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Fri Feb 13 01:36:51 CET 2015 · https://mail.python.org/pipermail/python-dev/2015-February/138184.html

I suspect the last big hurdle to making built-in data structures nicely subclassable is the insistence of such types to
return new instances as the base class instead of the derived class.

In case that wasn't clear ;)

--> class MyInt(int):
...   def __repr__(self):
...     return 'MyInt(%d)' % self
...
--> m = MyInt(42)
--> m
MyInt(42)
--> m + 1
43
--> type(m+1)
<class 'int'>

Besides the work it would take to rectify this, I imagine the biggest hurdle would be the performance hit in always
looking up the type of self.  Has anyone done any preliminary benchmarking?  Are there other concerns?

--
~Ethan~

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[Python-Dev] subclassing builtin data structures