fixedwidthparser.py

class FixedWidthParser(object):
    """A parser for fixed-width data files. Pass in a data file and 
    a list of field names and lengths, and get back a dictionary
    for each row.

    Useful for converting a fixed-width file to a CSV.

    See tests.py for a usage example.
    """

    def __init__(self, fields):
        """
        fields: a list of tuples in the form (fieldname, (startchar, endchar))
        """
        self.fields = fields

    def parse(self, fh):
        """
        fh: a file-like object
        """
        for line in fh:
            data = {}
            for fieldname, (start, end) in self.fields:
                data[fieldname] = line[start-1:end]
            yield data

tests.py

import unittest
from fixedwidthparser import FixedWidthParser

class TestFixedWidthParser(unittest.TestCase):
    def test_parser(self):
        try:
            from cStringIO import StringIO
        except ImportError:
            from StringIO import StringIO

        fields = [('filer_id', (1,9)),
                  ('amendment', (10, 10)),
                  ('report_type', (11, 13)),
                  ('primary_general', (14, 14)),
                  ('microfilm', (15, 25)),
                  ('transaction_type', (26, 28)),
                  ('contributor_name', (29, 62)),
                  ('city', (63, 80)),
                  ('state', (81, 82)),
                  ('zipcode', (83, 87)),
                  ('occupation', (88, 122)),
                  ('month', (123, 124)),
                  ('day', (125, 126)),
                  ('century', (127, 128)),
                  ('year', (129, 130)),
                  ('amount', (131, 137)),
                  ('other_id', (138, 146)),
                  ('fec_record', (147, 153)), ]

        data = """C00000042NYE P2893017625424KCongress for Judy Biggert         Clarendon Hills   IL60514                                   102220070002000C003302411367020
C00000042NYE P2893017625424KFriends of John Boehner           Hamilton          OH45011                                   102220070005000C002371981367021
C00000042NYE P2893017625424KCapito for Congress               Charleston        WV25314                                   102220070001000C003478491367022
C00000042NYE P2893017625524KCongressman for John Carter       Round Rock        TX78664                                   102220070001000C003712031367023"""
        fh = StringIO(data)

        parser = FixedWidthParser(fields)
        for row in parser.parse(fh):
            self.assertEqual(row['century'], '20')
            self.assertEqual(row['year'], '07')
            self.assertTrue(row['city'].strip() in ['Clarendon Hills', 'Hamilton', 'Charleston', 'Round Rock'])
            print row

if __name__ == '__main__':
    unittest.main()