Metaclass programming in Python, Part 3: Cleverness considered harmful -- Or, how to do metaprogramming without metaclasses --

Recently, co-author Michele attended the EuroPython 2006 conference. The conference was good, the organization perfect, the talks of very high level, the people extremely nice. Nonetheless, he noticed something of a distrurbing trend in the Python community that prompted this paper. About simultaneously, co-author David, was reflecting on a similar issue with some submitted patches to Gnosis Utilities. The trend at issue is the trend towards cleverness. Unfortunately, whereas once cleverness in the Python community was once largely confined to Zope and Twisted, it now is appearing everywhere.

We have nothing against cleverness in experimental projects and learning exercises. Our gripe is with cleverness in production frameworks that we are forced to cope with as users. This article makes a small contribution against cleverness, at least in a case where we have some expertise, i.e. against metaclass abuses.

For this article, at least, we take a ruthless stance: we consider metaclass abuse any usage of a metaclass where you could have solved the same problem equally well without a custom metaclass. Of course, the guilt of the authors is obvious here: our earlier installments on metaclasses in Python helped popularize their usage. Nostra culpa.

One of the most common metaprogramming scenarios is the creation of classes with attributes and methods which are dynamically generated. Contrarily to popular belief, this is a job where most of the time you do not need and you do not want a custom metaclass; or so we argue in this installment.

The paper is intended for two sets of readers: average programmers who would benefit from knowing a few meta-programming tricks, but are scared off by brain melting concepts; clever programmers who are too clever and should know better. The problem for the latter is that it is easy to be clever, whereas it takes a lot of time to become unclever. For instance, it took us a few months to understand how to use metaclasses, but a few years to understand how not to use them.

About Class Initialization

During class creation attributes and methods of classes are set once and for all. Or rather, in Python, methods and attributes can be changed at nearly any point, but only if naughty programmers sacrifice transparency.

In several various common situations, a programmer may want to create classes in more dynamic ways than simply running static code for their creation. For instance, she may want to set some default class attributes according to parameters read from a configuration file; or she may want to set class properties according to the fields in a database table. The easiest way to dynamically customize class behavior uses an imperative style: first creates the class, then add methods and attributes. For example, an excellent programmer of our acquaintance, Anand Pillai has proposed a path to Gnosis Utilities' subpackage gnosis.xml.objectify that does exactly this. A base class called gnosis.xml.objectify._XO_ that is specialized (at runtime) to hold "xml node objects" is "decorated" with a number of enhanced behaviors, e.g.:

Dynamic enhancement of a base class

setattr(_XO_, 'orig_tagname', orig_tagname)
setattr(_XO_, 'findelem', findelem)
setattr(_XO_, 'XPath', XPath)
setattr(_XO_, 'change_pcdata', change_pcdata)
setattr(_XO_,'addChild',addChild)

One might suggest, reasonably enough, that the same enhancement can be accomplished simply by subclassing the XO base class. True, in one sense, but Anand has provided about two dozen possible enhancements, and particular users might want some of them, but not others. There are too many permutations to easily create subclasses for every enhancement scenario. Still, the above code is not exactly pretty. One could accomplish the above sort of job with a custom metaclass, attached to XO, but with behavior determined dynamically, but that brings us back to the excessive cleverness (and opacity) that we hope to avoid.

A clean, and non-ugly, solution to the above need might be to add class decorators to Python. If we had those, we might write code similar to:

features = [('XPath',XPath), ('addChild',addChild), ('is_root',is_root)]
@enhance(features)
class _XO_plus(gnosis.xml.objectify._XO_): pass
gnosis.xml.objectify._XO_ = _XO_plus

When Metaclasses Become Complicated

It might seem like all the fuss in this paper, so far is about nothing. Why not just, e.g. define the metaclass of XO as Enhance, and be done with it. Enhance.__init__() can happily add whatever capabilities are needed for the particular use in question. This might look like, e.g.:

A fragile metaclass

class _XO_plus(gnosis.xml.objectify._XO_):
      __metaclass__ = Enhance
      features = [('XPath',XPath), ('addChild',addChild)]
gnosis.xml.objectify._XO_ = _XO_plus

Unfortunately, things are not so simple once you start to worry about inheritance. Once you have defined a custom metaclass for your base class, all the derived classes will inherit the metaclass, so the initialization code will be run on all derived classes, magically and implicitly. This may be fine in specific circumstances (for instance, suppose you have to register in your framework all the classes you define: using a metaclass ensures that you cannot forget to register a derived class), however, in many cases you may not like this behavior because:

In other words,you should use a custom metaclass only when your real intention is to have code running on derived classes without users of those classes noticing it. If this is not your case, skip the metaclass and make your life (and that of your users) happier.

The Classinitializer Decorator

What we present in the rest of this paper might be accused of cleverness. But the cleverness need not burden users, just us authors. Readers can do something much akin to the hypothetical (non-ugly) class decorator we propose, but without enountering the inheritance and metaclass conflict issues the metaclass approach raises. The "deep magic" decorator we give in full later generally just enhances the straightforward (but slightly ugly) imperative approach, and is "morally equivalent" to:

Wrapping imperative enhancement

def Enhance(cls, **kw):
    for k, v in kw.iteritems():
        setattr(cls, k, v)
class ClassToBeInitialized(object):
    pass
Enhance(ClassToBeInitialized, a=1, b=2)

The above imperative enhancer is not so bad. But it has a few drawbacks: It make you repeat the class name; readability is suboptimal since class definition and class initialization are separated--for long class definitions you can miss the last line; it feels wrong to first define something and then immediately mutate it.

The classinitializer decorator provides a declarative solution. The decorator converts Enhance(cls,**kw) into a method that can be used in a class definition:

The magic decarator in basic operation

>>> @classinitializer # add magic to Enhance
... def Enhance(cls, **kw):
...     for k, v in kw.iteritems():
...         setattr(cls, k, v)
>>> class ClassToBeInitialized(object):
...     Enhance(a=1, b=2)
>>> ClassToBeInitialized.a
1
>>> ClassToBeInitialized.b
2

If you have used Zope interfaces, you may have seen examples of class initializers (zope.interface.implements). In fact, classinitializer is implemented by using a trick copied from zope.interface.advice, which credits Phillip J. Eby. The trick uses the ``__metaclass__`` hook, but it does not use a custom metaclass. ClassToBeInitialized keeps its original metaclass, i.e. the plain built-in metaclass type of new style classes:

>>> type(ClassToBeInitialized)
<type 'type'>

In principle, the trick also works for old style classes, and it would be easy to write an implementation keeping old style classes old style. However, since according to Guido "old style classes are morally deprecated", the current implementation converts old style classes into new style classes:

Promotion to newstyle

>>> class WasOldStyle:
...     Enhance(a=1, b=2)
>>> WasOldStyle.a, WasOldStyle.b
(1, 2)
>>> type(WasOldStyle)
<type 'type'>

One of the motivations for the classinitializer decorator, is to hide the plumbing, and to make mere mortals able to implements their own class initializers in an easy way, without knowing the details of how class creation works and the secrets of the _metaclass_ hook. The other motivation, is that even for Python wizards it is very inconvenient to rewrite the code managing the _metaclass_ hook every time one writes a new class initializer.

As a final note, let us point out that the decorated version of Enhance is smart enough to continue to work as a non-decorated version outside a class scope, provided that you pass to it an explicit class argument:

>>> Enhance(WasOldStyle, a=2)
>>> WasOldStyle.a
2

The (overly) Deep Magic

Here is the code for classinitializer. You do not need to understand it to use the decorator:

The 'classinitializer' decorator

import sys
def classinitializer(proc):
   # basic idea stolen from zope.interface.advice, P.J. Eby
   def newproc(*args, **kw):
       frame = sys._getframe(1)
       if '__module__' in frame.f_locals and not \
           '__module__' in frame.f_code.co_varnames: # we are in a class
           if '__metaclass__' in frame.f_locals:
               raise SyntaxError("Don't use two class initializers or\n"
                 "a class initializer together with a __metaclass__ hook")
           def makecls(name, bases, dic):
               try:
                   cls = type(name, bases, dic)
               except TypeError, e:
                   if "can't have only classic bases" in str(e):
                       cls = type(name, bases + (object,), dic)
                   else:  # other strange errs, e.g. __slots__ conflicts
                       raise
               proc(cls, *args, **kw)
               return cls
           frame.f_locals["__metaclass__"] = makecls
       else:
           proc(*args, **kw)
 newproc.__name__ = proc.__name__
 newproc.__module__ = proc.__module__
 newproc.__doc__ = proc.__doc__
 newproc.__dict__ = proc.__dict__
 return newproc

From the implementation it is clear how class initializers work: when you call a class initializer inside a class, your are actually defining a _metaclass_ hook that will be called by the class' metaclass (typically type). The metaclass will create the class (as a new style one) and will pass it to the class initializer procedure.

Tricky points and caveats

Since class initializers (re)define the _metaclass_ hook, they don't play well with classes that define a _metaclass_ hook explicitly (as opposed to implicitly inheriting one). If a _metaclass_ hook is defined after the class initializer, it silently overrides it.

Local metaclass creates a problem

>>> class C:
...     Enhance(a=1)
...     def __metaclass__(name, bases, dic):
...         cls = type(name, bases, dic)
...         print 'Enhance is silently ignored'
...         return cls
...
Enhance is silently ignored
>>> C.a
Traceback (most recent call last):
  ...
AttributeError: type object 'C' has no attribute 'a'

While unfortunate, there is no general solution to this issue; we simply document it. On the other hand, if you call a class initializer after the _metaclass_ hook, you will get an exception:

Local metaclass raises an error

>>> class C:
...     def __metaclass__(name, bases, dic):
...         cls = type(name, bases, dic)
...         print 'calling explicit __metaclass__'
...         return cls
...     Enhance(a=1)
...
Traceback (most recent call last):
   ...
SyntaxError: Don't use two class initializers or
a class initializer together with a __metaclass__ hook

Raising an error is preferable to silently overriding your explicit _metaclass_ hook. As a consequence, you will get an error if you try to use two class initializers at the same time, or if you call twice the same one:

Doubled enhancement creates a problem

>>> class C:
...     Enhance(a=1)
...     Enhance(b=2)
Traceback (most recent call last):
  ...
SyntaxError: Don't use two class initializers or
a class initializer together with a__metaclass__ hook

On the plus side, all issues for inherited _metaclass_ hooks and for custom metaclasses are handled:

Happy to enhance inherited metaclass

>>> class B: # a base class with a custom metaclass
...     class __metaclass__(type):
...         pass
>>> class C(B): # class with both custom metaclass AND class initializer
...     Enhance(a=1)
>>> C.a
1
>>> type(C)
<class '_main.__metaclass__'>

The class initializer does not disturb the metaclass of C, which is the one inherited by base B, and the inherited metaclass does not disturb the class initializer, which does its job just fine. You would have run into trouble, instead, if you tried to call Enhance directly in the base class.

Putting It Together

With all this machinery defined, customizing class initialization becomes rather straightforward, and elegant looking. It might be something as simple as:

Simplest form enhancement

class _XO_plus(gnosis.xml.objectify._XO_):
    Enhance(XPath=XPath, addChild=addChild, is_root=is_root)
gnosis.xml.objectify._XO_ = _XO_plus

This example still uses the "injection" which is somewhat superfluous to the general case; i.e. we put the enhanced class back into a specific name in the module namespace. It is necessary for the particular module, but will not be needed most of the time. In any case, the argument to to Enhance() need not be fixed in code as above, you can equally use Enhance(**feature_set) for something completely dynamic.

The other point to keep in mind is that your Enhance() function can do rather more than the simple version suggested above. The decorator is more than happy to tweak more sophisticated enhancement functions. For example, here is one that adds "records" to a class:

Variations on class enhancement

@classinitializer
def def_properties(cls, schema):
    """
    Add properties to cls, according to the schema, which is a list
    of pairs (fieldname, typecast). A typecast is a
    callable converting the field value into a Python type.
    The initializer saves the attribute names in a list cls.fields
    and the typecasts in a list cls.types. Instances of cls are expected
    to have private attributes with names determined by the field names.
    """
    cls.fields = []
    cls.types = []
    for name, typecast in schema:
        if hasattr(cls, name): # avoid accidental overriding
            raise AttributeError('You are overriding %s!' % name)
        def getter(self, name=name):
            return getattr(self, '_' + name)
        def setter(self, value, name=name, typecast=typecast):
            setattr(self, '_' + name, typecast(value))
        setattr(cls, name, property(getter, setter))
        cls.fields.append(name)
        cls.types.append(typecast)

The differing concerns of (a) what is enhanced; (b) how the magic works; and (c) what the basic class itself does are kept orthogonal:

Customizing a record class

>>> class Article(object):
...    # fields and types are dynamically set by the initializer
...    def_properties([('title', str), ('author', str), ('date', date)])
...    def __init__(self, values): # add error checking if you like
...        for field, cast, value in zip(self.fields, self.types, values):
...            setattr(self, '_' + field, cast(value))

>>> a=Article(['How to use class initializers', 'M. Simionato', '2006-07-10'])
>>> a.title
'How to use class initializers'
>>> a.author
'M. Simionato'
>>> a.date
datetime.date(2006, 7, 10)

Resources

Co-author David recently wrote a Charming Python installment about using decorators instead of metaclasses. Take a look at "Decorators make magic easy A look at the newest Python facility for meta-programming" at:

Downloadhttp://www.phyast.pitt.edu/~micheles/classinitializer.zip which contains the source code in this article, and a doctester utility. Run python doctester.py classinitializer.txt to extract the self-test code from the paper.

About The Author

Michele Simionato is a plain, ordinary, theoretical physicist who was driven to Python by a quantum fluctuation that could well have passed without consequences, had he not met David Mertz. Now he has been trapped in Python's gravitational field. He will let his readers judge the final outcome. You can contact Michele at [email protected], or you can read his web site (http://www.phyast.pitt.edu/~micheles/).

David Mertz almost enjoys problems because of the solutions they enable. David may be reached at [email protected]; his life pored over athttp://gnosis.cx/publish/. Check out David's book Text Processing in Python (http://gnosis.cx/TPiP/).

Metaclass Programming In Python, Part 3: Cleverness Considered Harmful

Or, how to do metaprogramming without metaclasses

Introduction