Alex A. Naanou wrote:
> Hi!
>
> here is a simple piece of code
> <pre>
> ---cut---
> class Dict(dict):
> def __init__(self, dct={}):
> self._dict = dct
> def __getitem__(self, name):
> return self._dct[name]
> def __setitem__(self, name, value):
> self._dct[name] = value
> def __delitem__(self, name):
> del self._dct[name]
> def __contains__(self, name):
> return name in self._dct
> def __iter__(self):
> return iter(self._dct)
>
> class A(object):
> def __new__(cls, *p, **n):
> o = object.__new__(cls)
> o.__dict__ = Dict()
> return o
>
> a = A()
> a.xxx = 123
> print a.__dict__._dict
> a.__dict__._dict['yyy'] = 321
> print a.yyy
>
> --uncut--
> </pre>
>
> Here there are two problems, the first is minor, and it is that
> anything assigned to the __dict__ attribute is checked to be a
> descendant of the dict class (mixing this in does not seem to work)...
> and the second problem is a real annoyance, it is that the mapping
> protocol supported by the Dict object in the example above is not used
> by the attribute access mechanics (the same thing that once happened
> in exec)...
>
Actually, overriding __getattribute__() does work; __getattr__() and
__getitem__() doesn't. This was brought up last month at some point without
any resolve (I think Steve Bethard pointed it out).
> P.S. (IMHO) the type check here is not that necessary (at least in its
> current state), as what we need to assert is not the relation to the
> dict class but the support of the mapping protocol....
>
Semantically necessary, no. But simplicity- and performance-wise, maybe. If
you grep around in Objects/classobject.c, for instance, you will see
PyClassObject.cl_dict is accessed using PyDict_GetItem() and I spotted at least
one use of PyDict_DelItem(). To use the mapping protocol would require
changing all of these to PyObject_GetItem() and such.
Which will be a performance penalty compared to PyDict_GetItem(). So the
question is whether the flexibility is worth it.
-Brett
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