Metaclasses

Defined as “the class of a class”.

• Any class whose instances are themselves classes.
• type is a metaclass for instance; it creates classes!
• Used to construct classes. (always happens under hood)
• Can create dynamic classes with less lines using type.
• Think: instances are to classes as classes are to metaclasses.
• To create a metaclass, create a class that inherits from type

# in the most simple form
class Meta(type):
    pass

• Classes are normally created with the type metaclass
• We can create a class with a different metaclass:

# Python 2
class Final(object):
    __metaclass__ = Meta
# Python 3
class Final(metaclass=Meta):
    pass

__new__(): It’s a method which is called before __init__(). It creates the object and return it.

__init__(): This method just initialize the created object passed as parameter

Example:

class Meta(type):
    def __init__(cls, name, bases, dct):
        print "Creating class {} using Meta".format(name)
        super(Meta, cls).__init__(name, bases, dct)
class Foo(object):
    __metaclass__ = Meta
class FooBar(Foo):
    pass

First we create a new metaclass called Meta:

• On the construction of classes built using Meta, we print out that we are creating a class using Meta.

Next, create a class called Foo… built from Meta rather than type.

Finally, create a FooBar class from Foo.

• Notice how it too was built using Meta?

End of Python Subject