A search tool with O(n) complexity for finding something with its neighbours. Useful for a file like object.

fwn.py

#!/usr/bin/env python
from argparse import ArgumentParser, FileType
parser = ArgumentParser()
parser.add_argument('string', type=str, help='String to find')
parser.add_argument('file', type=FileType(), 
                    help='File to look in')
parser.add_argument('--neighbourhoodsize', '-ns', type=int, default=0,
                    help='Neighbourhood size')
parser.add_argument('--nolinenumbers', action='store_true', 
                    help='Line numbers')
parser.add_argument('--mark', action='store_true', 
                    help='Marks the important line.')
args = parser.parse_args()


def find_with_neighbours(iterator, detector, neighbourhood_size=2):
    detected_indexes = []
    current_queue = []
    for index, item in enumerate(iterator):
        current_queue.append(item)
        if len(current_queue) > 2 * neighbourhood_size + 1:
            current_queue.pop(0)

        if detector(item):
            detected_indexes.append(index + neighbourhood_size)

        if index in detected_indexes:
            yield current_queue[:], index + neighbourhood_size

    last_index = index
    for index in range(last_index, last_index + neighbourhood_size + 1):
        if index in detected_indexes:
            yield current_queue[:], index + len(current_queue) - neighbourhood_size - 1
        current_queue.pop(0)

def print_with_neighbours(neigbourhood, last_index=None, detector=None):
    if last_index is not None:
        first_index = last_index - len(neigbourhood)
        index_size = len(str(last_index))
        for index, line in enumerate(neigbourhood, start=first_index):
            if args.nolinenumbers:
                print(line.strip(), end='')
            else:
                print('{:{}}  {}'.format(index, index_size, line.strip()), end='')
            if detector is not None and detector(line):
                print(' <--')
            else:
                print('')
    print('\n')

if __name__ == '__main__':
    detector = lambda x: args.string in x
    g = find_with_neighbours(args.file, detector, getattr(args, 'neighbourhoodsize', 2))
    for r, i in g:
        print_with_neighbours(r, i, detector=detector if args.mark else None)