[Python-Dev] PEP 455: TransformDict

Steven D'Aprano steve at pearwood.info
Sat Sep 14 02:49:52 CEST 2013

On Fri, Sep 13, 2013 at 11:45:16PM +0200, Antoine Pitrou wrote:

> Ok, I have a better (IMO) proposal:
>     >>> d = TransformDict(str.casefold, {'Foo': 1})
>     >>> d.getitem('foo')
>     ('Foo', 1)
>     >>> d.getitem('bar')
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in <module>
>     KeyError: 'bar'

Is it more common to want both the canonical key and value at the same 
time, or to just want the canonical key? My gut feeling is that I'm 
likely to have code like this:

d = TransformDict(...)
for key in data:
    key = d.get_canonical(key)
    value = d[key]
    print("{}: {}".format(key, value))

in which case having a single call to return both will be great:

for key in data:
    key, value = d.getitem(key)
    print("{}: {}".format(key, value))

but I'm really not sure. Maybe Ethan is right.

I think these sorts of associated questions are why some people 
(Raymond, Nick) want to see the proposal live outside of the standard 
library for a while first. The general idea is great, but we're going to 
bike shed the API without having much idea of how it will actually be 

So, my suggestion is this:

- Let's add __transform__ to dicts for 3.4, similar to __missing__, and 
see how people end up using it in the real world.

- Then, in 3.5, we can make a much better informed decision about the 
best API for a TransformedDict front-end to __transform__.

+1 on __transform__ method on dicts.

+0 on TransformedDict in 3.4

+1 on waiting for 3.5 based on experience on using __transform__.


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