Question

class MType(type):
 pass

class MClass(object):
 __metaclass__ = MType

a = MType

From the above snippet is there anyway I can create an instance of MClass given a?

(It’s probably important to note that I won’t be able to simply contruct MClass())

Answer 1

There are two straightforward ways I can think of to find the instances of a metaclass. One is to have the metaclass keep track of a list of its instances as they are created, as follows:

class MType(type):
    instances = []    #list of instances of the metaclass

    def __init__(cls, name, bases, dct):
            MType.instances.append(cls)    #append to list of instances
            super(MType, cls).__init__(name, bases, dct)

class MClass(object):
    __metaclass__ = MType

Now MType.instances is [<class '__main__.MClass'>].

Another way is to use the
inspect module to look through the classes in, say, a given module, and use isinstance to check if they are instances of MType. This isn’t necessarily a crazy thing to do, either – for example, various unit testing suites work by looking through a module for classes and functions that appear to be tests, based on their names and/or inherited classes – I’m not really sure why you would want to do this with a metaclass, though.