[Python-Dev] Creating dicts from dict subclasses

[Armin Rigo]

Hi Walter,

On Wed, Dec 13, 2006 at 05:57:16PM +0100, Walter D?rwald wrote:
> I tried to reimplement weakref.WeakValueDictionary as a subclass of
> dict. The test passes except for one problem: To compare results
> test_weakref.py converts a weakdict to a real dict via dict(weakdict).
> This no longer works because PyDict_Merge() does a PyDict_Check() on the
> argument and then ignores all overwritten methods. (The old version
> worked because UserDict.UserDict was used).

This is an instance of a general problem in Python: if you subclass a
built-in type, then your overridden methods may or may not be used in
various situations.  In this case you might have subtle problems with
built-in functions and statements that expect a dict and manipulate it
directly, because they will see the underlying dict structure.  It is
also quite fragile: e.g. if a future version of CPython adds a new
method to dicts, then your existing code will also grow the new method
automatically - but as inherited from 'dict', which produces quite
surprizing results for the user.

>    for key in iter(arg.keys()):
>       self[key] = arg.__getitem__(key)
> 
> Why can't we use:
> 
>    for key in iter(arg):
>       self[key] = arg.__getitem__(key)

The latter would allow 'arg' to be a sequence instead of a mapping.  It
may even not crash but produce nonsense instead, e.g. if 'arg' is a list
of small integers.  Moreover there are multiple places in the code base
that assume that mappings are "something with a 'keys' and a
'__getitem__'", so I suppose any change in that should be done
carefully.


A bientot,

Armin