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Metaclasses in Python 3.0 [1 of 2]
by Michele Simionato
August 9, 2008
This is the English translation of an article I wrote some time ago for Stacktrace: For convenience, I have split it in two posts.


I heard for the first time the word "metaclass" in 2002, when I begun studying Python. I was very active in the Python newsgroup then, and I received from David Mertz an invitation to write a joint paper about metaclasses. That was my very first paper outside Physics and since then I have had a particular affection for the subject. At the time the topic was hot since the metaclass mechanism had been just reformed (that was around the time Python 2.2 came along). Nowadays the subject is still hot since with Python 3.0 coming in October we will have another reform of the metaclass mechanism. The reform is for the better. I could not miss the occasion to write something about it.

The sweeet changes

Here I assume my readers to be familiar with metaclass programming in Python, at least as it is intended from Python 2.2 to Python 2.6. If you need to refresh your memory, my advice is to give a look to the metaclass trilogy David and I wrote for IBM DeveloperWorks, Metaclass programming in Python, parts 1, 2 e 3 .

A metaclass is nothing else than a subclass of the builtin metaclass type:

>>> class DoNothingMeta(type): # a silly metaclass
...       pass

A metaclass can be instantiated by specifying a string (the name), a tuple of classes (the bases) and a dictionary (the dic):

>>> C = DoNothingMeta('C', (object,), {})

The metaclass instance is a class named C, with parent object and an empty dictionary. In Python 3.0 everything remains the same: the only change is in the syntactic sugar. In Python 2.2+ you have at your disposition the so-called __metaclass__ hook and that the previous line can be written also as

>>> class C(object): __metaclass__ = DoNothingMeta

In Python 3.0+ instead, you can use a somewhat nicer syntax:

>>> class C(object, metaclass=DoNothingMeta): pass

The important thing to notice is that in Python 3.0 you can write

>>> class C(object): __metaclass__ = DoNothingMeta

but the hook is not recognized: Python 3.0 will just add a __metaclass__ attribute to your class, but the attribute will not be treated in any special way and in particular your class will not be magically converted in an instance of the metaclass: the class of C will stay type and not DoNothingMeta:

>>> type(C) is DoNothingMeta

This is potentially risky when porting code from Python 2.X to Python 3.0. I guess the 2to3 tool will give some warning, but I have not tried it yet. If anybody who knows read this post, feel free to comment.

The hot changes

The syntactic changes, however useful, are still cosmetics. The substantial change between Python 2.2 and Python 3.0 is semantic: in Python 3.0 the dictionary passed to the metaclass can be replaced with a generic object with a __setitem__ method. In particular, you can pass to a metaclass an ordered dictionary instead of a ordinary dictionary. That means that it is possible to keep the order of the declarations in the class body, a much desired feature for metaclass practictioners. For instance, suppose we want to define a Book class with fields title and author (in that order). In Python 2.2 we would be forced to repeat the names of the fields:

class BookPython22(object):

    title = 'varchar(128)'
    author = 'varchar(64)'

    order = ['title', 'author']

The order list is redundant and annoying, but you cannot get rid of it in traditional Python, otherwise you would miss the relative ordering of title and author. In Python 3.0 instead, the order list can be avoided, provided we use an ordered dict. At the moment I am writing this is not clear if an ordered dict class will enter in the standard library of Python 3.0, therefore I will give a simple implementation here:

from collections import Mapping

class odict(Mapping):
   "A simple implementation of ordered dicts without __delitem__ functionality"

   def __init__(self, alist=None):
       self._inner = {} # inner dictionary
       self._keys = [] # the dictionary keys in order
       if alist:
           for k, v in alist:
               self[k] = v

   def __getitem__(self, key):
       return self._inner[key]

   def __setitem__(self, key, value):
       self._inner[key] = value

   def __iter__(self):
       return iter(self._keys)

   def __len__(self):
       return len(self._keys)

   def __repr__(self):
       return '{%s}' % list(self.items())

I am deriving here from the Mapping Abstract Base Class provided by the collections module, which takes the place of the traditional UserDict.DictMixin class. Talking about the ABCs would take too long and I will leave that for a future post.

The relevant thing to say for what concerns dictionaries is that the dictionary interface has changed in Python 3.0: the methods .iterkeys(), .itervalues() and .iteritems() were removed and the methods .keys(), .values() and .items() now return views instead of lists. Again, I have no time to discuss views right now, but you can look at the relevant PEP.

We still need to tell the metaclass to use an odict instead of dict. To this aim, Python 3.0 recognizes a special (class)method __prepare__(mcl, name, bases) returning the would be class dictionary. Therefore it is enough to add a suitable __prepare__ to the metaclass:

class MetaBookExample(type):
   def __prepare__(mcl, name, bases):
       return odict()
   def __new__(mcl, name, bases, odic):
       cls = super().__new__(mcl, name, bases, dict(odic))
       cls.odic = odic
       return cls

The tricky point here is that the __new__ method must convert the class "dictionary" returned by __prepare__ into a regular dictionary (this explains the dict(odic) expression). If your __prepare__ method does not return an instance of dict and you do not override __new__ properly, you will get an error. For instance in this example if you do not override __new__ you will get a TypeError: type() argument 3 must be dict, not odict when you try to instantiate the metaclass. If you do things correctly, instead,

class BookExample(metaclass=MetaBookExample):
   "An example"
   title = 'Varchar(128)'
   author = 'Varchar(64)'

you may verify that the order of the class attributes is preserved:

>>> BookExample.odic # this is an odict instance
{[('__module__', '__main__'), ('__doc__', 'An example'), ('title', 'Varchar(128)'), ('author', 'Varchar(64)')]}

We see here that the metaclass is also passing a couple of implicit magic attributes __module__ and __doc__ to the class. This is the usual behavior even in Python 2.2+ so it should not come as a surprise to you.

Keeping the order of class attributes can be useful for the builders of Object Relation Mappers (SQLAlchemy anyone?), since it becomes possible to match the order of the columns in the database with the order of the column attributes at the Python level. In the second part of this article I will give a pedagogical example of what you can do with Python 3.0 metaclasses, by implementing a clever (not necessarily in a good sense) record system. Stay tuned!

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About the Blogger

Michele Simionato started his career as a Theoretical Physicist, working in Italy, France and the U.S. He turned to programming in 2003; since then he has been working professionally as a Python developer and now he lives in Milan, Italy. Michele is well known in the Python community for his posts in the newsgroup(s), his articles and his Open Source libraries and recipes. His interests include object oriented programming, functional programming, and in general programming metodologies that enable us to manage the complexity of modern software developement.

This weblog entry is Copyright © 2008 Michele Simionato. All rights reserved.

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