Question

I’m trying to make a sum of a column in a csv file. The file looks like:

Date  Value
2012-11-20  12
2012-11-21  10
2012-11-22  3

This can be in the range of hundreds of rows. I need to get the total of Value (in this case it would be 25) printed on to a terminal. I so far have some code but it’s resulting in a much smaller figure than it should sum. When troubleshooting it, I did a print of the sum and realized that instead of summing 12 + 10 + 3, it actually breaks the numbers in each column and sums as 1 + 2 + 1 + 0 + 3, which obviously equals to a much smaller total. Here’s my code, if someone could make a recommendation would be great!

with open("file.csv")) as fin:
  headerline = fin.next()
  total = 0
  for row in csv.reader(fin):
    print col # for troubleshooting
    for col in row[1]:
      total += int(col)
  print total

Answer 1

The csv module loops over your rows one by one, there is no need to then loop over the column. Just sum int(row[1]):

with open("file.csv") as fin:
    headerline = next(fin)
    total = 0
    for row in csv.reader(fin):
        total += int(row[1])
    print(total)

You can use a shortcut with a generator expression and the sum() built-in function:

with open("file.csv") as fin:
    next(fin)
    total = sum(int(r[1]) for r in csv.reader(fin))

Note that in Python, strings are sequences too, so when you do for col in row[1]: you are looping over the individual characters of row[1]; so for your first row that’d be 1 and 2:

>>> for c in '123':
...     print(repr(c))
...
'1'
'2'
'3'