[Python-ideas] Override dict.__new__ to raise if cls is not dict; do the same for str, list, etc.

[Neil Girdhar]

Sometimes users inherit from builtin types only to find that their 
overridden methods are not called.  Instead of this being a trap for 
unsuspecting users, I suggest overriding the __new__ method of these types 
so that it will raise with an informative exception explaining that, e.g., 
instead of inheriting from dict, you should inherit from UserDict.

I suggest this modification to any Python implementation that has special 
versions of classes that cannot easily be extended, such as CPython.  If 
another Python implementation allows dict (e.g.) to be extended easily, 
then it doesn't have to raise.

Best,

Neil
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