Raymond Hettinger <rhettin...@users.sourceforge.net> added the comment:
Somewhere, we should document the-facts-of-life for subclassing builtins.
1) For the most part, C code has the pattern
if isinstance(obj, some_builtin_type):
call the built_type's methods directly using slotted methods
otherwise:
use slower getattribute style calls
2) A subclasser of a dict needs to actually populate the dict with the values
they want used. The built-in dict class is "open for extension and closed for
modification" -- the open/closed principle. This is necessary or else a
subclasser could easily break the built-in type's invariants and crash python.
3) For the most part, only something like subclassing UserDict gives you full
control.
----------
nosy: +rhettinger
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue8945>
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