Python singleton decorator class

example.py

@Singleton
class DBConnection(object):

    def __init__(self):
        """Initialize your database connection here."""
        pass

    def __str__(self):
        return 'Database connection object'
      


# Let’s access this connection class:

c1 = DBConnection.Instance()
c2 = DBConnection.Instance()

print("Id of c1 : {}".format(str(id(c1))))
print("Id of c2 : {}".format(str(id(c1))))

print("c1 is c2 ? " + str(c1 is c2))

# Output:

# Id of c1 : 139699882512960
# Id of c2 : 139699882512960
# c1 is c2 ? True

# If we do as below, we will get TypeError:

try:
    p = DBConnection()
except TypeError as ex:
    print("ERROR: {}".format(ex.message))

singleton.py

class Singleton:

    def __init__(self, cls):
        self._cls = cls

    def Instance(self, *args, **kwargs):
        try:
            return self._instance
        except AttributeError:
            self._instance = self._cls(*args, **kwargs)
            return self._instance

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

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