Namedtuples problem

[Deborah Swanson]

> -----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.

> 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.
> 
> E.
>