Lintel Technologies: Python: __new__ magic method explained

Python is Object oriented language, every thing is an object in python. Python is having special type of  methods called magic methods named with preceded and trailing double underscores.

When we talk about magic method __new__ we also need to talk about __init__

These methods will be called when you instantiate(The process of creating instance from class is called instantiation). That is when you create instance. The magic method __new__ will be called when instance is being created. Using this method you can customize the instance creation. This is only the method which will be called first then __init__ will be called to initialize instance when you are creating instance.

Method __new__ will take class reference as the first argument followed by arguments which are passed to constructor(Arguments passed to call of class to create instance). Method __new__ is responsible to create instance, so you can use this method to customize object creation. Typically method __new__ will return the created instance object reference. Method __init__ will be called once __new__ method completed execution.

You can create new instance of the class by invoking the superclass’s __new__ method using super. Something like super(currentclass, cls).__new__(cls, [,….])

Usual class declaration and instantiation

class Foo(object):
    def __init__(self, a, b):
        self.a = a
        self.b = b

    def bar(self):
        pass
    
i = Foo(2, 3)

A class implementation with __new__ method overridden

class Foo(object):
    def __new__(cls, *args, **kwargs):
        print "Creating Instance"
        instance = super(Foo, cls).__new__(cls, *args, **kwargs)
        return instance

    def __init__(self, a, b):
        self.a = a
        self.b = b

    def bar(self):
        pass

OutPut:

>>> i = Foo(2, 3)
Creating Instance

Note:

You can create instance inside __new__  method either by using super function or by directly calling __new__ method over object  Where if parent class is object. That is,

instance = super(MyClass, cls).__new__(cls, *args, **kwargs)

or

instance = object.__new__(cls, *args, **kwargs)

Things to remember

If __new__ return instance of  it’s own class, then the __init__ method of newly created instance will be invoked with instance as first (like __init__(self, [, ….]) argument following by arguments passed to __new__ or call of class.  So, __init__ will called implicitly.

If __new__ method return something else other than instance of class,  then instances __init__ method will not be invoked. In this case you have to call __init__ method yourself.

Applications

Usually it’s uncommon to override __new__ method, but some times it is required if you are writing APIs or customizing class or instance creation or abstracting something using classes.

Singleton using __new__

You can implement the singleton design pattern using __new__ method. Where singleton class is a class that can only have one object. That is, instance of class.

Here is how you can restrict creating more than one instance by overriding __new__

class Singleton(object):
    _instance = None  # Keep instance reference 
    
    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = object.__name__(cls, *args, **kwargs)
        return cls._instance

It is not limited to singleton. You can also impose limit on total number created instances

class LimitedInstances(object):
    _instances = []  # Keep track of instance reference
    limit = 5 

    def __new__(cls, *args, **kwargs):
        if not len(cls._instances) <= cls.limit:
            raise RuntimeError, "Count not create instance. Limit %s reached" % cls.limit    
        instance = object.__name__(cls, *args, **kwargs)
        cls._instances.append(instance)
        return instance
    
    def __del__(self):
        # Remove instance from _instances 
        self._instance.remove(self)

Customize Instance Object

You can customize the instance created and make some operations over it before initializer __init__  being called.Also you can impose restriction on instance creation based on some constraints

ef createInstance():
    # Do what ever you want to determie if instance can be created
    return True 

class CustomizeInstance(object):

    def __new__(cls, a, b):
        if not createInstance():
            raise RuntimeError, "Count not create instance"
        instance = super(CustomizeInstance, cls).__new__(cls, a, b)
        instance.a = a
        return instance

    def __init__(self, a, b):
        pass

Customize Returned Object

Usually when you instantiate class it will return the instance of that class.You can customize this behaviour and you can return some random object you want.

Following  one is simple example to demonstrate that returning random object other than class instance

class AbstractClass(object):

    def __new__(cls, a, b):
        instance = super(AbstractClass, cls).__new__(cls)
        instance.__init__(a, b)
        return 3

    def __init__(self, a, b):
        print "Initializing Instance", a, b

Output:

>>> a = AbstractClass(2, 3)
Initializing Instance 2 3
>>> a 
3

Here you can see when we instantiate class it returns  3 instead of instance reference. Because we are returning 3 instead of created instance from __new__ method. We are calling __init__ explicitly.  As I mentioned above, we have to call __init__ explicitly if we are not returning instance object from __new__ method.

The __new__ method is also used in conjunction with meta classes to customize class creation

Conclusion

There are many possibilities on how you can use this feature.  Mostly it is not always required to override __new__ method unless you are doing something regarding instance creation.

Simplicity is better than complexity. Try to make life easier use this method only if it is necessary to use.

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