Calling a class method upon creation of Python classes

Question

I'd like to automatically run some code upon class creation that can call other class methods. I have not found a way of doing so from within the class declaration itself and end up creating a @classmethod called __clsinit__ and call it from the defining scope immediately after the class declaration. Is there a method I can define such that it will get automatically called after the class object is created?

Leralerch

Class creation or instance creation? Because both can be done, but the answer is different depending. – Meaning

You can use metaclasses for this. See for instance this question. – Degression

Aside: Please don't make up __special_methods__. That's reserved for the Python language. Use a _single_underscore_prefix if you want it to be private. – Onaonager

Answer 1

You can do this with a metaclass or a class decorator.

A class decorator (since 2.6) is probably easier to understand:

def call_clsinit(cls):
    cls._clsinit()
    return cls

@call_clsinit
class MyClass:
    @classmethod
    def _clsinit(cls):
        print "MyClass._clsinit()"

Metaclasses are more powerful; they can call code and modify the ingredients of the class before it is created as well as afterwards (also, they can be inherited):

def call_clsinit(*args, **kwargs):
    cls = type(*args, **kwargs)
    cls._clsinit()
    return cls;

class MyClass(object):
    __metaclass__ = call_clsinit

    @classmethod
    def _clsinit(cls):
        print "MyClass._clsinit()"

Rosin

I'd add that at least from the text of the question, it's hard to tell why __new__ or __init__ don't do the trick. – Meaning

Answer 2

Since Python 3.6 there is __init_subclass__ as defined in PEP 487.

Judaist