Python's object model

General Definition Of Object

In computer science, an object is a location in memory having a value and possibly referenced by an identifier. An object can be a variable, a data structure, or a function. In the class-based object-oriented programming paradigm, object refers to a particular instance of a class where the object can be a combination of variables, functions, and data structures.

The Class type Is The Base-Metaclass

• A metaclass is an object able to create classes
• The class type is a metaclass, and it's the default one:

        
        >>> class MyClass: # By default metaclass=type
        ...     a = 33
        ... 
        >>> class MyClass(metaclass=type): # Explicit definition
        ...     a = 33
        ... 
        >>> MyClass = type('MyClass', (), {'a': 33}) # No __prepare__()
        
        

User-Defined Metaclasses

• It is easy to create a metaclass: we can just define a subclass of type

        
        >>> class MyMetaclass(type): pass
        ... 
        >>> # To create a class, we can directly call MyMetaclass
        >>> MyClass = MyMetaclass('MyClass', (), {})
        >>> 
        >>> # We can also indicate MyMetaclass in the class definition
        >>> class MyClass(metaclass=MyMetaclass): pass
        ... 
        >>> type(MyClass) is MyMetaclass
        True
        
        

An Example Of Metaclass

        
        >>> class SingletonMeta(type):
        ...     def __call__(cls, *args, **kwargs):
        ...         if not hasattr(cls, '_instance'):
        ...             cls._instance = super().__call__(*args, **kwargs)
        ...         return cls._instance
        ... 
        >>> class MyClass(metaclass=SingletonMeta): pass
        ... 
        >>> c1 = MyClass()
        >>> c2 = MyClass()
        >>> c1 is c2
        True
        
        

The built-in isinstance() function

            
            >>> class ArtMeta(type): pass
            ... 
            >>> class Painting(metaclass=ArtMeta): pass
            ... 
            >>> class Fresco(Painting): pass
            ... 
            >>> adorazione_magi = Fresco()
            >>> isinstance(adorazione_magi, Fresco)
            True
            >>> isinstance(adorazione_magi, Painting)
            True
            >>> isinstance(adorazione_magi, ArtMeta)
            False
            
        

Every Class Is A Subclass Of object

            
            >>> issubclass(int, object)
            True
            >>> issubclass(list, object)
            True
            >>> class MyClass: pass
            ... 
            >>> issubclass(MyClass, object)
            True
            >>> issubclass(type, object)
            True
            >>> issubclass(object, object)
            True
            
        

Every Class Is A Subclass Of object

The Class type Is The Default Metaclass

            
            >>> type(int) is type
            True
            >>> type(list) is type
            True
            >>> type(object) is type
            True
            >>> type(type) is type
            True
            >>> import sys
            >>> module_type = type(sys)
            >>> type(module_type) is type
            True
            
        

The Class type Is The Default Metaclass

The Class type Is Subclass Of Itself

In Python, Everything Is An Object

            
            >>> import sys
            >>> isinstance(sys, object) # Modules are objects
            True
            >>> isinstance(abs, object) # Functions are objects
            True
            >>> class MyClass: pass
            ... 
            >>> isinstance(MyClass, object) # Classes are objects
            True
            >>> isinstance(object, object)
            True
            
        

Python's Object Model Diagram

Object Is Synonymous for Instance

            
            >>> class MyClass:
            ...     pass
            ... 
            >>> MyClass.__name__
            'MyClass'
            >>> MyClass().__name__
            Traceback (most recent call last):
                ...
            AttributeError: 'MyClass' object has no attribute '__name__'
            
        

How come MyClass has the attribute __name__ but MyClass() has not?

From Metaclass To Non-Class Objects

            
            >>> '__name__' in type.__dict__
            True
            >>> '__name__' in MyClass.__dict__
            False
            >>> hasattr(MyClass, '__name__') # The right way 
            True
            >>> isinstance(MyClass, type)
            True
            >>> isinstance(MyClass(), type)
            False
            
        

Classes and Non-Classes

• The difference between classes and non-classes matters: the attribute lookup for an object obj depends on whether obj is a class or not

        
        # If obj is not a class:
        obj.foo # -> Is type(obj).__dict__['foo'] a descriptor?
         
        # If obj is a class
        obj.foo # -> Is obj.__dict__['foo'] a descriptor?
        
        

Summary

• In Python, everything is an object
• The metaclasses are objects that create classes
• Instance is synonymous for object, and classes are instances: it is a nonsense to differentiate between classes and objects
• The difference between classes and non-classes matters
• If isinstance(obj, type) is true, then obj is a class
• If issubclass(obj, type) is true, then obj is a metaclass