metaclass

Here is a python session. >>> class Z(type): def __new__(cls, name, bases, attrs): print cls print name return type(name, bases, attrs) ... >>> class Y(object): __metaclass__ = Z ... <class '__main__.Z'> Y >>> class X(Y): ... pass ... >>> class W(Y): ... __metaclass__ = Z ... <class '__main__.Z'> W >>> After I define class X I expect Z._new__ to be called for it, and to print the two line, which is not happening, (as metaclass are inherited?) The problem is that the cls argument (which is the metaclass object) is not passed on when you call type , therefore the class object Y that is created

I tried to subclass threading.Condition earlier today but it didn't work out. Here is the output of the Python interpreter when I try to subclass the threading.Condition class: >>> import threading >>> class ThisWontWork(threading.Condition): ... pass ... Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: Error when calling the metaclass bases function() argument 1 must be code, not str Can someone explain this error? Thanks! You're getting that exception because, despite its class-like name, threading.Condition is a function, and you cannot subclass functions. >

I've been messing around with python's enum library and have come across a conundrum. In the docs, they show an example of an auto-numbering enum , wherein something is defined: class Color(AutoNumber): red = () green = () ... I want to make a similar class, but the value would automatically be set from the name of the member AND keep the functionality that you get from doing the str and enum mixin stuff So something like: class Animal(MagicStrEnum): horse = () dog = () Animal.dog == 'dog' # True I've looked at the source code of the enum module and tried a lot of variations messing around

I need to create a class that uses a different base class depending on some condition. With some classes I get the infamous: TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases One example is sqlite3 , here is a short example you can even use in the interpreter: >>> import sqlite3 >>> x = type('x', (sqlite3,), {}) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

Is it possible to chain metaclasses? I have class Model which uses __metaclass__=ModelBase to process its namespace dict. I'm going to inherit from it and "bind" another metaclass so it won't shade the original one. First approach is to subclass class MyModelBase(ModelBase) : MyModel(Model): __metaclass__ = MyModelBase # inherits from `ModelBase` But is it possible just to chain them like mixins, without explicit subclassing? Something like class MyModel(Model): __metaclass__ = (MyMixin, super(Model).__metaclass__) ... or even better: create a MixIn that will use __metaclass__ from the direct

This is a follow-up question for unittest and metaclass: automatic test_* method generation : For this (fixed) unittest.TestCase layout: #!/usr/bin/env python import unittest class TestMaker(type): def __new__(cls, name, bases, attrs): callables = dict([ (meth_name, meth) for (meth_name, meth) in attrs.items() if meth_name.startswith('_test') ]) for meth_name, meth in callables.items(): assert callable(meth) _, _, testname = meth_name.partition('_test') # inject methods: test{testname}_v4,6(self) for suffix, arg in (('_false', False), ('_true', True)): testable_name = 'test{0}{1}'.format

I want to be able to create a python decorator that automatically "registers" class methods in a global repository (with some properties). Example code: class my_class(object): @register(prop1,prop2) def my_method( arg1,arg2 ): # method code here... @register(prop3,prop4) def my_other_method( arg1,arg2 ): # method code here... I want that when loading is done, somewhere there will be a dict containing: { "my_class.my_method" : ( prop1, prop2 ) "my_class.my_other_method" : ( prop3, prop4 ) } Is this possible? Not with just a decorator, no. But a metaclass can automatically work with a class