It’s pretty easy to implement a sloppy Singleton. You just need to hide the constructor and implement a static creation method.
The same class behaves incorrectly in a multithreaded environment. Multiple threads can call the creation method simultaneously and get several instances of Singleton class.
main.py: Conceptual example
class SingletonMeta(type):
"""
The Singleton class can be implemented in different ways in Python. Some
possible methods include: base class, decorator, metaclass. We will use the
metaclass because it is best suited for this purpose.
"""
_instances = {}
def __call__(cls, *args, **kwargs):
"""
Possible changes to the value of the `__init__` argument do not affect
the returned instance.
"""
if cls not in cls._instances:
instance = super().__call__(*args, **kwargs)
cls._instances[cls] = instance
return cls._instances[cls]
class Singleton(metaclass=SingletonMeta):
def some_business_logic(self):
"""
Finally, any singleton should define some business logic, which can be
executed on its instance.
"""
# ...
if __name__ == "__main__":
# The client code.
s1 = Singleton()
s2 = Singleton()
if id(s1) == id(s2):
print("Singleton works, both variables contain the same instance.")
else:
print("Singleton failed, variables contain different instances.")
Output.txt: Execution result
Singleton works, both variables contain the same instance.