pattern 

singleton-class-diagram

Advantages of using the Singleton Method: 

1. Initializations: An object created by the Singleton method is initialized only when it is requested for the first time.
2. Access to the object: We got global access to the instance of the object.
3. Count of instances: In singleton, method classes can’t have more than one instance

Disadvantages of using the Singleton Method: 

1. Multithread Environment: It’s not easy to use the singleton method in a multithread environment, because we have to take care that the multithread wouldn’t create a singleton object several times.
2. Single responsibility principle: As the Singleton method is solving two problems at a single time, it doesn’t follow the single responsibility principle.
3. Unit testing process: As they introduce the global state to the application, it makes the unit testing very hard.

Applicability

1. Controlling over global variables: In the projects where we specifically need strong control over the global variables, it is highly recommended to use Singleton Method
2. Daily Developers use: Singleton patterns are generally used in providing the logging, caching, thread pools, and configuration settings and are often used in conjunction with Factory design patterns.

Further read: Singleton method in Java, Singleton Design Pattern Practices with Examples

chaudhary_19

Follow

Improve