The way I learn things, I can read about something a number of times, and intellectually understand it, but it won’t really sink in until I have a real reason to try it out myself. Toy examples don’t do it for me, I have to have an actual problem in hand before the solution becomes part of my repertoire. Recently I finally had a use for metaclasses.

I wanted to create an in-memory list of items that I could reference by key. It was a micro-database of languages:

class Language(object):

    # The class attribute of all languages, mapped by id.    
    _db = {}
    
    def __init__(self, **kwargs):
        for k, v in kwargs.iteritems():
            setattr(self, k, v)
        self._db[self.id] = self
        
    @classmethod
    def get(cls, key):
        return cls._db.get(key)

Language(
    id = 'en',
    name = _('English'),
    native = u'English',
    )

Language(
    id = 'fr',
    name = _('French'),
    native = u'Fran\u00E7ais',
    )

Language(
    id = 'nl',
    name = _('Dutch'),
    native = u'Nederlands',
    )

# Some time later:
lang = Language.get(langcode)
lang.native # blah blah

This worked well, it gave me a simple schema-less set of constant items that I could look up by id. And the class attribute _db is used implicitly in the constructor, so I get a clean declarative syntax for building my list of languages.

But then I wanted another another set, for countries, so I made a MiniDbItem class to derive both Language and Country from:

class MiniDbItem(object):
    def __init__(self, **kwargs):
        for k, v in kwargs.iteritems():
            setattr(self, k, v)
        self._db[self.id] = self
        
    @classmethod
    def get(cls, key):
        return cls._db.get(key)

class Language(MiniDbItem):
    _db = {}

Language(id='en', ...)
Lanugage(id='fr', ...)

class Country(MiniDbItem):
    _db = {}
    
Country(id='US', ...)
Country(id='FR', ...)

This works, but the unfortunate part is that each derived class has to define it’s own _db class attribute to keep the Languages separate from the Countries. Each derived class is obligated to do that little bit of redundant work, or the MiniDbItem base class isn’t used properly.

The way to avoid that is to use a metaclass. The metaclass provides an __init__ method. In a class, __init__ is called when new class instances are created, but in a metaclass, __init__ is called when new classes are created.

class MetaMiniDbItem(type):
    """ A metaclass to give every class derived from MiniDbItem
        a _db attribute.
    """
    def __init__(cls, name, bases, dict):
        super(MetaMiniDbItem, cls).__init__(name, bases, dict)
        # Each class has its own _db, a dict of its items
        cls._db = {}

class MiniDbItem(object):
    
    __metaclass__ = MetaMiniDbItem

    def __init__(self, **kwargs):
        for k, v in kwargs.iteritems():
            setattr(self, k, v)
        self._db[self.id] = self
        
    @classmethod
    def get(cls, key):
        return cls._db.get(key)

class Language(MiniDbItem): pass

Language(id='en', ...)
Lanugage(id='fr', ...)

class Country(MiniDbItem): pass
    
Country(id='US', ...)
Country(id='FR', ...)

Now MetaMiniDbItem.__init__ is invoked twice: once when class Language is defined, and again when class Country is defined. The class being constructed is passed in as the cls parameter. We use super to invoke the regular class creation machinery, then simply set the _db attribute on the class like we want.

Of course, metaclasses can be used to do many more things than simply setting a class attribute, but this example was the first time in my work that metaclasses seemed like a natural solution to a problem rather than an advanced-magic Stupid Python Trick.