tax_balances.py

import sys
import csv
from collections import defaultdict

reader = csv.reader(iter(sys.stdin.readline, ''), delimiter=';')

# Skip header row
reader.next()

grouped_dict = defaultdict(lambda: ['', '', '', 0.0, 0.0, 0.0, 0.0, 0.0])

for row in reader:
  # Trim whitespace in each field and remove last column (it's empty)
  row = [field.strip() for field in row[:-1]]

  # Strip month and day from tax period, leaving only the year
  row[2] = row[2][:4]

  # Group by parcel number and tax period, summing the numeric columns
  group = grouped_dict[(row[0], row[2])]
  for index in range(3, 8):
    # If there's a minus at the end, move it to the front so it can be parsed
    if row[index][-1] == '-':
      row[index] = '-' + row[index][:-1]

    group[index] += float(row[index])

# Convert grouped_rows dict back to list of lists
grouped_rows = [list(key) + totals for key, totals in grouped_dict.iteritems()]

for row in grouped_rows: print row