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python singleton class

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David Eisen answered on March 4, 2021 Popularity 9/10 Helpfulness 3/10

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from threading import Lock, Thread


class SingletonMeta(type):
    """
    This is a thread-safe implementation of Singleton.
    """

    _instances = {}

    _lock: Lock = Lock()
    """
    We now have a lock object that will be used to synchronize threads during
    first access to the Singleton.
    """

    def __call__(cls, *args, **kwargs):
        """
        Possible changes to the value of the `__init__` argument do not affect
        the returned instance.
        """
        # Now, imagine that the program has just been launched. Since there's no
        # Singleton instance yet, multiple threads can simultaneously pass the
        # previous conditional and reach this point almost at the same time. The
        # first of them will acquire lock and will proceed further, while the
        # rest will wait here.
        with cls._lock:
            # The first thread to acquire the lock, reaches this conditional,
            # goes inside and creates the Singleton instance. Once it leaves the
            # lock block, a thread that might have been waiting for the lock
            # release may then enter this section. But since the Singleton field
            # is already initialized, the thread won't create a new object.
            if cls not in cls._instances:
                instance = super().__call__(*args, **kwargs)
                cls._instances[cls] = instance
        return cls._instances[cls]


class Singleton(metaclass=SingletonMeta):
    value: str = None
    """
    We'll use this property to prove that our Singleton really works.
    """

    def __init__(self, value: str) -> None:
        self.value = value

    def some_business_logic(self):
        """
        Finally, any singleton should define some business logic, which can be
        executed on its instance.
        """


def test_singleton(value: str) -> None:
    singleton = Singleton(value)
    print(singleton.value)


if __name__ == "__main__":
    # The client code.

    print("If you see the same value, then singleton was reused (yay!)\n"
          "If you see different values, "
          "then 2 singletons were created (booo!!)\n\n"
          "RESULT:\n")

    process1 = Thread(target=test_singleton, args=("FOO",))
    process2 = Thread(target=test_singleton, args=("BAR",))
    process1.start()
    process2.start()

Popularity 9/10 Helpfulness 3/10 Language python

Source: refactoring.guru

Tags: class python singleton

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Contributed on Oct 05 2022

David Eisen

0 Answers Avg Quality 2/10

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python singleton

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class Singleton:
    """
    A non-thread-safe helper class to ease implementing singletons.
    This should be used as a decorator -- not a metaclass -- to the
    class that should be a singleton.

    The decorated class can define one `__init__` function that
    takes only the `self` argument. Also, the decorated class cannot be
    inherited from. Other than that, there are no restrictions that apply
    to the decorated class.

    To get the singleton instance, use the `instance` method. Trying
    to use `__call__` will result in a `TypeError` being raised.

    """

    def __init__(self, decorated):
        self._decorated = decorated

    def instance(self):
        """
        Returns the singleton instance. Upon its first call, it creates a
        new instance of the decorated class and calls its `__init__` method.
        On all subsequent calls, the already created instance is returned.

        """
        try:
            return self._instance
        except AttributeError:
            self._instance = self._decorated()
            return self._instance

    def __call__(self):
        raise TypeError('Singletons must be accessed through `instance()`.')

    def __instancecheck__(self, inst):
        return isinstance(inst, self._decorated)

Popularity 10/10 Helpfulness 4/10 Language python

Source: stackoverflow.com

Tags: python py

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Contributed on Mar 04 2021

Nervous Nightingale

0 Answers Avg Quality 2/10

python singleton

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@Singleton
class Foo:
   def __init__(self):
       print 'Foo created'

f = Foo() # Error, this isn't how you get the instance of a singleton

f = Foo.instance() # Good. Being explicit is in line with the Python Zen
g = Foo.instance() # Returns already created instance

print f is g # True

Popularity 10/10 Helpfulness 4/10 Language python

Source: stackoverflow.com

Tags: python py

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Contributed on Mar 04 2021

Nervous Nightingale

0 Answers Avg Quality 2/10

singleton in python

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class Singleton:
    _instance = None

    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super().__new__(cls, *args, **kwargs)
        return cls._instance

# Example usage
obj1 = Singleton()
obj2 = Singleton()

print(obj1 is obj2)  # Output: True

Popularity 8/10 Helpfulness 3/10 Language python

Source: Grepper

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Contributed on Sep 11 2023

YOU.com

0 Answers Avg Quality 2/10

python singleton module

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# mymodule.py

__all__ = ["mysingleton"]

class __MySingletonLike(object):
    def __init__(self):
        self._variable = 42

    @property
    def variable(self):
        return self._variable

    @variable.setter
    def variable(self, value):
        check_value(value) # imaginary validation
        self._variable = value


mysingleton = __MySingleton()

Popularity 10/10 Helpfulness 2/10 Language python

Source: stackoverflow.com

Tags: module mo

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Contributed on May 15 2022

0 Answers Avg Quality 2/10

python singleton

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class Logger(object):
    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, '_logger'):
            cls._logger = super(Logger, cls
                    ).__new__(cls, *args, **kwargs)
        return cls._logger

Popularity 8/10 Helpfulness 2/10 Language python

Source: www.toptal.com

Tags: python singleton singlet

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Contributed on Jun 25 2023

Modern Macaque

0 Answers Avg Quality 2/10

python singleton class

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