How are Python metaclasses different from regular class inheritance?

Question

This might be too much of an open ended question, but I'm just now learning about metaclasses in Python and I don't understand how a metaclass is any different than just having a child class inherit from the parent class, like

class child(parent):

Wouldn't this serve the same purpose of a metaclass? I guess maybe I don't understand the purpose of a metaclass.

How is this a duplicate? I'm not asking 'what is a metaclass', and I have read that thread a couple times. — user2897013, Oct 19 '13 at 07:41
I am specifically asking what is the difference between a metaclass and a child class inheriting from parent class, as stated in my question. — user2897013, Oct 19 '13 at 08:07
What you are asking is a subset of what is asked in that question, and if you read and understand the answer to that question, you'll understand the difference you're asking about. — BrenBarn, Oct 19 '13 at 08:12

score 12 · Answer 1 · answered Oct 19 '13 at 08:12

The difference is that inheriting from a class does not affect how the class is created, it only affects how instances of the class are created. If you do:

class A(object):
    # stuff

class B(A):
    # stuff

then A does not have any opportunity to "hook in" when B is created. Methods of A may be called when an instance of B is created, but not when the class B itself is created.

Metaclasses allow you to define custom behavior for when a class is created. Refer to the question I marked as duplicate for examples of how metaclasses work, and convince yourself that there are effects in those examples that you can't achieve with normal inheritance.

Try looking on youtube for a video tutorial, that may be more helpful. — david, Oct 19 '13 at 08:17

score 4 · Accepted Answer · edited Oct 19 '13 at 08:29

class AccessorType(type):
    def __init__(self, name, bases, d):
        type.__init__(self, name, bases, d)
        accessors = {}
        prefixs = ["get_", "set_", "del_"]

        for k in d.keys():
            v = getattr(self, k)
            for i in range(3):
                if k.startswith(prefixs[i]):
                    accessors.setdefault(k[4:], [None, None, None])[i] = v

        for name, (getter, setter, deler) in accessors.items():
            # create default behaviours for the property - if we leave
            # the getter as None we won't be able to getattr, etc..
            # [...] some code that implements the above comment
            setattr(self, name, property(getter, setter, deler, ""))

How are Python metaclasses different from regular class inheritance?

2 Answers2