Using reverse iteration to clean up a list

[tkpmep at hotmail.com]

Thank you all so much for the generous dollop of help: the dictionary
suggestion is particularly helpful. The problem arises as follows: A
software application stores the securities held in a portfolio in a
.csv file, one row per security, with three colulmns.

The first has a security identifier or descriptor (such as a ticker)
the second has a single letter that identifies the type of the
identifier (T for ticker, C for cusip etc.) and the third has the
number of shares. A typical line looks like this:

IBM, T, 500

I need to read in one or more portfolios and aggregate their holdings.
To do so, I read in the portfolios using the csv package, convert each
line to a list and then append it to a list of lists. Eventually the
list of lists contains all the securities, and can then be sorted and
aggregated.

I suppose I could convert it instead to a dictionary, and the most
natural key would be the first two items, i.e. a portfolio containing
just 500 shares of IBM ought to be represented as
{("IBM", "T") : 500 }

How can I translate the data I read in using csv.reader into a
dictionary?

Thomas Philips