Python metaclasses vs class decorators

Question

What are the main differences between Python metaclasses and class decorators? Is there something I can do with one but not with the other?

Asked By: gpilotino

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Answer 1

Decorators are much, much simpler and more limited — and therefore should be preferred whenever the desired effect can be achieved with either a metaclass or a class decorator.

Anything you can do with a class decorator, you can of course do with a custom metaclass (just apply the functionality of the “decorator function”, i.e., the one that takes a class object and modifies it, in the course of the metaclass’s __new__ or __init__ that make the class object!-).

There are many things you can do in a custom metaclass but not in a decorator (unless the decorator internally generates and applies a custom metaclass, of course — but that’s cheating;-)… and even then, in Python 3, there are things you can only do with a custom metaclass, not after the fact… but that’s a pretty advanced sub-niche of your question, so let me give simpler examples).

For example, suppose you want to make a class object X such that print X (or in Python 3 print(X) of course;-) displays peekaboo!. You cannot possibly do that without a custom metaclass, because the metaclass’s override of __str__ is the crucial actor here, i.e., you need a def __str__(cls): return "peekaboo!" in the custom metaclass of class X.

The same applies to all magic methods, i.e., to all kinds of operations as applied to the class object itself (as opposed to, ones applied to its instances, which use magic methods as defined in the class — operations on the class object itself use magic methods as defined in the metaclass).

Answered By: Alex Martelli

Answer 2

As given in the chapter 21 of the book ‘fluent python’, one difference is related to inheritance. Please see these two scripts. The python version is 3.5. One point is that the use of metaclass affects its children while the decorator affects only the current class.

The script use class-decorator to replace/overwirte the method ‘func1’.

def deco4cls(cls):
    cls.func1 = lambda self: 2
    return cls


@deco4cls
class Cls1:
    pass


class Cls1_1(Cls1):
    def func1(self):
        return 3


obj1_1 = Cls1_1()
print(obj1_1.func1())  # 3

The script use metaclass to replace/overwrite the method ‘func1’.

class Deco4cls(type):
    def __init__(cls, name, bases, attr_dict):
        # print(cls, name, bases, attr_dict)
        super().__init__(name, bases, attr_dict)
        cls.func1 = lambda self: 2


class Cls2(metaclass=Deco4cls):
    pass


class Cls2_1(Cls2):
    def func1(self):
        return 3


obj2_1 = Cls2_1()
print(obj2_1.func1())  # 2!! the original Cls2_1.func1 is replaced by metaclass

Answered By: Hobin C.

Python metaclasses vs class decorators

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