Referencing Items in a List of Tuples

[Bruno Desthuilliers]

rshepard at nospam.appl-ecosys.com a écrit :
>   While working with lists of tuples is probably very common, none of my
> five Python books or a Google search tell me how to refer to specific items
> in each tuple.

t = "a", "b", "c"
assert t[0] == "a"
assert t[1] == "b"
assert t[2] == "c"


> I find references to sorting a list of tuples, but not
> extracting tuples based on their content.

loft = [(1, 2, 3), (1, 2, 4), (2, 3, 4)]
starting_with_one = [t for t in loft if t[0] == 1]

>   In my case, I have a list of 9 tuples. Each tuple has 30 items. The first
> two items are 3-character strings, the remaining 28 itmes are floats.
> 
>   I want to create a new list from each tuple. But, I want the selection of
> tuples, and their assignment to the new list, to be based on the values of
> the first two items in each tuple.
> 
>   If I try, for example, writing:
> 
> for item in mainlist:
>     if mainlist[item][0] == 'eco' and mainlist[item][1] == 'con':
>       ec.Append(mainlist[item][2:])
> 
> python doesn't like a non-numeric index.

Hem... I'm afraid you don't understand Python's for loops. The above 
should be:

ec = []
for item in mainlist:
     if [item][0] == 'eco' and item[1] == 'con':
         ec.append(item[2:])

which can also be written:
ec = [item[2:] for item in mainList \
       if item[0] == 'eco' and item[1] == 'con']

>   I would really appreciate a pointer so I can learn how to manipulate lists
> of tuples by referencing specific items in each tuple (string or float).

Did you actually tried reading some Python 101 material ?