Namedtuples problem

[MRAB]

On 2017-02-23 23:00, Deborah Swanson wrote:
>
>
>> -----Original Message-----
>> From: Erik [mailto:python at lucidity.plus.com]
>> Sent: Thursday, February 23, 2017 2:09 AM
>> To: python at deborahswanson.net; python-list at python.org
>> Subject: Re: Namedtuples problem
>>
>>
>> Hi,
>>
>> On 23/02/17 09:38, Deborah Swanson wrote:
>> > group[[idx][records_idx[label]]]
>> > gets an IndexError: list index out of range
>>
>> [snip]
>>
>> > Can anyone see why I'm getting this Index error? and how to fix it?
>>
>> It looks to me like you are indexing into a single-element
>> list that you
>> are creating using the literal list syntax in the middle of
>> the expression.
>
> Actually, group is essentially a 2-element list. Each group has a list
> of rows, and each row has a set of fields. group has to be indexed by
> row index and field index. (This is a namedtuple configuration.)
>
> The weirdness is that
>
> group[0][4]
>
> gets the right answer, but
>
> group[[idx][records_idx[label]]],
> where idx = 0 and records_idx[label]] = 4
>
> gets the IndexError.
>
It's not weird. As Erik says below, and I'll repeat here, you have:

     group[[idx][records_idx[label]]]

That consists of the expression:

     [idx][records_idx[label]]

within:

     group[...]

The [idx] creates a one-element list.

You then try to subscript it with records_idx[label]. If 
records_idx[label] is anything other than 0 (or -1), it'll raise 
IndexError because there's only one element in that list.

If you substitute idx == 0 and records_idx[label]] == 4 into:

     group[[idx][records_idx[label]]]

you'll get:

     group[[0][4]]

which is not the same thing as group[0][4]!

>> If we were to break the expression into parts to make it a
>> bit simpler
>> to refer to discuss:
>>
>> ridx = records_idx[label]
>> group[[idx][ridx]]
>>
>> You can now more easily see that 'group' is being indexed by the
>> expression "[idx][ridx]". What does that mean?
>>
>> [idx] is creating a single-element list using literal list
>> syntax. This
>> is then indexed using 'ridx' (using, perhaps confusingly, the
>> exact same
>> syntax to do a different thing).
>>
>> The result of *that* expression is then being used to index
>> 'group', but
>> it won't get that far because you'll get the exception if 'ridx' is
>> anything but zero.
>>
>> So the initial problem at least is the extra [] around 'idx' which is
>> creating a list on the fly for you.
>>