larry-discuss team mailing list archive

[Keith Goodman <kwgoodman@xxxxxxxxx>]

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'), :]