On Apr 16, 2020, at 12:26, Andrew Barnert <abarn...@yahoo.com> wrote:
> 
> Somewhere I have some code for a set of class decorators that help 
> implementing mappings and sequences: you provide basic __fooitem__ methods, 
> and it wraps them with methods that do all the extra stuff dict, tuple, and 
> list do. IIRC, the mutable sequence stuff is the only place where it gets 
> complicated, but there may be others I haven’t remembered. I can dig this up 
> if you‘re interested. Maybe it’s even worth cleaning up and posting to PyPI, 
> or even proposing for somewhere in the stdlib (collections or functools?).

Ok, I found it, and it’s in better shape than I thought it was. (It even passes 
all the relevant tests from the stdlib test suite.) So I posted it on 
https://github.com/collectionhelpers in case anyone wants to play with it. If 
there’s any demand for turning it into a PyPI package, I can do that, but 
otherwise I won’t bother.

Anyway, if you look at the code, most of it is devoted to the mutable sequence 
__setitem__, but making mapping __getitem__ handle __missing__ wasn’t quite as 
trivial as you’d expect (it breaks the __contains__ and get methods you inherit 
from Mapping…). Whether that counts as an argument for or against any version 
of the various proposals in this thread, I’m not sure.

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