Simple Python Singleton pattern

This module provides a simple way to define a class as a singleton.

Install

You can install this python module via pip:

pip install simple-singleton

Otherwise the module can be downloaded from PyPI: https://pypi.org/project/simple-singleton/

Usage

1. Import the module:

   from simple_signleton import Singleton

   or:

   from simple_signleton import SingletonArgs

2. Create a class that uses one of the above meta classes:

   class NewClass(metaclass=Singleton):
       pass

   or:

   class NewClass(metaclass=SingletonArgs):
       pass

Difference between Singleton and SingletonArgs

The Singleton class is a very basic implementation of the singleton pattern. All instances of a class are equal. Even if they are initialized with different parameters:

instance1 = SingletonClass(param="value")
instance2 = SingletonClass(param="different_value")

assert instance1 == instance2  # True
print(instance2.param)         # "value"

If you do not want this behavior, use the SingletonArgs meta class. With this class only instances that are initialized with the same parameters are the equal:

instance1 = SingletonArgsClass(param="value")
instance2 = SingletonArgsClass(param="different_value")
instance3 = SingletonArgsClass(param="value")

assert instance1 == instance2  # False
assert instance1 == instance3  # True

print(instance2.param)         # "different_value"

Usage in multi-threaded environments

The Singleton and SingletonArgs meta classes are not thread-safe!

To use them in a multi-threaded environment, please use the

• ThreadSingleton and
• ThreadSingletonArgs

meta classes. They can be used exactly like the standard meta classes.