[Python-3000] PEP for Metaclasses in Python 3000 (original) (raw)

Brett Cannon brett at python.org
Fri Mar 9 22:31:02 CET 2007

On 3/9/07, Talin <talin at acm.org> wrote:

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: RevisionRevisionRevision Last-Modified: DateDateDate Author: Talin 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.

I think you mean "via logical proofs" or "logically via 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.

Do the keywords have to follow the metaclass keyword, or is order irrelevant? While order makes sense, it would be a new precedent for keyword arguments to have an important order.

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.

That last sentence felt a little clumsy. I think if you ditch that last comma it reads more easily.

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.

Does the language spec guarantee that the body of a class will be executed in definition order? Or is that considered implicit by the fact that the class body is executed as code?

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.

Seems good, although I hardly ever use metaclasses so that doesn't say a whole lot. =)

-Brett

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