larry-discuss team mailing list archive

[josef.pktd@xxxxxxxxx]

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.

Josef