[Persistence-sig] "Straw Baby" Persistence API

[Jeremy Hylton]

>>>>> "SM" == Steve Menard <smenard@bigfoot.com> writes:

  GvR> But perhaps these should be rewritten to derive from dict and
  GvR> list instead of UserDict and UserList?
  >>
  >> One small comment.  (I owe more substantial comment on Phillip's
  >> earlier proposals.)  The persistent versions of dict and list
  >> can't extend the builtin types, because they need to hook
  >> __getitem__() and __setitem__().  The overridden methods may not
  >> be called if we extend the builtin types.
  >>

  SM> hum, if those method are not guaranteed to be called by
  SM> subclassing dict or list, then there is something broken. Either
  SM> that or there is a subtle thing I do not understand.

The latter.  For performance reasons, most C code uses calls like
PyDict_GetItem(), which operates directly on the C representation of a
dict.  If you inherit from dict, you'll get the same C representation
for your object.  That allows PyDict_GetItem() to be called, but
doesn't arrange to call your __getitem__() method.  The indirection
required to invoke a subclass's __getitem__() would cause serious
performance problems.

Normally Guido only recommends inheriting from dict to add new
behavior (as opposed to customizing existing behavior).

Jeremy