Why aren't OrderedDicts comparable with < etc?

Mark list at qtrac.plus.com
Thu Jul 16 12:58:08 CEST 2009


On 16 July, 08:51, Steven D'Aprano
<ste... at REMOVE.THIS.cybersource.com.au> wrote:
> On Thu, 16 Jul 2009 00:30:26 -0700, Chris Rebert wrote:
> > On Wed, Jul 15, 2009 at 11:21 PM, Mark Summerfield<l... at qtrac.plus.com>
> > wrote:
> >> Hi,
>
> >> I'm just wondering why <, <=, >=, and > are not supported by
> >> collections.OrderedDict:
>
> >>    >>> d1 = collections.OrderedDict((("a",1),("z",2),("k",3))) d2 =
> >>    >>> d1.copy()
> >>    >>> d2["z"] = 4
> >>    >>> d1 == d2
> >>    False
> >>    >>> d1 < d2
> >>    Traceback (most recent call last):
> >>    File "<pyshell#6>", line 1, in <module>
> >>        d1 < d2
> >>    TypeError: unorderable types: OrderedDict() < OrderedDict()
>
> >> It just seems to me that since the items in ordered dictionaries are
> >> ordered, it would make sense to do an item by item comparison from
> >> first to last item in exactly the same way that Python compares lists
> >> or tuples?
>
> > Use case? I'm curious.
>
> Surely it would be the same use case as for comparing two lists? There
> doesn't need to be a special "OrderedDict use case" beyond "an
> OrderedDict is just like a list of (key,value) tuples, but searches are
> faster".
>
> Or maybe not. If OrderedDicts are sequences as well as mappings, then we
> should be able to sort them. And that seems a bit much even for me.
>
> --
> Steven

One thing that I've just noticed is that you can use <, <=, >=, and >
with sets:

>>> s1 = {1,2,3}
>>> s2 = {1,2,4}
>>> s1 == s2
False
>>> s1 < s2
False
>>> s1 <= s2
False
>>> s2 < s1
False
>>> s2 <= s1
False
>>> s1 != s2
True

It seems a bit inconsistent that with sets you always get False when
using an ordering operator but with an ordered dict you get an
exception?



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