Difference between __init__() and __new__() method in Python

Background

In Pyhton, there are some built-in functions when creating a class that looks like __func_name__. There are two of them, __new__ and __init__. I found it’s very interesting that we seldomly use __new__ but mostly just use __init__. But actually they are bit different and are all useful for different senarios.

Most object-orented programming languages such as Java, C#, etc have the concept of a constructor - a method that creates and initialise the instance that created by the class. However, Python is different, it splits these into two different methods, the constructor is __new__ while the initializer is __init__. But why we usually only use __init__ but not __new__? Because the __new__ is kind like hidden but actually already automatially excuted when we create a new instance. Let’s start with a simple example:

Example 1:

class A:
  def __init__(self):
    print("The init method has been excuted")

a = A()

Output:

The init method has been excuted

Ok, definately the init method has been excuted. Now let’s add a __new__ method that similar to __init__:

class A:
  def __new__(self):
    print("The new method has been excuted")

  def __init__(self):
    print("The init method has been excuted")

a = A()

Output:

The new method has been excuted

Now we can see the __new__ method kind like block the __init__. That’s because what the __new__ does is return a object(self) and pass to __init__. Also, we need to keep in mind that, __new__ is a cls method that should pass cls instead of self. Although this is not that essential in Python but highly recommended to use the right word.

Let’s see what actually the __new__ method does that inherit from base class or Object from Python 3. (Although we didn’t say anything when define a class in Python 3, but class A: = class A(object):, or we can say all the class inherit from class object).

Example 2:

class A:
  def __new__(cls):
    print("The new method has been excuted")
    return object.__new__(cls)

  def __init__(self):
    print("The init method has been excuted")

a = A()

Output:

The new method has been excuted

The init method has been excuted

Cool now we can see both methods have run. This is because a self instance was returned/created by __new__ method and then passed to the __init__. This is how the Python actually create a object from a class. We usually don’t write the __new__ method but can directly initialize an object using __init__ is because it will inherit the new method from base(Object).

Let’s see another example that init a instance with params:

Example 3:

class A:
  def __new__(cls, *args, **kwargs):
    return object.__new__(cls)

  def __init__(self, x, y, z):
      self.x = x
      self.y = y
      self.z = z
    

a = A(x=1, y=2, z=3)
print(a.x)
print(a.y)
print(a.z)

Output

1

2

3

When to use __new__

Now the question is when shall we define a __new__ method ourselves but not using the default one? Acutally it all depends. As we can see above, if we implement a __new__ without any return value, it will not reture a self so that all the method below that used for instances will not be available anymore.

Another very useful senario is when we just allow the class to create only one instance or Singleton in OO Design Patterns

Example 4:

Let’s first see a class without __new__:

class Singleton:
    ...

s1 = Singleton()
s2 = Singleton()
print(s1)
print(s2) 

Output

<__main__.Singleton object at 0x7f86b0a93370>

<__main__.Singleton object at 0x7f86b0a935e0>

We can see the two instance s1 and s2 are point to two different memory address.

Now we add a refined __new__ method:

class Singleton:
    _instance = None
    def __new__(cls, *args, **kwargs):
        if cls._instance is None:
            cls._instance = object.__new__(cls)

        return cls._instance

s1 = Singleton()
s2 = Singleton()
print(s1)
print(s2) 

Output

<__main__.Singleton object at 0x7f870061d4c0>

<__main__.Singleton object at 0x7f870061d4c0>

Now we see we can only create one instance from the class.