larry-discuss team mailing list archive

[Keith Goodman <kwgoodman@xxxxxxxxx>]

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))