singletons.md

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Singletons

How are singletons implemented?

PHP

There exists no native support for singletons in PHP, but they can be approximated programmatically most easily by implementing a factory class. The following example comes from a stack overflow response:

<?php
    final class UserFactory {
        public static function Instance() {
            static $instance = NULL;
            if($instance === NULL) {
                $instance = new UserFactory();
            }
            return $instance;
        }
        private function __construct() { } // Empty constructor to prevent unintended initialization
    }

    // Foo and bar are comparatively equal
    $foo = UserFactory::Instance();
    $bar = UserFactory::Instance();
?>

Python

Singletons are not native to Python, but their design pattern can be approximated. This can be done due to Python's ability to have an object inherit properties of two super classes, but it can be approximated in a more clean matter by using metaclasses. The following example is taken from this stackoverflow response:

class Singleton:
    __instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

class myClass(SuperClass, metaclass=Singleton):
    pass

Can they be made thread safe?

PHP

Not easily or natively. PHP has less well defined threading functions than Python so while the Singleton behavior can be approximated with classes and constructors, making them strictly thread-safe is more difficult without something similar to Python's built-in thread locking.

Python

Yes, by leveraging some library tools from Python's threading abilities:

import threading
class Singleton:
    __singleton_lock = threading.Lock()
    __singleton_instance = None

    @classmethod
    def instance(cls):
        if not cls.__singleton_instance:
            with cls.__singleton_lock:
                if not cls.__singleton_instance:
                    cls.__singleton_instance = cls() # See https://docs.python.org/3/library/functions.html#classmethod
        return cls.__singleton_instance

Can the singleton instance be lazily instantiated?

PHP

Yes, and the previous examples employ this kind of instantiation checking. Singleton data structures can maintain whether or not they have been instantiated in their internal scopes and spawn them only once.

Python

Yes, and both previous examples approximate this behavior in similar ways. By having the base Singleton class keep track of if the instance is initialized it can be spawned only once, either in a __init__() method, or an overloaded __call__() method.