How to efficiently parse fixed width files?

Using the Python standard library's struct module would be fairly easy as well as extremely fast since it's written in C.

Here's how it could be used to do what you want. It also allows columns of characters to be skipped by specifying negative values for the number of characters in the field.

import struct

fieldwidths = (2, -10, 24)  # negative widths represent ignored padding fields
fmtstring = ' '.join('{}{}'.format(abs(fw), 'x' if fw < 0 else 's')
                        for fw in fieldwidths)
fieldstruct = struct.Struct(fmtstring)
parse = fieldstruct.unpack_from
print('fmtstring: {!r}, recsize: {} chars'.format(fmtstring, fieldstruct.size))

line = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789\n'
fields = parse(line)
print('fields: {}'.format(fields))

Output:

fmtstring: '2s 10x 24s', recsize: 36 chars
fields: ('AB', 'MNOPQRSTUVWXYZ0123456789')

The following modifications would adapt it work in Python 2 or 3 (and handle Unicode input):

import struct
import sys

fieldstruct = struct.Struct(fmtstring)
if sys.version_info[0] < 3:
    parse = fieldstruct.unpack_from
else:
    # converts unicode input to byte string and results back to unicode string
    unpack = fieldstruct.unpack_from
    parse = lambda line: tuple(s.decode() for s in unpack(line.encode()))

Here's a way to do it with string slices, as you were considering but were concerned that it might get too ugly. The nice thing about it is, besides not being all that ugly, is that it works unchanged in both Python 2 and 3, as well as being able to handle Unicode strings. Speed-wise it is, of course, slower than the versions based the struct module, but could be sped-up slightly by removing the ability to have padding fields.

try:
    from itertools import izip_longest  # added in Py 2.6
except ImportError:
    from itertools import zip_longest as izip_longest  # name change in Py 3.x

try:
    from itertools import accumulate  # added in Py 3.2
except ImportError:
    def accumulate(iterable):
        'Return running totals (simplified version).'
        total = next(iterable)
        yield total
        for value in iterable:
            total += value
            yield total

def make_parser(fieldwidths):
    cuts = tuple(cut for cut in accumulate(abs(fw) for fw in fieldwidths))
    pads = tuple(fw < 0 for fw in fieldwidths) # bool values for padding fields
    flds = tuple(izip_longest(pads, (0,)+cuts, cuts))[:-1]  # ignore final one
    parse = lambda line: tuple(line[i:j] for pad, i, j in flds if not pad)
    # optional informational function attributes
    parse.size = sum(abs(fw) for fw in fieldwidths)
    parse.fmtstring = ' '.join('{}{}'.format(abs(fw), 'x' if fw < 0 else 's')
                                                for fw in fieldwidths)
    return parse

line = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789\n'
fieldwidths = (2, -10, 24)  # negative widths represent ignored padding fields
parse = make_parser(fieldwidths)
fields = parse(line)
print('format: {!r}, rec size: {} chars'.format(parse.fmtstring, parse.size))
print('fields: {}'.format(fields))

Output:

format: '2s 10x 24s', rec size: 36 chars
fields: ('AB', 'MNOPQRSTUVWXYZ0123456789')