larry-discuss team mailing list archive

[Keith Goodman <kwgoodman@xxxxxxxxx>]

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.