larry-discuss team mailing list archive

[Keith Goodman <kwgoodman@xxxxxxxxx>]

On Sun, Feb 7, 2010 at 6:55 PM,  <josef.pktd@xxxxxxxxx> wrote:
> On Sun, Feb 7, 2010 at 9:46 PM, Keith Goodman <kwgoodman@xxxxxxxxx> wrote:
>> On Sun, Feb 7, 2010 at 6:43 PM, Keith Goodman <kwgoodman@xxxxxxxxx> wrote:
>>> On Sun, Feb 7, 2010 at 6:39 PM, Keith Goodman <kwgoodman@xxxxxxxxx> wrote:
>>>> On Sun, Feb 7, 2010 at 6:35 PM, Keith Goodman <kwgoodman@xxxxxxxxx> wrote:
>>>>> On Sun, Feb 7, 2010 at 6:28 PM,  <josef.pktd@xxxxxxxxx> wrote:
>>>>>> On Sun, Feb 7, 2010 at 8:46 PM, Keith Goodman <kwgoodman@xxxxxxxxx> wrote:
>>>>>>> On Sun, Feb 7, 2010 at 5:26 PM, Keith Goodman <kwgoodman@xxxxxxxxx> wrote:
>>>>>>>> On Sat, Feb 6, 2010 at 6:53 PM,  <josef.pktd@xxxxxxxxx> wrote:
>>>>>>>>> On Sat, Feb 6, 2010 at 9:48 PM, Keith Goodman <kwgoodman@xxxxxxxxx> wrote:
>>>>>>>>>> On Sat, Feb 6, 2010 at 5:38 PM, Keith Goodman <kwgoodman@xxxxxxxxx> wrote:
>>>>>>>>>>> In a blueprint titled "index-by-label" I proposed a way to index
>>>>>>>>>>> larrys by lists of label elements. Here's a simpler, but less
>>>>>>>>>>> versatile, proposal. On the whole, due to its simplicity, I think it
>>>>>>>>>>> is more powerful.
>>>>>>>>>>
>>>>>>>>>> I commit this proposal in r187. Please give it a try.
>>>>>>>>>
>>>>>>>>> I will try it tomorrow and look at the implementation.
>>>>>>>>> My first reaction: very convenient but potentially fragile for arbitrary labels.
>>>>>>>>
>>>>>>>> The rule is simple for indexing with a string S:
>>>>>>>>
>>>>>>>> 1. Look for string S in the label. If found you are done. If not found...
>>>>>>>> 2. Map the labels to strings and look again
>>>>>>>>
>>>>>>>> Although the rule is simple, the result can be unexpected in corner
>>>>>>>> cases. For example, you may try to index with str(1) to access the
>>>>>>>> label integer 1 but the label could also contain string '1'. So in
>>>>>>>> that case you'd get an unexpected result even though the rule is
>>>>>>>> simple.
>>>>>>>>
>>>>>>>> I could add a check: len(set(strlabel)) == len(set(label)). And raise
>>>>>>>> an IndexError (or is that ValueError?) if they are not equal. That
>>>>>>>> will slow things down but only for indexing by strings.
>>>>>>>>
>>>>>>>> Would that address your fragile comment? Or do you have something else in mind?
>>>>>>>
>>>>>>> Wait, that's being too restrictive. We don't care if there are
>>>>>>> duplicates in strlabel. We only care if S appears more than once in
>>>>>>> strlabel. For example, if we are indexing with str(1) and the label is
>>>>>>> [2, str(2), 1], then we don't care that strlabel = [str(2), str(2),
>>>>>>> str(1)] has duplicates; we only care that str(1) only appears once. If
>>>>>>> we were indexing with str(2), on the other hand, then there would be a
>>>>>>> problem and we'd raise a ValueError.
>>>>>>>
>>>>>>> I can add that check and then you can take a look.
>>>>>>>
>>>>>>
>>>>>> I just started to look at it. I saw in str2labelindex  you use
>>>>>> str(labelobject) to identify the label.
>>>>>> I don't think __string__ is very save to use in general, I don't think
>>>>>> it is guaranteed to remain unchanged. e.g. in numpy you can affect the
>>>>>> str result with the print options for numbers in arrays, e.g.
>>>>>> np.set_printoptions(precision=2).
>>>>>>
>>>>>> another example objects that don't define a unique string or use a
>>>>>> default string
>>>>>>>>> class MyA(object):pass
>>>>>>
>>>>>>>>> aaa = MyA()
>>>>>>>>> str(aaa)
>>>>>> '<__main__.MyA object at 0x01A57DD0>'
>>>>>>
>>>>>> I'm not very familiar with datetime, Is the string representation
>>>>>> locale or timezone dependent ?
>>>>>> decimal point is local dependent from some messages on the mailing
>>>>>> lists, I assume that in some cases the default in german is 5,4
>>>>>> instead of 5.4
>>>>>>
>>>>>> So, relying on the string representation imposes quite a lot of
>>>>>> restrictions for which type of labels this would work.
>>>>>>
>>>>>> I look some more.
>>>>>
>>>>> Sure, indexing with things like '(3,4)' will be a problem since
>>>>> str((3,4)) is '(3, 4)' (note the space). So the safe way to index is,
>>>>> for example, y[str(1)].
>>>>>
>>>>> I like the general idea of using __getitem__ to index both the regular
>>>>> and the label way. One thing I am wondering about is if there is
>>>>> another way to signify indexing by labels other than with strings. It
>>>>> would have to be something that numpy arrays can't be indexed by.
>>>>
>>>> I suppose dictionaries could be used. It does take quite a bit more
>>>> typing. For example:
>>>>
>>>>>> class eli(object):
>>>>   ...:     def __init__(self):
>>>>   ...:         pass
>>>>   ...:     def __getitem__(self, index):
>>>>   ...:         print index
>>>>   ...:
>>>>   ...:
>>>>
>>>>>> e[{'label': 'a'},:]
>>>> ({'label': 'a'}, slice(None, None, None))
>>>>
>>>> On the plus side: no need to map labels to strings.
>>>
>>> Or any two element sequence where the first element is 'label:
>>>
>>>>> e = eli()
>>>
>>>>> e[('label', 'a'), :]
>>> (('label', 'a'), slice(None, None, None))
>>>
>>>>> e[['label', 'a'], :]
>>> (['label', 'a'], slice(None, None, None))
>>
>> Or:
>>
>>>> from la import ix
>>>> e[ix('a'), :]
>
> You got to this while I was writing my reply to an earlier message. If
> you agree with this version, we can look at it more closely, I think
> it's the safest bet.

Yes, I like it. I also ended up using the name lix. Now, one problem.
One of the features I like about string indexing is that you can do
slices like this:

y['2010-02-01':]

So let's think on that.