Why are there no ordered dictionaries?
bokr at oz.net
Tue Nov 22 00:32:25 CET 2005
On Mon, 21 Nov 2005 01:27:22 +0100, Christoph Zwerschke <cito at online.de> wrote:
>Fredrik Lundh wrote:
>> if you restructure the list somewhat
>> d = (
>> ('pid', ('Employee ID', 'int')),
>> ('name', ('Employee name', 'varchar')),
>> ('sal', ('Salary', 'float'))
>> you can still loop over the list
>> but you can easily generate an index when you need it:
>> index = dict(d)
>That's exactly the kind of things I find myself doing too often and what
>I was talking about: You are using *two* pretty redundant data
>structures, a dictionary and a list/tuple to describe the same thing.
>Ok, you can use a trick to automatically create the dictionary from the
>tuple, but still it feels somewhat "unnatural" for me. A "ordered
>dictionary" would be the more "natural" data structure here.
But, as has been mentioned**n, this is only one example of an ordering one
could make default for an "ordered" dictionary. Suppose you say it should
be ordered by insertion order, so
d = OrderedDict(); d='one'; d='two' =>> list(d) => [1, 2]
ok, now we do d='ein' and what is the order? list(d) => [2, 1] ??
Or do replacements not count as "insertions"? The devil is always going
to be in the details. Maybe you want a model that works more like a list
of key:value pairs with just optimized access to a pair by key name as
well as position in the list. Or maybe you want to permit append and
NOT prevent [('a',1), ('a':2)] and maybe d['a'] => [1, 2] ???
The point is that Python is a nice lego set, and pre-molded castles
don't re-use well, even if they suit a particular you to a t ;-)
Note that is isn't hard to snap a few pieces together to make an ordered
dict to your own specs. But IMO it belongs in pyPI or such, not in the system
library. At least until it gets a lot of mileage -- and MMV ;-)
>I also wanted to mention the uglyness in the definition (nested tuples),
>but then I understood that even an ordered dictionary would not
>eliminate that uglyness, since the curly braces are part of the Python
>syntax and cannot be used for creating ordered dictionaries anyway. I
>would have to define the ordered dictionary in the very same ugly way:
>d = odict(('pid', ('Employee ID', 'int')),
> ('name', ('Employee name', 'varchar')),
> ('sal', ('Salary', 'float')))
>(Unless the Python syntax would be extend to use double curly braces or
>something for ordered dictionaries - but I understand that this is not
Whatever your odict does, if I had type a lot of definitions for it
I think I would write a QnD helper to make this work:
d = odict(prep("""
pid, Employee ID, int
name, Employee name, varchar # (comments to be ignored)
sal, Salary, float # alignment as above not mandatory
other, Something else, long, additional elements, allowed in second tuple?
( posting delayed >12 hrs due to news server prob ;-/ )
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