[Python-Dev] PEP 455: TransformDict

[Steven D'Aprano]

On Mon, Oct 07, 2013 at 06:17:09PM -0700, Mark Janssen wrote:
> Sorry I missed the original discussion, but isn't this a simple case
> of putting a decorator around the getitem method (to transform the
> input key) and a single line in the body of the setitem method, making
> this very easy adaptation of the existing dict class?

Not really. We can try what you suggest to implement a case insensitive 
dict (decorator is not needed):


py> class CaseInsensitiveDict(dict):
...     def __getitem__(self, key):
...             key = key.casefold()  # use .lower() before Python 3.3
...             return super().__getitem__(key)
...     def __setitem__(self, key, value):
...             key = key.casefold()
...             super().__setitem__(key, value)
...
py> d = CaseInsensitiveDict()
py> d['X'] = 42
py> d
{'x': 42}


Well, that's not exactly what I was hoping for... I was hoping that the 
dict would preserve case, rather than just convert it. But that's only 
the start of the problem:

py> d['X']  # this works
42
py> d.pop('X')  # expecting 42
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'X'


So no, it isn't just a matter of a trivial wrapper around __getitem__, 
__setitem__ and __delitem__.

Check the bug tracker for more detail:

http://bugs.python.org/issue18986




-- 
Steven