A Singleton pattern in python is a design pattern that allows you to create just one instance of a class, throughout the lifetime of a program. Using a singleton pattern has many benefits. A few of them are:
- To limit concurrent access to a shared resource.
- To create a global point of access for a resource.
- To create just one instance of a class, throughout the lifetime of a program.
Different ways to implement a Singleton:
A singleton pattern can be implemented in three different ways. They are as follows:
- Module-level Singleton
- Classic Singleton
- Borg Singleton
All modules are singleton, by definition. Let’s create a simple module-level singleton where the data is shared among other modules. Here we will create three python files – singleton.py, sample_module1.py, and sample_module2.py – in which the other sample modules share a variable from singleton.py.
## singleton.py shared_variable = "Shared Variable"
## samplemodule1.py import singleton print(singleton.shared_variable) singleton.shared_variable += "(modified by samplemodule1)"
##samplemodule2.py import singleton print(singleton.shared_variable)
Let’s look into the output.
Here, the value changed by samplemodule1 is also reflected in samplemodule2.
Classic Singleton creates an instance only if there is no instance created so far; otherwise, it will return the instance that is already created. Let’s take a look at the below code.
True Singleton Variable
Here, in the __new__ method, we will check whether an instance is created or not. If created, it will return the instance; otherwise, it will create a new instance. You can notice that singleton and new_singleton return the same instance and have the same variable.
Let’s check what happens when we subclass a singleton class.
True Singleton Variable
Here, you can see that SingletonChild has the same instance of SingletonClass and also shares the same state. But there are scenarios, where we need a different instance, but should share the same state. This state sharing can be achieved using Borg singleton.
Borg singleton is a design pattern in Python that allows state sharing for different instances. Let’s look into the following code.
False Shared Variable
Along with the new instance creation process, a shared state is also defined in the __new__ method. Here the shared state is retained using the shared_borg_state attribute and it is stored in the __dict__ dictionary of each instance.
If you want a different state, then you can reset the shared_borg_state attribute. Let’s see how to reset a shared state.
Here, we have reset the shared state and tried to access the shared_variable. Let’s see the error.
Traceback (most recent call last): File "/home/329d68500c5916767fbaf351710ebb13.py", line 16, in <module> print(newChildBorg.shared_variable) AttributeError: 'NewChildBorg' object has no attribute 'shared_variable'
Use cases of a Singleton:
Let’s list a few of the use cases of a singleton class. They are as follows:
- Managing a database connection
- Global point access to writing log messages
- File Manager
- Print spooler
Create a Web Crawler using Classic Singleton:
Let’s create a webcrawler that uses the benefit of a classic singleton. In this practical example, the crawler scans a webpage, fetch the links associated with the same website, and download all the images in it. Here, we have two main classes and two main functions.
- CrawlerSingleton: This class acts a classic singleton
- ParallelDownloader: This class provides thread functionality to download images
- navigate_site: This function crawls the website and fetches the links that belong to the same website. And, finally, it arranges the link to download images.
- download_images: This function crawls the page link and downloads the images.
Apart from the above classes and functions, we use two sets of libraries to parse the web page – BeautifulSoap and HTTP Client.
Have a look at the below code.
Note: Execute the code in your local machine
Let’s look into the downloaded images and python shell output.
Singleton pattern is a design pattern in Python that restricts the instantiation of a class to one object. It can limit concurrent access to a shared resource, and also it helps to create a global point of access for a resource.
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