singleton-classes 1.0

package for different Singleton classes and metaclasses

singleton_classes - package for different Singleton classes and metaclasses

This package contains different classes to handle singleton instances. The following type of singleton class are available :

• Singleton : A single instance is available
• SingletonByClass : A single instance is available per class using this pattern
• SingletonByClassAndId : A single instance with a given ID is available per class using this pattern

Basic Use

Example with the Singleton class

from singleton_classes import Singleton

# =============================================================================
# You just have to inherit from the Singleton class to benefit from the feature

class A(Singleton):
    def __init__(self, a):
        self.a = a

a = A(8)


# =====================================================================================
# Once a first instance is created, you can acces it from anywhere by calling the class
# with no arguemnt

b = A()
b is a
>>> True


# =======================================================
# Trying to create a new instance will raise an Exception

b = A(8)
>>> singleton_classes.exceptions.singleton_exception.SingletonException: <class '__main__.A'> has been called with arguments. Unauthorized operation for Singleton sub classes !

Example with the SingletonByClass class

from singleton_classes import SingletonByClass

# =============================================================================
# You just have to inherit from the SingletonByClass class to benefit from the feature

class A(SingletonByClass):
    def __init__(self, a):
        self.a = a

class B(SingletonByClass):
    def __init__(self, a):
        self.a = a

class C(B):
    pass

a = A(8)

# =========================================================
# You can create an instance per class even for sub classes
b = B(8)
c = C(8)


# =====================================================================================
# Just like before, once a first instance is created, you can acces it from anywhere
# by calling the class with no arguemnt

a2 = A()
b2 = B()
c2 = C()

a2 is a
>>> True

b2 is b
>>> True

c2 is c
>>> True

# =======================================================
# Trying to create a new instance will raise an Exception
c = C(8)
>>> singleton_classes.exceptions.singleton_by_class_exception.SingletonByClassException: <class '__main__.C'> has been called with arguments. Unauthorized operation for SingletonByClass sub classes !

Example with the SingletonByClassAndId class

Sometime, you just want to avoid data multiplication. You want your data to be easily accessible, but you do not want to create an new instance each time and gather the related data.
For instance, you have clients for your comapany, each client has a unique ID. You want to easily access the data of client through its ID, the SingletonByClassAndId class is designed for this case.

from singleton_classes import SingletonByClassAndId

# ====================================================================================
# You have to inherit from the SingletonByClassAndId class to benefit from the feature
#  * the first argument of the __init__ method must be the ID
#  * don't forget to add the call to the super __init__ method with the ID !

class SingleClient(SingletonByClassAndId):
    def __init__(self, client_id, a):
        super().__init__(client_id)
        self.a = a

class GroupClient(SingletonByClassAndId):
    def __init__(self, client_id, a):
        super().__init__(client_id)
        self.a = a

john = SingleClient("John Doe", 8)

# ====================================================================
# You can create an instance per class AND per ID even for sub classes
jane = SingleClient("Jane Doe", 8)
company_john = GroupClient("John Doe", 8)  # ID is unique per class


# =====================================================================================
# Just like before, once a first instance is created, you can acces it from anywhere
# by calling the class with no arguemnt
john2 = SingleClient("John Doe")
jane2 = SingleClient("Jane Doe")
company_john2 = GroupClient("John Doe")

john2 is john
>>> True

jane2 is jane
>>> True

company_john2 is company_john
>>> True

# =======================================================
# Trying to create a new instance will raise an Exception
john3 = SingleClient("John Doe", 8)
>>> singleton_classes.exceptions.singleton_by_class_and_id_exception.SingletonByClassAndIdException: <class '__main__.SingleClient'> has been called with more arguments than just id. Unauthorized operation for SingletonByClassAndId sub classes !

Special method

First, for the SingletonByClassAndId, you might require to get the id from an instance. Use the 'get_singleton_id' method for this.

john.get_singleton_id()
>>> John Doe

Sometime, the singleton feature is a bit to restrictive. You may want to create a new instance for some reason.
Two class methods allow such a mechanism :

• new_unregistered_instance : creates a new instance but do not register it : If you call your class, you will still get the previous instance.
• new_ref_instance : creates a new instance and register it : If you call your class, you will get this precise instance

These two methods have to be called with the same arguments as when you create an instance and are available for the three singleton classes

from singleton_classes import SingletonByClass

class MySingletonClass(SingletonByClass):
    def __init__(self, a):
        self.a = a

a = MySingletonClass("Ref instance")
a2 = MySingletonClass.new_unregistered_instance("Unregistred instance")
a3 = MySingletonClass()

a2 is a
>>> False

a3 is a
>>> True

a2 = MySingletonClass.new_ref_instance("Registred Instance")
a3 = MySingletonClass()

a2 is a
>>> False

a3 is a2
>>> True