[Python-Dev] PEP 455: TransformDict

[MRAB]

On 14/09/2013 01:49, Steven D'Aprano wrote:
> 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))
>
I think must be missing something. I thought that iterating over the
dict would yield the original keys, so if you wanted the original key
and value you would write:

for key, value in data.items():
     print("{}: {}".format(key, value))

and if you wanted the transformed key you would apply the transform
function to the key.

>
> 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
> used.
>
> 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__.
>