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Wed Dec 13 17:57:16 CET 2006 · https://mail.python.org/pipermail/python-dev/2006-December/070334.html

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).

The simplest solution is to replace the PyDict_Check() call with
PyDict_CheckExact(), but this might slow things down too much, because
the fallback code basically does:

   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)

instead?

Servus,
   Walter

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[Python-Dev] Creating dicts from dict subclasses