Searching for a string from a line number in both directions

Using Python 2.4, I am reading a large flat file and picking a particular line number. Now I want to search for a string, say, START before that line number, and a string END, after that line n开发者_如何学Cumber.

How do I get the line numbers of the nearest occurrences of the strings START (before current line number) and END (after current line number)?

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How about this:

line_no = 1

# Seek the last START before reaching the target line.
start_line_no = -1
while line_no != target_line_no:
    line = input.readline()
    if line == "":
        # File is shorter than you think.
        break
    line_no += 1
    if START in line:
        start_line_no = line_no

# Seek the first END after the target line.    
end_line_no = -1
while true:
    line = input.readline()
    if line == "":
        # END could not be found.
        break
    line_no += 1
    if END in line:
        end_line_no = line_no
        break

print start_line_no, end_line_no