Question

Here’s a sample csv file

id, serial_no
2, 500
2, 501
2, 502
3, 600
3, 601

This is the output I’m looking for (list of serial_no withing a list of ids):

[2, [500,501,502]]
[3, [600, 601]]

I have implemented my solution but it’s too much code and I’m sure there are better solutions out there. Still learning Python and I don’t know all the tricks yet.

file = 'test.csv'

data = csv.reader(open(file))
fields = data.next()

for row in data:
  each_row = []     
    each_row.append(row[0])
    each_row.append(row[1])
    zipped_data.append(each_row)
for rec in zipped_data:
  if rec[0] not in ids:
    ids.append(rec[0])
for id in ids:
    for rec in zipped_data:
      if rec[0] == id:
        ser_no.append(rec[1])
  tmp.append(id)
  tmp.append(ser_no)
  print tmp
  tmp = []
  ser_no = []

**I’ve omitted var initializing for simplicity of code

print tmp

Gives me output I mentioned above. I know there’s a better way to do this or pythonic way to do it. It’s just too messy! Any suggestions would be great!

Answer 1

from collections import defaultdict

records = defaultdict(list)

file = 'test.csv'

data = csv.reader(open(file))
fields = data.next()

for row in data:
    records[row[0]].append(row[1])

#sorting by ids since keys don't maintain order
results = sorted(records.items(), key=lambda x: x[0])
print results

If the list of serial_nos need to be unique just replace defaultdict(list) with defaultdict(set) and records[row[0]].append(row[1]) with records[row[0]].add(row[1])