Python Metaclasses

Metaclasses in python are way of supporting metaprogramming. Python has built-in metaclasses and also supports creating custom metaclasses. Understanding metaclasses in python helps to understand under the hood of classes and objects in python.

In python, to get the instance of or type of or class of an object, there are two ways → accessing __class__attribute of the object and calling type(obj)function. Both returns the same value in new-style classes. Old-style classes before 3 have different result for those though

So in python, we can refer to the object’s type and it’s class interchangeably. Everything in python is an object. If so, then a class should also be an object. Then what is the type if the class object or what is the parent class of class object? We will use type() to find the type of or class of the class object

class Foo:
	pass

x = Foo()

type(x) // <class '__main__.Foo'>
type(Foo) // <class 'type'>

As shown in the code above type(x) will print <class '__main__.Foo'>because x is of type Foo or x is an object of class Foo. But the type(Foo) prints <class 'type'> which say the Foo is of type type. Foo is an object of type class. Not only the user-defined classes, but all the built-in classes will also have the same result which is <class 'type'>. And again as mentioned, all classes are instances/objects of type class, then type class should also be an object of a class which is type itself. In the short, the class of or parent class of typeclass is type itself. And this type is called a metaclass. How classes are created with metaclass type? Using the type function. The type function when passed a single argument, it returns the type of the argument. The same function can also be used to create a class dynamically as below.

class Foo:
	pass

Bar = type('Bar', (Foo,), dict(attr=100)) 

The above code create a new class named Bar, inheriting from base class Foo

Let see what those argument are. The syntax is type(<name>, <base classes>, <attibutes>)

<name>: the name of the class to be created.

<base classes>: a tuple object of all classed to inherit from.

<attibutes>: a dictinary object of all the attribute to be added to the created class.

Instance(class and object of class) creation flow

Knowing the flow of instance creation helps understand metaclasses better.

Creating a class instance which is basically creating objects from classes can be depicted from the below diagram

The diagram is more self explantory. The flow is

• When a new object for a class is created, the __call__ method of the metaclass is called. In this case, the metaclass will be type
• The call method calls the __new__ method of the class. This is the method which creates an instance and returns it to the __call__ method
• Then the __init__method of the class is called with the created instance as the first parameter.
• Then the instance is returned from the __call__ method

Not lets look at the creation of instance of metaclass which is creating a class itself.

Let’s see the flow

• Initially, __prepare__ method is called which can return a dictionary that will be used as a local namespace in the created class.
• Meta-metaclass, which is in normal cases the type class __call__ method is invoked, which calls __new__ method of the metaclass. This is where the class object is created and returned.
• Then the __init__ method is invoked with created class a first parameter
• Once __init__ returns, the created class is returned