Using python to write specific lines from one file to another file

Question

I have ~200 short text files (50kb) that all have a similar format. I want to find a line in each of those files that contains a certain string and then write that line plus

Answer 1

ajon has the right answer, but so long as you are looking for guidance, your solution doesn't take advantage of the high-level constructs that Python can offer. How about:

searchquery = 'am\n'

with open('Test.txt') as f1:
  with open(Output.txt, 'a') as f2:

    Lines = f1.readlines()

    try:
      i = Lines.index(searchquery)
      for iline in range(i, i+3):
        f2.write(Lines[iline])
    except:
      print "not in file"

The two "with" statements will automatically close the files at the end, even if an exception happens.

A still better solution would be to avoid reading in the whole file at once (who knows how big it could be?) and, instead, process line by line, using iteration on a file object:

  with open('Test.txt') as f1:
    with open(Output.txt, 'a') as f2:
      for line in f1:
        if line == searchquery:
          f2.write(line)
          f2.write(f1.next())
          f2.write(f1.next())

All of these assume that there are at least two additional lines beyond your target line.