New submission from Daniel Urban <urban.dani...@gmail.com>:
As Nick Coghlan proposed [1, 2], there should be a way to dynamically create
classes, which handles metaclasses correctly (see also issue1294232).
Here is my first attempt at creating an operator.build_class method. It only
includes very simple tests and no documentation, but I will write them if
needed.
With this patch there are two functions for creating a class object:
1. __build_class__ (no change)
2. operator.build_class(name, bases=(), kwds=None, eval_body=None): finds the
correct metaclass and calls its __prepare__. If eval_body is given, calls it
with the namespace returned by __prepare__. Then calls the correct metaclass,
and returns the created class object.
Both of these functions (after parsing their arguments) call
_PyType_BuildClass, a new C API function. The first argument of this function
is a callable, that will be called with the namespace returned by __prepare__
(it also can be NULL, in that case nothing will be called). __build_class__
passes the function that is the body of the class statement.
operator.build_class passes the callable given by the user (or NULL, if the
user didn't pass the eval_body argument). The implementation of
_PyType_BuildClass is approximately the following:
def _PyType_BuildClass(func=None, name, bases, kwds={}):
meta = kwds.pop('metaclass', None)
if meta is None:
if not bases:
meta = type
else:
meta = type(bases[0])
ns, meta = prepare_namespace(name, meta, bases, kwds)
if func is not None:
func(ns)
return meta(name, bases, ns, kwds)
(Actually the return value of the func is used if it's a cell object. I'm not
sure, why and when this is needed, this code comes from __build_class__.)
The changes are in the following files:
1. object.h: the exported function is _PyType_BuildClass instead of
_PyType_CalculateMetaclass (that doesn't need to be in the include file
anymore).
2. operator.c: the build_class method checks its arguments, then calls
_PyType_BuildClass.
3. typeobject.c:
_PyType_CalculateMetaclass is renamed to calculate_metaclass, because now it is
only called from this file.
prepare_namespace calls calculate_metaclass to determine the correct metaclass,
then calls its __prepare__ method. (This code is moved here mostly from
__build_class__). It also passes back the correct metaclass to its caller.
_PyType_BuildClass gets the starting metaclass from its arguments. Then it
calls prepare_namespace to get the namespace and the correct metaclass. If it
received a non-NULL first argument (the function that is the class body or the
eval_body argument of operator.build_class), then calls it, passing the
namespace. Then it calls the correct metaclass. (Most of this code is also from
__build_class__.)
4. bltinmodule.c: builtin___build_class__ now only parses its arguments, and
simply calls _PyType_BuildClass.
5. test_operator.py: a simple test for operator.build_class
[1] http://mail.python.org/pipermail/python-dev/2011-April/110874.html
[2] http://mail.python.org/pipermail/python-dev/2012-April/118732.html
----------
components: Extension Modules, Interpreter Core
files: operator_build_class.patch
keywords: patch
messages: 158382
nosy: durban, ncoghlan
priority: normal
severity: normal
status: open
title: PEP 3115 compliant dynamic class creation
type: enhancement
versions: Python 3.3
Added file: http://bugs.python.org/file25231/operator_build_class.patch
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<http://bugs.python.org/issue14588>
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