[Python-Dev] PEP 455: TransformDict

[Antoine Pitrou]

On Tue, 8 Oct 2013 08:31:46 +1000
Nick Coghlan <ncoghlan at gmail.com> wrote:
> 
> That said, with the current plan to lower the barrier to entry for PyPI
> dependencies (I should have the 3.4 only ensurepip proposal written up some
> time this week), I think it makes sense to let this one bake on PyPI for a
> while.

"the current plan to lower the barrier to entry for PyPI" sounds a lot
like the obsession du jour to me :-) It's not like "ensurepip" makes it
cheaper / more attractive to add dependencies. It just provides a
better experience for those who *want* to use pip (and would
otherwise have installed it using an explicit download).

> I think there *is* a potentially worthwhile generalisation here, but I'm
> far from sure "is-a-dict" is the right data model - for composability
> reasons, it feels like a "has-a" relationship with an underlying data store
> may make more sense.

It doesn't work. Your "underlying mapping" can show too much variation
for the wrapper/proxy to have sane semantics. For example, how do you
combine with a defaultdict or a WeakKeyDictionary, knowing that the
wrapper/proxy has to have its own internal mapping as well?

> (If performance is critical, you're going to write a
> dedicated type anyway, so composability and simplicity strike me as more
> important criteria at this point).

A dedicated CaseInsensitiveDict won't be much faster than
TransformDict(str.casefold).

Regards

Antoine.