Singleton in python

[AnnaYasenova]

Implemented singleton in different ways:

• using inner class
• using new method
• using dict
• inctance of a class variable
• as class decorator
• with metaclasses

[tshemsedinov]

🌷 ☭ ☭ ☭ 🌷 thanks for you contribution to the world revolution 🌷 ☭ ☭ ☭ 🌷

[agil]

invalid formating

[agil]

following print statements do not verify that a, b, c are singletones:
a, b and c have different addresses and they are different objects
only a.instance b.instance and c.instance is a singleton

[agil]

inconsistent class declaration, redundant object inheritance for python 3 or missing object as superclass in other places for python2. But who uses python2 in 2018.
use copy-pasta smarter

[agil]

would be nice to have consistent tests in all examples, at least

print(a is b is c) 

check

[agil]

class SingletoneDict is redundant
_state_dict can be stored directly in Singletone class

class Singletone:
    _state_dict = {}

    def __init__(self, arg):
        self.__dict__ = self._state_dict
        self.val = arg

    def __str__(self):
        return self.val

[agil]

inconsistent class declaration, redundant object inheritance for python 3 or missing object as superclass in other places for python2. But who uses python2 in 2018.
use copy-pasta wiser

[agil]

would be nice to have consistent string representations along all examples

def __str__(self):
    return self.val

[agil]

prefer decorator syntax

@SingletoneDecorator
class MyClass:

[tshemsedinov]

@agil thanks, @AnnaYasenova see review please