[Python-3000] PEP for Metaclasses in Python 3000

[Talin]

I had a conversation with Guido last night at the Python user's group 
meeting, and we hashed out some of the details of how metaclasses should 
work. I've gone ahead and written up a PEP, which I present for your review.
--------------------------------------------
PEP: xxx
Title: Metaclasses in Python 3000
Version: $Revision$
Last-Modified: $Date$
Author: Talin <talin at acm.org>
Status: Draft
Type: Standards
Content-Type: text/plain
Created: 07-Mar-2007
Python-Version: 3.0
Post-History:

Abstract

     This PEP proposes changing the syntax for declaring metaclasses,
     and alters the semantics for how classes with metaclasses are
     constructed.


Rationale

     There are two rationales for this PEP, both of which are somewhat
     subtle.

     The primary reason for changing the way metaclasses work, is that
     there are a number of interesting use cases that require the
     metaclass to get involved earlier in the class construction process
     than is currently possible. Currently, the metaclass mechanism is
     essentially a post-processing step. With the advent of class
     decorators, much of these post-processing chores can be taken over
     by the decorator mechanism.

     In particular, there is an important body of use cases where it
     would be useful to preserve the order in which a class members are
     declared. Ordinary Python objects store their members in a
     dictionary, in which ordering is unimportant, and members are
     accessed strictly by name. However, Python is often used to
     interface with external systems in which the members are organized
     according to an implicit ordering. Examples include declaration of C
     structs; COM objects; Automatic translation of Python classes into
     IDL or database schemas, such as used in an ORM; and so on.

     In such cases, it would be useful for a Python programmer to specify
     such ordering directly using the declaration order of class members.
     Currently, such orderings must be specified explicitly, using some
     other mechanism (see the ctypes module for an example.)

     Unfortunately, the current method for declaring a metaclass does
     not allow for this, since the ordering information has already been
     lost by the time the metaclass comes into play. By allowing the
     metaclass to get involved in the class construction process earlier,
     the new system allows the ordering or other early artifacts of
     construction to be preserved and examined.

     The other, weaker, rationale is purely cosmetic: The current method
     for specifying a metaclass is by assignment to the special variable
     __metaclass__, which is considered by some to be aesthetically less
     than ideal. Others disagree strongly with that opinion. This PEP
     will not address this issue, other than to note it, since aesthetic
     debates cannot be resolved via logically proofs.


Specification

     In the new model, the syntax for specifying a metaclass is via a
     keyword argument in the list of base classes:

       class Foo(base1, base2, metaclass=mymeta):
         ...

     Additional keywords will also be allowed here, and will be passed to
     the metaclass, as in the following example:

       class Foo(base1, base2, metaclass=mymeta, private=True):
         ...

     Note that this PEP makes no attempt to define what these other
     keywords might be - that is up to metaclass implementors to
     determine.

Invoking the Metaclass

     In the current metaclass system, the metaclass object can be any
     callable type. This does not change, however in order to fully
     exploit all of the new features, the metaclass will need to have an
     extra attribute which is used during class pre-construction.

     This attribute is a method named __metacreate__, which is invoked
     before the evaluation of the class body, and which has the
     following form:

      classdict = metaclass.__metacreate__(name, bases, keywords)

     Where:

       'name' is the name of the class being created.
       'bases' is the list of base classes.
       'keywords' is the dictionary of keywords in the base class list.
       'classdict' is a custom dictionary object which is created by the
           metaclass, and which is used to store the class members as
           they are declared.

     Note that the Python interpreter will check to insure that the
     __metacreate__ attribute exists before calling it. This preserves
     backwards compatibility with existing metaclasses.

     The 'classdict' object can be a regular dictionary or a custom
     mapping type. It does not need to implement the full dictionary
     interface; only the ability to insert items and retrieve them are
     required. (Note: double check that this is true). When the body of
     the class is evaluated, the dictionary will be used as the
     'locals()' dict for that evaluation.

     Once the class body has finished evaluating, the metaclass will be
     called (as a callable) with the class dictionary, which is no
     different from the current metaclass mechanism.

     Typically, a metaclass will create a custom dictionary - either a
     subclass of dict, or a wrapper around it - that will contain
     additional properties that are set either before or during the
     evaluation of the class body. Then in the second phase, the
     metaclass can use these additional properties to further customize
     the class.

     An example would be a metaclass that uses information about the
     ordering of member declarations to create a C struct. The metaclass
     would provide a custom dictionary that simply keeps a record of the
     order of insertions. This does not need to be a full 'ordered dict'
     implementation, but rather just a Python list of (key,value) pairs
     that is appended to for each insertion.

     Note that in such a case, the metaclass would be required to deal
     with the possibility of duplicate keys, but in most cases that is
     trivial. The metaclass can use the first declaration, the last,
     combine them in some fashion, or simply throw an exception. It's up
     to the metaclass to decide how it wants to handle that case.


Alternate Proposals

     Josiah Carlson proposed using the name 'type' instead of
     'metaclass', on the theory that what is really being specified is
     the type of the type. While this is technically correct, it is also
     confusing from the point of view of a programmer creating a new
     class. From the application programmer's point of view, the 'type'
     that they are interested in is the class that they are writing; the
     type of that type is the metaclass.

     There were some objections in the discussion to the 'two-phase'
     creation process, where the metaclass is invoked twice, once to
     create the class dictionary and once to 'finish' the class. Some
     people felt that these two phases should be completely separate, in
     that there ought to be separate syntax for specifying the custom
     dict as for specifying the metaclass. However, in most cases, the
     two will be intimately tied together, and the metaclass will most
     likely have an intimate knowledge of the internal details of the
     class dict. Requiring the programmer to insure that the correct dict
     type and the correct metaclass type are used together creates an
     additional and unneeded burden on the programmer.

     Another good suggestion was to simply use an ordered dict for all
     classes, and skip the whole 'custom dict' mechanism. This was based
     on the observation that most use cases for a custom dict were for
     the purposes of preserving order information. However, this idea has
     two drawbacks, first because it means that an ordered dict
     implementation would have to be added to the set of built-in types
     in Python, and second because it would impose a slight speed (and
     complexity) penalty on all class declarations.


Backwards Compatibility

     It would be possible to leave the existing __metaclass__ syntax in
     place. Alternatively, it would not be too difficult to modify the
     syntax rules of the Py3K translation tool to convert from the old to
     the new syntax.


References

     [1] [Python-3000] Metaclasses in Py3K (original proposal)
 
http://mail.python.org/pipermail/python-3000/2006-December/005030.html

     [2] [Python-3000] Metaclasses in Py3K (Guido's suggested syntax)
 
http://mail.python.org/pipermail/python-3000/2006-December/005033.html

     [3] [Python-3000] Metaclasses in Py3K (Objections to two-phase init)
 
http://mail.python.org/pipermail/python-3000/2006-December/005108.html

     [4] [Python-3000] Metaclasses in Py3K (Always use an ordered dict)
 
http://mail.python.org/pipermail/python-3000/2006-December/005118.html

     [5] PEP 359: The 'make' statement -
         http://www.python.org/dev/peps/pep-0359/

Copyright

     This document has been placed in the public domain.


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