9/13/2023 0 Comments Itertools permutationsYou write class Foo(object) first, but the class object Foo is not created If you do so, Python will use the metaclass to create the class Foo. In Python 2, you can add a _metaclass_ attribute when you write a class (see next section for the Python 3 syntax): class Foo(object): Type is the built-in metaclass Python uses, but of course, you can create your You can call it a 'class factory' if you wish. So, a metaclass is just the stuff that creates class objects. Now, what is the _class_ of any _class_ ? > age._class_._class_ You see that by checking the _class_ attribute.Įverything, and I mean everything, is an object in Python. Just the class that creates class objects. Strings objects, and int the class that creates integer objects. Well, I guess it's a matter of consistency with str, the class that creates Now you wonder "why the heck is it written in lowercase, and not Type?" Metaclass Python uses to create all classes behind the scenes. It's because the function type is in fact a metaclass. attrs: dictionary containing attributes names and valuesĬan be created manually this way: > MyShin圜lass = type('MyShin圜lass', (), ).bases: tuple of the parent class (for inheritance, can be empty).Type works this way: type(name, bases, attrs) (I know, it's silly that the same function can have two completely different uses according to the parameters you pass to it. type can take the description of a class as parameters, Well, type has a completely different ability, it can also create classes on the fly. Remember the function type? The good old function that lets you know what With most things in Python, it gives you a way to do it manually. When you use the class keyword, Python creates this object automatically. Since classes are objects, they must be generated by something. > print(M圜lass()) # you can create an object from this classīut it's not so dynamic, since you still have to write the whole class yourself. > print(M圜lass) # the function returns a class, not an instance return Foo # return the class, not an instance Since classes are objects, you can create them on the fly, like any object.įirst, you can create a class in a function using class: > def choose_class(name): > print(ObjectCreatorMirror.new_attribute) > ObjectCreatorMirror = ObjectCreator # you can assign a class to a variable > ObjectCreator.new_attribute = 'foo' # you can add attributes to a class > print(hasattr(ObjectCreator, 'new_attribute')) > echo(ObjectCreator) # you can pass a class as a parameter you can pass it as a function parameterĮ.g.: > print(ObjectCreator) # you can print a class because it's an object.This object (the class) is itself capable of creating objects (the instances),īut still, it's an object, and therefore: The instruction > class ObjectCreator(object):Ĭreates in memory an object with the name ObjectCreator. Classes are objects too.Īs soon as you use the keyword class, Python executes it and createsĪn object. That's kinda true in Python too: > class ObjectCreator(object):īut classes are more than that in Python. In most languages, classes are just pieces of code that describe how to produce an object. And Python has a very peculiar idea of what classes are, borrowed from the Smalltalk language. Before understanding metaclasses, you need to master classes in Python.
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