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How to create a class which can be iniitalized by its own instance

Task:
Implement some class that accepts at least one argument and can be either initialized by original data, or its own instance.
Minimal example of usage:
arg = {} # whatever necessary for the real object
instance1 = NewClass(arg)
instance2 = NewClass(instance1)
assert instance2 is instance1 # or at least, ==
More complex example of usage:
from typing import Mapping, Union
class NewClass:
"""
Incomplete
Should somehow act like described in the task
"""
def __init__(self, data: Mapping):
self.data = data
def cool_method(self):
assert isinstance(self.data, Mapping)
# do smth with self.data
return ...
...
class AnotherClass:
"""
Accepts both mappings and NewClass instances,
but needs NewClass internally
"""
def __init__(self, obj: Union[Mapping, NewClass]):
self.cool = NewClass(obj).cool_method()
...

One just have to make use of the __new__ method on the class, instead of __init__ to be able to change what is instantiated.
In this case, all you need is to write your NewClass like this:
from typing import Union, Mapping, Self
class NewClass:
"""
acts like described in the task
"""
# typing.Self is available in Python 3.11.
# For previous versions, just put the class name quoted
# in a string: `"NewClass"` instead of `Self`
def __new__(cls, data: Union[Mapping, Self]):
if isinstance(data, NewClass):
return data
self = super().__new__(cls)
self.data = data
return self
def cool_method(self):
assert isinstance(self.data, Mapping)
# do smth with self.data
return ...
Avoiding a metaclass is interesting because it avoid metaclasses conflicts, in larger projects, and it is an abstraction level most
projects simply does not need. Actually, static type checkers such
as "Mypy" can't even figure out behavior changes coded into
the metaclasses.
On the other hand, __new__ is a common special method sibling to __init__, readily available, just not used more commonly because Python also provides
__init__, which suffices when the default behavior of __new__, of
always creating a new instance, is not the desired one.
For some reason I do not know, making use of a metaclass to create a "singleton" got wildly popular in tutorials and answers. It is a design pattern much less important and less used in Python than in languages which do not allow "stand alone" functions. Metaclasses are not needed for singletons either, by the way - one can just create a top-level instance of whatever class should have a single instance, and use that instance from that point on, instead of creating new instances. Other languages also restrict the existence of top-level, importable, instances, making that a need that was artificially imported into Python.

Metaclass solution:
Actual for python 3.8
class SelfWrapperMeta(type):
"""
Metaclass, allowing to return previously created user class instance,
if the user class init receives it as the first positional argument
Other arguments are just ignored in that self-wrapping case
Otherwise, the user class init calls normally
"""
def __call__(cls, arg, /, *args, **kwargs):
if isinstance(arg, cls):
return arg
return super().__call__(arg, *args, **kwargs)
Example of usage:
class A(metaclass=SelfWrapperMeta):
def __init__(self, data):
self.data = data
example = {}
a = A(example)
b = A(a)
c = A(example)
assert a is b
assert c is not a

How to understand the `org = super(Singleton, cls)` and `org.__new__(cls, *args, **kw)`?

How to understand this code:
I am researching the Singleton of Python implementation:
class Singleton(object):
''''' A python style singleton '''
def __new__(cls, *args, **kw):
if not hasattr(cls, '_instance'):
org = super(Singleton, cls) # what does this line mean, what's the funciton of it?
cls._instance = org.__new__(cls, *args, **kw) # and this line
return cls._instance
if __name__ == '__main__':
class SingleSpam(Singleton):
def __init__(self, s):
self.s = s
def __str__(self):
return self.s
s1 = SingleSpam('spam')
print id(s1), s1
s2 = SingleSpam('spa')
print id(s2), s2
print id(s1), s1
I do not understand the two lines code:
org = super(Singleton, cls)
cls._instance = org.__new__(cls, *args, **kw)
whether the cls equals self?
what's the meaning of org = super(Singleton, cls)?
why use org.__new__(cls, *args, **kw) for creating the Singleton instance's property _instance?

whether the cls equals self?
No, __new__ is a static method (automatically, without needing the #staticmethod decorator) that doesn't take a self. Which is good, because there is no self to pass it—the whole point of __new__ is to create that self and return it.1
So, its first argument, cls, is the type of object Python wants it to create. Here, that will be Singleton.
In particular, when you write this:
obj = Singleton()
… what Python turns that into is something like this (slightly oversimplified):
obj = Singleton.__new__(Singleton)
if isinstance(obj, Singleton):
obj.__init__()
what's the meaning of org = super(Singleton, cls)?
What super does is:
Return a proxy object that delegates method calls to a parent or sibling class of type. This is useful for accessing inherited methods that have been overridden in a class. The search order is same as that used by getattr() except that the type itself is skipped.
In other words, super(Singleton, cls) is an object that acts just like cls—which, remember, is Singleton—except that when you try to call methods on it, it skips methods defined in Singleton and only finds methods defined in the next class in method resolution order—in this case, that's object.
why use org.__new__(cls, *args, **kw) for creating the Singleton instance's property _instance?
Remember that cls is Singleton, and org is a super proxy object, that references the same class object as cls, but has the methods of object instead of the methods of Singleton.2
So, when you call org.__new__, instead of calling the Singleton.__new__ code again and getting stuck in an infinite loop, it calls the object.__new__ code. But we pass it cls—that is, Singleton—as its argument. So, we're asking the object type to create a new object of type Singleton. The result, then, is a Singleton instance, which we return.
But first, we store it cls._instance—that is, Singleton._instance—so the next time we try to create a Singleton, we can just return the same cls._instance.
If we just wanted a single Singleton class that didn't do anything, it could be a little bit simpler. But Singleton can be subclassed, and each of its subclasses will be a singleton class. Start with this:
class SingleSub(Singleton):
pass
obj = SingleSub()
So this is going to call SingleSub.__new__(SingleSub). Since SingleSub doesn't override __new__, that calls Singleton.__new__(SingleSub), so, inside that code, cls is now SingleSub instead of Singleton.
And you should be able to trace things through from there and see how SingleSub ends up with a single _instance, distinct from any other subclass of Singleton (and its __init__ gets called exactly when you'd expect, and so on).
1. Of course, unless you're writing an extension module (e.g., a C module for CPython), you have no way of creating a self. That's why you delegate it to your super—ultimately it gets to object, or some other builtin/extension type, which does know how to create an instance. And of course you can modify the arguments passed up the chain, or the self passed down the chain.
2. Under the covers, this works much the same way methods themselves work, which should make things a lot clearer if you already know how methods work, but if you don't it'll probably be more confusing than enlightening, so don't worry about it. But at some point, you should work through the Descriptor HOWTO. It's tough going, but once you get that idea, all of the "magic" in Python becomes clear.

Clearing a MetaClass Singleton

I've created a Singleton using a MetaClass as discussed in Method 3 of this answer
class Singleton(type):
_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 MySing(metaclass=Singleton): ...
I'd like to be able to clear the Singleton in the setUp() method of a unittest.TestCase so that each test starts with a clean Singleton.
I guess I don't really understand what this metaClass is doing because I can't get the correct incantation for a clear() method:
def clear(self):
try:
del(Singleton._instances[type(self)]
except KeyError:
pass #Sometimes we clear before creating
Any thoughts on what I'm doing wrong here? My singleton is not getting cleared.
sing=MySing()
sing.clear()
The type call above returns Singleton not MySing.

Let's walk through a (corrected) definition of Singleton and a class defined using it. I'm replacing uses of cls with Singleton where the lookup is passed through anyway.
class Singleton(type):
_instances = {}
# Each of the following functions use cls instead of self
# to emphasize that although they are instance methods of
# Singleton, they are also *class* methods of a class defined
# with Singleton
def __call__(cls, *args, **kwargs):
if cls not in Singleton._instances:
Singleton._instances[cls] = super().__call__(*args, **kwargs)
return Singleton._instances[cls]
def clear(cls):
try:
del Singleton._instances[cls]
except KeyError:
pass
class MySing(metaclass=Singleton):
pass
s1 = MySing() # First call: actually creates a new instance
s2 = MySing() # Second call: returns the cached instance
assert s1 is s2 # Yup, they are the same
MySing.clear() # Throw away the cached instance
s3 = MySing() # Third call: no cached instance, so create one
assert s1 is not s3 # Yup, s3 is a distinct new instance
First, _instances is a class attribute of the metaclass, meant to map a class to a unique instance of that class.
__call__ is an instance method of the metaclass; its purpose is to make instances of the metaclass (i.e., classes) callable. cls here is the class being defined, not the metaclass. So each time you call MyClass(), that converts to Singleton.__call__(MyClass).
clear is also a instance method of the metaclass, meaning it also takes a instance of the meta class (i.e again, a class) as an argument (not an instance of the class defined with the metaclass.) This means MyClass.clear() is the same as Singleton.clear(MyClass). (This also means you can, but probably shouldn't for clarity, write s1.clear().)
The identification of metaclass instance methods with "regular" class class methods also explains why you need to use __call__ in the meta class where you would use __new__ in the regular class: __new__ is special-cased as a class method without having to decorate it as such. It's slightly tricky for a metaclass to define an instance method for its instances, so we just use __call__ (since type.__call__ doesn't do much, if anything, beyond invoking the correct __new__ method).

I see three useful test cases for this metaclass at first glance.
Test whether a single class creation works properly.
Test whether no new class is created after the initial one.
Test whether multiple classes can use this metaclass in conjunction.
All of these tests can be achieved without a "reset" button. After which you'll have covered most of your bases. (I might have forgotten one).
Simply create a few different TestClasses that use this metaclass and check their Id's and types.

Creating a singleton in Python

This question is not for the discussion of whether or not the singleton design pattern is desirable, is an anti-pattern, or for any religious wars, but to discuss how this pattern is best implemented in Python in such a way that is most pythonic. In this instance I define 'most pythonic' to mean that it follows the 'principle of least astonishment'.
I have multiple classes which would become singletons (my use-case is for a logger, but this is not important). I do not wish to clutter several classes with added gumph when I can simply inherit or decorate.
Best methods:
Method 1: A decorator
def singleton(class_):
instances = {}
def getinstance(*args, **kwargs):
if class_ not in instances:
instances[class_] = class_(*args, **kwargs)
return instances[class_]
return getinstance
#singleton
class MyClass(BaseClass):
pass
Pros
Decorators are additive in a way that is often more intuitive than multiple inheritance.
Cons
While objects created using MyClass() would be true singleton objects, MyClass itself is a function, not a class, so you cannot call class methods from it. Also for
x = MyClass();
y = MyClass();
t = type(n)();
then x == y but x != t && y != t
Method 2: A base class
class Singleton(object):
_instance = None
def __new__(class_, *args, **kwargs):
if not isinstance(class_._instance, class_):
class_._instance = object.__new__(class_, *args, **kwargs)
return class_._instance
class MyClass(Singleton, BaseClass):
pass
Pros
It's a true class
Cons
Multiple inheritance - eugh! __new__ could be overwritten during inheritance from a second base class? One has to think more than is necessary.
Method 3: A metaclass
class Singleton(type):
_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]
#Python2
class MyClass(BaseClass):
__metaclass__ = Singleton
#Python3
class MyClass(BaseClass, metaclass=Singleton):
pass
Pros
It's a true class
Auto-magically covers inheritance
Uses __metaclass__ for its proper purpose (and made me aware of it)
Cons
Are there any?
Method 4: decorator returning a class with the same name
def singleton(class_):
class class_w(class_):
_instance = None
def __new__(class_, *args, **kwargs):
if class_w._instance is None:
class_w._instance = super(class_w,
class_).__new__(class_,
*args,
**kwargs)
class_w._instance._sealed = False
return class_w._instance
def __init__(self, *args, **kwargs):
if self._sealed:
return
super(class_w, self).__init__(*args, **kwargs)
self._sealed = True
class_w.__name__ = class_.__name__
return class_w
#singleton
class MyClass(BaseClass):
pass
Pros
It's a true class
Auto-magically covers inheritance
Cons
Is there not an overhead for creating each new class? Here we are creating two classes for each class we wish to make a singleton. While this is fine in my case, I worry that this might not scale. Of course there is a matter of debate as to whether it aught to be too easy to scale this pattern...
What is the point of the _sealed attribute
Can't call methods of the same name on base classes using super() because they will recurse. This means you can't customize __new__ and can't subclass a class that needs you to call up to __init__.
Method 5: a module
a module file singleton.py
Pros
Simple is better than complex
Cons
Not lazily instantiated

Use a Metaclass
I would recommend Method #2, but you're better off using a metaclass than a base class. Here is a sample implementation:
class Singleton(type):
_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 Logger(object):
__metaclass__ = Singleton
Or in Python3
class Logger(metaclass=Singleton):
pass
If you want to run __init__ every time the class is called, add
else:
cls._instances[cls].__init__(*args, **kwargs)
to the if statement in Singleton.__call__.
A few words about metaclasses. A metaclass is the class of a class; that is, a class is an instance of its metaclass. You find the metaclass of an object in Python with type(obj). Normal new-style classes are of type type. Logger in the code above will be of type class 'your_module.Singleton', just as the (only) instance of Logger will be of type class 'your_module.Logger'. When you call logger with Logger(), Python first asks the metaclass of Logger, Singleton, what to do, allowing instance creation to be pre-empted. This process is the same as Python asking a class what to do by calling __getattr__ when you reference one of its attributes by doing myclass.attribute.
A metaclass essentially decides what the definition of a class means and how to implement that definition. See for example http://code.activestate.com/recipes/498149/, which essentially recreates C-style structs in Python using metaclasses. The thread What are some (concrete) use-cases for metaclasses? also provides some examples, they generally seem to be related to declarative programming, especially as used in ORMs.
In this situation, if you use your Method #2, and a subclass defines a __new__ method, it will be executed every time you call SubClassOfSingleton() -- because it is responsible for calling the method that returns the stored instance. With a metaclass, it will only be called once, when the only instance is created. You want to customize what it means to call the class, which is decided by its type.
In general, it makes sense to use a metaclass to implement a singleton. A singleton is special because is created only once, and a metaclass is the way you customize the creation of a class. Using a metaclass gives you more control in case you need to customize the singleton class definitions in other ways.
Your singletons won't need multiple inheritance (because the metaclass is not a base class), but for subclasses of the created class that use multiple inheritance, you need to make sure the singleton class is the first / leftmost one with a metaclass that redefines __call__ This is very unlikely to be an issue. The instance dict is not in the instance's namespace so it won't accidentally overwrite it.
You will also hear that the singleton pattern violates the "Single Responsibility Principle" -- each class should do only one thing. That way you don't have to worry about messing up one thing the code does if you need to change another, because they are separate and encapsulated. The metaclass implementation passes this test. The metaclass is responsible for enforcing the pattern and the created class and subclasses need not be aware that they are singletons. Method #1 fails this test, as you noted with "MyClass itself is a a function, not a class, so you cannot call class methods from it."
Python 2 and 3 Compatible Version
Writing something that works in both Python2 and 3 requires using a slightly more complicated scheme. Since metaclasses are usually subclasses of type type, it's possible to use one to dynamically create an intermediary base class at run time with it as its metaclass and then use that as the baseclass of the public Singleton base class. It's harder to explain than to do, as illustrated next:
# works in Python 2 & 3
class _Singleton(type):
""" A metaclass that creates a Singleton base class when called. """
_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 Singleton(_Singleton('SingletonMeta', (object,), {})): pass
class Logger(Singleton):
pass
An ironic aspect of this approach is that it's using subclassing to implement a metaclass. One possible advantage is that, unlike with a pure metaclass, isinstance(inst, Singleton) will return True.
Corrections
On another topic, you've probably already noticed this, but the base class implementation in your original post is wrong. _instances needs to be referenced on the class, you need to use super() or you're recursing, and __new__ is actually a static method that you have to pass the class to, not a class method, as the actual class hasn't been created yet when it is called. All of these things will be true for a metaclass implementation as well.
class Singleton(object):
_instances = {}
def __new__(class_, *args, **kwargs):
if class_ not in class_._instances:
class_._instances[class_] = super(Singleton, class_).__new__(class_, *args, **kwargs)
return class_._instances[class_]
class MyClass(Singleton):
pass
c = MyClass()
Decorator Returning A Class
I originally was writing a comment but it was too long, so I'll add this here. Method #4 is better than the other decorator version, but it's more code than needed for a singleton, and it's not as clear what it does.
The main problems stem from the class being its own base class. First, isn't it weird to have a class be a subclass of a nearly identical class with the same name that exists only in its __class__ attribute? This also means that you can't define any methods that call the method of the same name on their base class with super() because they will recurse. This means your class can't customize __new__, and can't derive from any classes that need __init__ called on them.
When to use the singleton pattern
Your use case is one of the better examples of wanting to use a singleton. You say in one of the comments "To me logging has always seemed a natural candidate for Singletons." You're absolutely right.
When people say singletons are bad, the most common reason is they are implicit shared state. While with global variables and top-level module imports are explicit shared state, other objects that are passed around are generally instantiated. This is a good point, with two exceptions.
The first, and one that gets mentioned in various places, is when the singletons are constant. Use of global constants, especially enums, is widely accepted, and considered sane because no matter what, none of the users can mess them up for any other user. This is equally true for a constant singleton.
The second exception, which get mentioned less, is the opposite -- when the singleton is only a data sink, not a data source (directly or indirectly). This is why loggers feel like a "natural" use for singletons. As the various users are not changing the loggers in ways other users will care about, there is not really shared state. This negates the primary argument against the singleton pattern, and makes them a reasonable choice because of their ease of use for the task.
Here is a quote from http://googletesting.blogspot.com/2008/08/root-cause-of-singletons.html:
Now, there is one kind of Singleton which is OK. That is a singleton where all of the reachable objects are immutable. If all objects are immutable than Singleton has no global state, as everything is constant. But it is so easy to turn this kind of singleton into mutable one, it is very slippery slope. Therefore, I am against these Singletons too, not because they are bad, but because it is very easy for them to go bad. (As a side note Java enumeration are just these kind of singletons. As long as you don't put state into your enumeration you are OK, so please don't.)
The other kind of Singletons, which are semi-acceptable are those which don't effect the execution of your code, They have no "side effects". Logging is perfect example. It is loaded with Singletons and global state. It is acceptable (as in it will not hurt you) because your application does not behave any different whether or not a given logger is enabled. The information here flows one way: From your application into the logger. Even thought loggers are global state since no information flows from loggers into your application, loggers are acceptable. You should still inject your logger if you want your test to assert that something is getting logged, but in general Loggers are not harmful despite being full of state.

class Foo(object):
pass
some_global_variable = Foo()
Modules are imported only once, everything else is overthinking. Don't use singletons and try not to use globals.

Use a module. It is imported only once. Define some global variables in it - they will be singleton's 'attributes'. Add some functions - the singleton's 'methods'.

You probably never need a singleton in Python. Just define all your data and functions in a module and you have a de facto singleton:
import datetime
file_name=None
def set_file_name(new_file_name: str):
global file_name
file_name=new_file_name
def write(message: str):
global file_name
if file_name:
with open(file_name, 'a+') as f:
f.write("{} {}\n".format(datetime.datetime.now(), message))
else:
print("LOG: {}", message)
To use:
import log
log.set_file_name("debug.log")
log.write("System starting")
...
If you really absolutely have to have a singleton class then I'd go with:
class MySingleton(object):
def foo(self):
pass
my_singleton = MySingleton()
To use:
from mysingleton import my_singleton
my_singleton.foo()
where mysingleton.py is your filename that MySingleton is defined in. This works because after the first time a file is imported, Python doesn't re-execute the code.

Here's a one-liner for you:
singleton = lambda c: c()
Here's how you use it:
#singleton
class wat(object):
def __init__(self): self.x = 1
def get_x(self): return self.x
assert wat.get_x() == 1
Your object gets instantiated eagerly. This may or may not be what you want.

Check out Stack Overflow question Is there a simple, elegant way to define singletons in Python? with several solutions.
I'd strongly recommend to watch Alex Martelli's talks on design patterns in python: part 1 and part 2. In particular, in part 1 he talks about singletons/shared state objects.

If one wants to have multiple number of instances of the same class, but only if the args or kwargs are different, one can use the third-party python package Handy Decorators (package decorators).
Ex.
If you have a class handling serial communication, and to create an instance you want to send the serial port as an argument, then with traditional approach won't work
Using the above mentioned decorators, one can create multiple instances of the class if the args are different.
For same args, the decorator will return the same instance which is already been created.
>>> from decorators import singleton
>>>
>>> #singleton
... class A:
... def __init__(self, *args, **kwargs):
... pass
...
>>>
>>> a = A(name='Siddhesh')
>>> b = A(name='Siddhesh', lname='Sathe')
>>> c = A(name='Siddhesh', lname='Sathe')
>>> a is b # has to be different
False
>>> b is c # has to be same
True
>>>

You just need a decorator, depending on the python version:
Python 3.2+
Implementation
from functools import lru_cache
#lru_cache(maxsize=None)
class CustomClass(object):
def __init__(self, arg):
print(f"CustomClass initialised with {arg}")
self.arg = arg
Usage
c1 = CustomClass("foo")
c2 = CustomClass("foo")
c3 = CustomClass("bar")
print(c1 == c2)
print(c1 == c3)
Output
>>> CustomClass initialised with foo
>>> CustomClass initialised with bar
>>> True
>>> False
Notice how foo got printed only once
Python 3.9+
Implementation:
from functools import cache
#cache
class CustomClass(object):
...

Using a function attribute is also very simple
def f():
if not hasattr(f, 'value'):
setattr(f, 'value', singletonvalue)
return f.value

I prefer this solution which I found very clear and straightforward.
It is using double check for instance, if some other thread already created it.
Additional thing to consider is to make sure that deserialization isn't creating any other instances.
https://gist.github.com/werediver/4396488
import threading
# Based on tornado.ioloop.IOLoop.instance() approach.
# See https://github.com/facebook/tornado
class SingletonMixin(object):
__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()
return cls.__singleton_instance
if __name__ == '__main__':
class A(SingletonMixin):
pass
class B(SingletonMixin):
pass
a, a2 = A.instance(), A.instance()
b, b2 = B.instance(), B.instance()
assert a is a2
assert b is b2
assert a is not b
print('a: %s\na2: %s' % (a, a2))
print('b: %s\nb2: %s' % (b, b2))

Here's my own implementation of singletons. All you have to do is decorate the class; to get the singleton, you then have to use the Instance method. Here's an example:
#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
And here's the code:
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. 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.
Limitations: The decorated class cannot be inherited from.
"""
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)

I will recommend an elegant solution using metaclasses
class Singleton(type):
# Inherit from "type" in order to gain access to method __call__
def __init__(self, *args, **kwargs):
self.__instance = None # Create a variable to store the object reference
super().__init__(*args, **kwargs)
def __call__(self, *args, **kwargs):
if self.__instance is None:
# if the object has not already been created
self.__instance = super().__call__(*args, **kwargs) # Call the __init__ method of the subclass (Spam) and save the reference
return self.__instance
else:
# if object (Spam) reference already exists; return it
return self.__instance
class Spam(metaclass=Singleton):
def __init__(self, x):
print('Creating Spam')
self.x = x
if __name__ == '__main__':
spam = Spam(100)
spam2 = Spam(200)
Output:
Creating Spam
As you can see from the output, only one object is instantiated

from functools import cache
#cache
class xxx:
....
Dead easy and works!

Use a class variable (no decorator)
By overriding the __new__ method to return the same instance of the class. A boolean to only initialize the class for the first time:
class SingletonClass:
_instance = None
def __new__(cls, *args, **kwargs):
# If no instance of class already exits
if cls._instance is None:
cls._instance = object.__new__(cls)
cls._instance._initialized = False
return cls._instance
def __init__(self, *args, **kwargs):
if self._initialized:
return
self.attr1 = args[0]
# set the attribute to `True` to not initialize again
self._initialized = True

Method 3 seems to be very neat, but if you want your program to run in both Python 2 and Python 3, it doesn't work. Even protecting the separate variants with tests for the Python version fails, because the Python 3 version gives a syntax error in Python 2.
Thanks to Mike Watkins: http://mikewatkins.ca/2008/11/29/python-2-and-3-metaclasses/. If you want the program to work in both Python 2 and Python 3, you need to do something like:
class Singleton(type):
_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]
MC = Singleton('MC', (object), {})
class MyClass(MC):
pass # Code for the class implementation
I presume that 'object' in the assignment needs to be replaced with the 'BaseClass', but I haven't tried that (I have tried code as illustrated).

I'll toss mine into the ring. It's a simple decorator.
from abc import ABC
def singleton(real_cls):
class SingletonFactory(ABC):
instance = None
def __new__(cls, *args, **kwargs):
if not cls.instance:
cls.instance = real_cls(*args, **kwargs)
return cls.instance
SingletonFactory.register(real_cls)
return SingletonFactory
# Usage
#singleton
class YourClass:
... # Your normal implementation, no special requirements.
Benefits I think it has over some of the other solutions:
It's clear and concise (to my eye ;D).
Its action is completely encapsulated. You don't need to change a single thing about the implementation of YourClass. This includes not needing to use a metaclass for your class (note that the metaclass above is on the factory, not the "real" class).
It doesn't rely on monkey-patching anything.
It's transparent to callers:
Callers still simply import YourClass, it looks like a class (because it is), and they use it normally. No need to adapt callers to a factory function.
What YourClass() instantiates is still a true instance of the YourClass you implemented, not a proxy of any kind, so no chance of side effects resulting from that.
isinstance(instance, YourClass) and similar operations still work as expected (though this bit does require abc so precludes Python <2.6).
One downside does occur to me: classmethods and staticmethods of the real class are not transparently callable via the factory class hiding it. I've used this rarely enough that I've never happen to run into that need, but it would be easily rectified by using a custom metaclass on the factory that implements __getattr__() to delegate all-ish attribute access to the real class.
A related pattern I've actually found more useful (not that I'm saying these kinds of things are required very often at all) is a "Unique" pattern where instantiating the class with the same arguments results in getting back the same instance. I.e. a "singleton per arguments". The above adapts to this well and becomes even more concise:
def unique(real_cls):
class UniqueFactory(ABC):
#functools.lru_cache(None) # Handy for 3.2+, but use any memoization decorator you like
def __new__(cls, *args, **kwargs):
return real_cls(*args, **kwargs)
UniqueFactory.register(real_cls)
return UniqueFactory
All that said, I do agree with the general advice that if you think you need one of these things, you really should probably stop for a moment and ask yourself if you really do. 99% of the time, YAGNI.

Maybe I missunderstand the singleton pattern but my solution is this simple and pragmatic (pythonic?). This code fullfills two goals
Make the instance of Foo accessiable everywhere (global).
Only one instance of Foo can exist.
This is the code.
#!/usr/bin/env python3
class Foo:
me = None
def __init__(self):
if Foo.me != None:
raise Exception('Instance of Foo still exists!')
Foo.me = self
if __name__ == '__main__':
Foo()
Foo()
Output
Traceback (most recent call last):
File "./x.py", line 15, in <module>
Foo()
File "./x.py", line 8, in __init__
raise Exception('Instance of Foo still exists!')
Exception: Instance of Foo still exists!

Well, other than agreeing with the general Pythonic suggestion on having module-level global, how about this:
def singleton(class_):
class class_w(class_):
_instance = None
def __new__(class2, *args, **kwargs):
if class_w._instance is None:
class_w._instance = super(class_w, class2).__new__(class2, *args, **kwargs)
class_w._instance._sealed = False
return class_w._instance
def __init__(self, *args, **kwargs):
if self._sealed:
return
super(class_w, self).__init__(*args, **kwargs)
self._sealed = True
class_w.__name__ = class_.__name__
return class_w
#singleton
class MyClass(object):
def __init__(self, text):
print text
#classmethod
def name(class_):
print class_.__name__
x = MyClass(111)
x.name()
y = MyClass(222)
print id(x) == id(y)
Output is:
111 # the __init__ is called only on the 1st time
MyClass # the __name__ is preserved
True # this is actually the same instance

How about this:
def singleton(cls):
instance=cls()
cls.__new__ = cls.__call__= lambda cls: instance
cls.__init__ = lambda self: None
return instance
Use it as a decorator on a class that should be a singleton. Like this:
#singleton
class MySingleton:
#....
This is similar to the singleton = lambda c: c() decorator in another answer. Like the other solution, the only instance has name of the class (MySingleton). However, with this solution you can still "create" instances (actually get the only instance) from the class, by doing MySingleton(). It also prevents you from creating additional instances by doing type(MySingleton)() (that also returns the same instance).

This answer is likely not what you're looking for. I wanted a singleton in the sense that only that object had its identity, for comparison to. In my case it was being used as a Sentinel Value. To which the answer is very simple, make any object mything = object() and by python's nature, only that thing will have its identity.
#!python
MyNone = object() # The singleton
for item in my_list:
if item is MyNone: # An Example identity comparison
raise StopIteration

Pros
It's a true class Auto-magically covers inheritance Uses metaclass
for its proper purpose (and made me aware of it) Cons
Are there any?
This will be problem with serialziation. If you try to deserialize object from file (pickle) it will not use __call__ so it will create new file, you can use base class inheritance with __new__ to prevent that.

I also prefer decorator syntax to deriving from metaclass. My two cents:
from typing import Callable, Dict, Set
def singleton(cls_: Callable) -> type:
""" Implements a simple singleton decorator
"""
class Singleton(cls_): # type: ignore
__instances: Dict[type, object] = {}
__initialized: Set[type] = set()
def __new__(cls, *args, **kwargs):
if Singleton.__instances.get(cls) is None:
Singleton.__instances[cls] = super().__new__(cls, *args, **kwargs)
return Singleton.__instances[cls]
def __init__(self, *args, **kwargs):
if self.__class__ not in Singleton.__initialized:
Singleton.__initialized.add(self.__class__)
super().__init__(*args, **kwargs)
return Singleton
#singleton
class MyClass(...):
...
This has some benefits above other decorators provided:
isinstance(MyClass(), MyClass) will still work (returning a function from the clausure instead of a class will make isinstance to fail)
property, classmethod and staticmethod will still work as expected
__init__() constructor is executed only once
You can inherit from your decorated class (useless?) using #singleton again
Cons:
print(MyClass().__class__.__name__) will return Singleton instead of MyClass. If you still need this, I recommend using a metaclass as suggested above.
If you need a different instance based on constructor parameters this solution needs to be improved (solution provided by siddhesh-suhas-sathe provides this).
Finally, as other suggested, consider using a module in python. Modules are objects. You can even pass them in variables and inject them in other classes.

I just made a simple one by accident and thought I'd share it...
class MySingleton(object):
def __init__(self, *, props={}):
self.__dict__ = props
mything = MySingleton()
mything.test = 1
mything2 = MySingleton()
print(mything2.test)
mything2.test = 5
print(mything.test)

It is slightly similar to the answer by fab but not exactly the same.
The singleton pattern does not require that we be able to call the constructor multiple times. As a singleton should be created once and once only, shouldn't it be seen to be created just once? "Spoofing" the constructor arguably impairs legibility.
So my suggestion is just this:
class Elvis():
def __init__(self):
if hasattr(self.__class__, 'instance'):
raise Exception()
self.__class__.instance = self
# initialisation code...
#staticmethod
def the():
if hasattr(Elvis, 'instance'):
return Elvis.instance
return Elvis()
This does not rule out the use of the constructor or the field instance by user code:
if Elvis() is King.instance:
... if you know for sure that Elvis has not yet been created, and that King has.
But it encourages users to use the the method universally:
Elvis.the().leave(Building.the())
To make this complete you could also override __delattr__() to raise an Exception if an attempt is made to delete instance, and override __del__() so that it raises an Exception (unless we know the program is ending...)
Further improvements
My thanks to those who have helped with comments and edits, of which more are welcome. While I use Jython, this should work more generally, and be thread-safe.
try:
# This is jython-specific
from synchronize import make_synchronized
except ImportError:
# This should work across different python implementations
def make_synchronized(func):
import threading
func.__lock__ = threading.Lock()
def synced_func(*args, **kws):
with func.__lock__:
return func(*args, **kws)
return synced_func
class Elvis(object): # NB must be subclass of object to use __new__
instance = None
#classmethod
#make_synchronized
def __new__(cls, *args, **kwargs):
if cls.instance is not None:
raise Exception()
cls.instance = object.__new__(cls, *args, **kwargs)
return cls.instance
def __init__(self):
pass
# initialisation code...
#classmethod
#make_synchronized
def the(cls):
if cls.instance is not None:
return cls.instance
return cls()
Points of note:
If you don't subclass from object in python2.x you will get an old-style class, which does not use __new__
When decorating __new__ you must decorate with #classmethod or __new__ will be an unbound instance method
This could possibly be improved by way of use of a metaclass, as this would allow you to make the a class-level property, possibly renaming it to instance

Method: override __new__ after single use
class Singleton():
def __init__(self):
Singleton.instance = self
Singleton.__new__ = lambda _: Singleton.instance
Pros
Extremely simple and concise
True class, no modules needed
Proper use of lambda and pythonic monkey patching
Cons
__new__ could be overridden again

I can't remember where I found this solution, but I find it to be the most 'elegant' from my non-Python-expert point of view:
class SomeSingleton(dict):
__instance__ = None
def __new__(cls, *args,**kwargs):
if SomeSingleton.__instance__ is None:
SomeSingleton.__instance__ = dict.__new__(cls)
return SomeSingleton.__instance__
def __init__(self):
pass
def some_func(self,arg):
pass
Why do I like this? No decorators, no meta classes, no multiple inheritance...and if you decide you don't want it to be a Singleton anymore, just delete the __new__ method. As I am new to Python (and OOP in general) I expect someone will set me straight about why this is a terrible approach?

Code based on Tolli's answer.
#decorator, modyfies new_cls
def _singleton(new_cls):
instance = new_cls() #2
def new(cls):
if isinstance(instance, cls): #4
return instance
else:
raise TypeError("I can only return instance of {}, caller wanted {}".format(new_cls, cls))
new_cls.__new__ = new #3
new_cls.__init__ = lambda self: None #5
return new_cls
#decorator, creates new class
def singleton(cls):
new_cls = type('singleton({})'.format(cls.__name__), (cls,), {} ) #1
return _singleton(new_cls)
#metaclass
def meta_singleton(name, bases, attrs):
new_cls = type(name, bases, attrs) #1
return _singleton(new_cls)
Explanation:
Create new class, inheriting from given cls
(it doesn't modify cls in case someone wants for example singleton(list))
Create instance. Before overriding __new__ it's so easy.
Now, when we have easily created instance, overrides __new__ using method defined moment ago.
The function returns instance only when it's what the caller expects, otherwise raises TypeError.
The condition is not met when someone attempts to inherit from decorated class.
If __new__() returns an instance of cls, then the new instance’s __init__() method will be invoked like __init__(self[, ...]), where self is the new instance and the remaining arguments are the same as were passed to __new__().
instance is already initialized, so function replaces __init__ with function doing nothing.
See it working online

If you don't need lazy initialization of the instance of the Singleton, then the following should be easy and thread-safe:
class A:
instance = None
# Methods and variables of the class/object A follow
A.instance = A()
This way A is a singleton initialized at module import.

After struggling with this for some time I eventually came up with the following, so that the config object would only be loaded once, when called up from separate modules. The metaclass allows a global class instance to be stored in the builtins dict, which at present appears to be the neatest way of storing a proper program global.
import builtins
# -----------------------------------------------------------------------------
# So..... you would expect that a class would be "global" in scope, however
# when different modules use this,
# EACH ONE effectively has its own class namespace.
# In order to get around this, we use a metaclass to intercept
# "new" and provide the "truly global metaclass instance" if it already exists
class MetaConfig(type):
def __new__(cls, name, bases, dct):
try:
class_inst = builtins.CONFIG_singleton
except AttributeError:
class_inst = super().__new__(cls, name, bases, dct)
builtins.CONFIG_singleton = class_inst
class_inst.do_load()
return class_inst
# -----------------------------------------------------------------------------
class Config(metaclass=MetaConfig):
config_attr = None
#classmethod
def do_load(cls):
...<load-cfg-from-file>...

You can use a metaclass if you want to use instance as a property. For example;
class SingletonMeta(type):
def __init__(cls, *args, **kwargs):
super().__init__(*args, **kwargs)
cls._instance = None
cls._locker = threading.Lock()
#property
def instance(self, *args, **kwargs):
if self._instance is None:
with self._locker:
if self._instance is None:
self._instance = self(*args, **kwargs)
return self._instance
class MyClass(metaclass=SingletonMeta):
def __init__(self):
# init here
pass
# get the instance
my_class_instance = MyClass.instance

python decorator to modify variable in current scope

Goal: Make a decorator which can modify the scope that it is used in.
If it worked:
class Blah(): # or perhaps class Blah(ParentClassWhichMakesThisPossible)
def one(self):
pass
#decorated
def two(self):
pass
>>> Blah.decorated
["two"]
Why? I essentially want to write classes which can maintain specific dictionaries of methods, so that I can retrieve lists of available methods of different types on a per class basis. errr.....
I want to do this:
class RuleClass(ParentClass):
#rule
def blah(self):
pass
#rule
def kapow(self):
pass
def shazam(self):
class OtherRuleClass(ParentClass):
#rule
def foo(self):
pass
def bar(self):
pass
>>> RuleClass.rules.keys()
["blah", "kapow"]
>>> OtherRuleClass.rules.keys()
["foo"]

You can do what you want with a class decorator (in Python 2.6) or a metaclass. The class decorator version:
def rule(f):
f.rule = True
return f
def getRules(cls):
cls.rules = {}
for attr, value in cls.__dict__.iteritems():
if getattr(value, 'rule', False):
cls.rules[attr] = value
return cls
#getRules
class RuleClass:
#rule
def foo(self):
pass
The metaclass version would be:
def rule(f):
f.rule = True
return f
class RuleType(type):
def __init__(self, name, bases, attrs):
self.rules = {}
for attr, value in attrs.iteritems():
if getattr(value, 'rule', False):
self.rules[attr] = value
super(RuleType, self).__init__(name, bases, attrs)
class RuleBase(object):
__metaclass__ = RuleType
class RuleClass(RuleBase):
#rule
def foo(self):
pass
Notice that neither of these do what you ask for (modify the calling namespace) because it's fragile, hard and often impossible. Instead they both post-process the class -- through the class decorator or the metaclass's __init__ method -- by inspecting all the attributes and filling the rules attribute. The difference between the two is that the metaclass solution works in Python 2.5 and earlier (down to 2.2), and that the metaclass is inherited. With the decorator, subclasses have to each apply the decorator individually (if they want to set the rules attribute.)
Both solutions do not take inheritance into account -- they don't look at the parent class when looking for methods marked as rules, nor do they look at the parent class rules attribute. It's not hard to extend either to do that, if that's what you want.

Problem is, at the time the decorated decorator is called, there is no object Blah yet: the class object is built after the class body finishes executing. Simplest is to have decorated stash the info "somewhere else", e.g. a function attribute, then a final pass (a class decorator or metaclass) reaps that info into the dictionary you desire.
Class decorators are simpler, but they don't get inherited (so they wouldn't come from a parent class), while metaclasses are inherited -- so if you insist on inheritance, a metaclass it will have to be. Simplest-first, with a class decorator and the "list" variant you have at the start of your Q rather than the "dict" variant you have later:
import inspect
def classdecorator(aclass):
decorated = []
for name, value in inspect.getmembers(aclass, inspect.ismethod):
if hasattr(value, '_decorated'):
decorated.append(name)
del value._decorated
aclass.decorated = decorated
return aclass
def decorated(afun):
afun._decorated = True
return afun
now,
#classdecorator
class Blah(object):
def one(self):
pass
#decorated
def two(self):
pass
gives you the Blah.decorated list you request in the first part of your Q. Building a dict instead, as you request in the second part of your Q, just means changing decorated.append(name) to decorated[name] = value in the code above, and of course initializing decorated in the class decorator to an empty dict rather than an empty list.
The metaclass variant would use the metaclass's __init__ to perform essentially the same post-processing after the class body is built -- a metaclass's __init__ gets a dict corresponding to the class body as its last argument (but you'll have to support inheritance yourself by appropriately dealing with any base class's analogous dict or list). So the metaclass approach is only "somewhat" more complex in practice than a class decorator, but conceptually it's felt to be much more difficult by most people. I'll give all the details for the metaclass if you need them, but I'd recommend sticking with the simpler class decorator if feasible.