Newbie Question: Obtain element from list of tuples

[HoneyMonster]

On Mon, 19 Dec 2011 10:40:06 +1100, Chris Angelico wrote:

> On Mon, Dec 19, 2011 at 9:55 AM, HoneyMonster
> <someone at someplace.invalid> wrote:
>> When the user selects a row and clicks a button, I am using:
>> pos = self.grid_1.GetGridCursorRow() to establish which tuple in recs
>> is involved, and then pid = recs[pos][4] to determine the key value (I
>> suspected that accessing recs directly would be more efficient that
>> trying to determine the wxGrid column value, and in any case the pid is
>> in recs but not in the wxGrid).
> 
> Yep, this looks like a sensible way to do it!
> 
> I would recommend, though, that you avoid "magic numbers" - the process
> ID might happen to be in cell 4, but what if your column list changes?
> This is especially problematic when you use "select * from table",
> because changes outside your code can break things.
> 
> There's two solutions. One is to carefully match your SELECT statement
> to a set of constants:
> 
> exec("SELECT FOO,BAR,QUUX,ASDF,PID,WHATEVER FROM table")
> FOO,BAR,QUUX,ASDF,PID,WHATEVER,*_=range(20) # Cool trick to avoid having
> to count the columns
> 
> But a better way is to let the database handle it. Check your interface
> library to see if you can have it return dictionaries or objects instead
> of tuples - then you could use:
> 
> pid = recs[pos]["pid"]
> # or pid = recs[pos]->pid
> 
> which avoids the need to count columns at all.
> 
> Other than that, though, yep - your code concept looks fine.

Thanks again, Chris. I'm completely new to Python, but am an old had at 
databases. I *never* use "select *" in a production environment; rather 
an explicit list of columns. Thus it doesn't matter if a column is added 
to the table, nor if the column order in the table is changed.

But I have used this at the start if my code to make it more readable 
anyway:

# Define constants (position in tuple)
Title = 0
Episode = 1
Categories = 2
Channel = 3
PID = 4
Index = 5

so that in the bit where I populate the grid, the code is now:

cnt = 0
for tup in recs:
    self.grid_1.SetCellValue(cnt, Title, tup[Title])
    self.grid_1.SetCellValue(cnt, Episode, tup[Episode])
    self.grid_1.SetCellValue(cnt, Categories, tup[Categories])
    self.grid_1.SetCellValue(cnt, Channel, tup[Channel])
    cnt = cnt + 1

just so that is more legible.

Cheers,
HM