New submission from Eric Snow:
Currently type_new() in Objects/typeobject.c enforces a restriction that the
namespace be a dict or dict subclass. It does this via the
PyArg_ParseTupleAndKeywords() call there.
This means that valid mappings may not be used even though they should work
just fine. A demonstration of the problem is below.
I've attached a patch that relaxes this restriction. Should we also add a note
in the docs that type() will take anything for namespace that dict() will take?
Demonstration
-------------
class Meta(type):
@classmethod
def __prepare__(cls, name, bases, **kwargs):
return ClassMapping()
from collections import MutableMapping
class ClassMapping(MutableMapping):
def __init__(self, *args, **kwargs):
self._dict = dict(*args, **kwargs)
def __len__(self):
return len(self._dict)
def __iter__(self):
return iter(self._dict)
def __getitem__(self, key):
return self._dict[key]
def __setitem__(self, key, value):
self._dict[key] = value
def __delitem__(self, key):
del self._dict[key]
>>> class X(metaclass=Meta):
... pass
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: type() argument 3 must be dict, not ClassMapping
----------
files: type-namespace-restriction.diff
keywords: patch
messages: 184187
nosy: eric.snow, ncoghlan
priority: normal
severity: normal
stage: patch review
status: open
title: Drop restriction that meta.__prepare__() must return a dict (subclass)
type: enhancement
versions: Python 3.4
Added file: http://bugs.python.org/file29408/type-namespace-restriction.diff
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue17421>
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