Question

I have a file csv file in this format, I would like to generate the average for each step:

elapsed,label
120,Step 01
260,Step 02
113,Step 03
100,Step 01
200,Step 02
103,Step 03

but am having a hard time figuring the lists out. The Python script that I am using is:

for file in sys.argv[1:]:
  for row in csv.DictReader(open(file)):
    label = row['label']
    elapsed = row['elapsed']
    print elapsed,label

I’ve tried

label.append(row['elapsed']) and label.append('elapsed'), but in each case I get this error

AttributeError: 'str' object has no attribute 'append'

I am unsure how to get past this error. I’m new to Python, so perhaps I am missing something in the way lists work? If I can get a list like [120,100] for step 01, I can easily sum and average it, but it is the list creation part that I am stuck on.

Edit: Sum of result list

Now, I have this:

  for item in result:
    for n in result[item]:
      int(n)
      print sum(float(result[item][n]))

but get this error

TypeError: list indices must be integers, not str

But “n” is an integer already, no … ? Or is this referring to “item”? I think I am more confused now than before.

Answer 1

If what you want is to make lists,

from collections import defaultdict
steps = defaultdict(list)

for file in sys.argv[1:]:
  for row in csv.DictReader(open(file)):
    steps[row['label']].append(row['elapsed'])

averages = dict((key, sum(value)) for key, value in steps.iteritems())

Will give you the averages.

What you’re doing is attempting to add the string in row['elapsed'] to the string in label, but strings are immutable so can’t be appended to. You can join them with string1 + string2 or `”.join((string1, string2)).

EDIT: Also, for your new error, See the last line of my code, or, try:

averages = {}
for item in result.iteritems():
    print item[0], sum(item[1])