[Python-Dev] Add a "transformdict" to collections

[Nigel Small]

Could a more generic variant of this class work? In the same way that
`sorted` can accept a comparison function, similar could be done for a
dictionary-like class:

d = transformdict(key=str.lower)

Strictly speaking, this would provide case-insensitive but not
case-preserving behaviour. For any given use case, though, a function could
instead be supplied to "normalise" the key (upper, lower, title case, etc)
in a way that fits that case. I can't think of many real cases where
multiple types of capitalisation would be useful within the same dictionary.

Nigel



On 10 September 2013 15:40, Richard Oudkerk <shibturn at gmail.com> wrote:

> On 10/09/2013 3:15pm, Armin Rigo wrote:
>
>> Hi Richard,
>>
>> On Tue, Sep 10, 2013 at 3:42 PM, Richard Oudkerk <shibturn at gmail.com>
>> wrote:
>>
>>> I guess another example is creating an "identity dict" (see
>>> http://code.activestate.com/**lists/python-ideas/7161/<http://code.activestate.com/lists/python-ideas/7161/>)
>>> by doing
>>>
>>>      d = transformdict(id)
>>>
>>
>> This is bogus, because only the id will be stored, and the original
>> key object will be forgotten (and its id probably reused).
>>
>
> Seems to work for me:
>
> >>> import collections
> >>> d = collections.transformdict(id)
> >>> L = [1,2,3]
> >>> d[L] = None
> >>> L in d
> True
> >>> [1,2,3] in d
> False
> >>> print(d[L])
> None
> >>> d._data
> {41444136: ([1, 2, 3], None)}
> >>> list(d)
> [[1, 2, 3]]
>
> However __repr__() is broken:
>
> >>> d
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "C:\Repos\cpython-dirty\lib\**collections\abc.py", line 444, in
> __repr__
>     return '{0.__class__.__name__}({0._**mapping!r})'.format(self)
>   File "C:\Repos\cpython-dirty\lib\**collections\__init__.py", line 944,
> in __repr__
>     self._transform, repr(dict(self)))
> TypeError: unhashable type: 'list'
>
> --
> Richard
>
> ______________________________**_________________
> Python-Dev mailing list
> Python-Dev at python.org
> https://mail.python.org/**mailman/listinfo/python-dev<https://mail.python.org/mailman/listinfo/python-dev>
> Unsubscribe: https://mail.python.org/**mailman/options/python-dev/**
> nigel%40nigelsmall.com<https://mail.python.org/mailman/options/python-dev/nigel%40nigelsmall.com>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-dev/attachments/20130910/5107ee06/attachment.html>