Python: Search text file and write block of lines including previous line to another file

Question

I am searching through a text file and want to copy and write a block of lines associated with a match in another text file. Once i find the search criteria, i want to copy/write out the previous line and next 9 lines (total of 10 lines) out to a file for each match.

Example input file to be search

Line 1: File sent to xyz blah blah:
                             Line 2: Search Criteria here
                             Line 3
                             Line 4
                             Line 5
                             Line 6
                             Line 7
                             Line 8
                             Line 9
                             Line 10

Line 1: File sent to xyz blah blah:
                             Line 2: Search Criteria here
                             Line 3
                             Line 4
                             Line 5
                             Line 6
                             Line 7
                             Line 8
                             Line 9
                             Line 10

Code i have started:

searchList = []
searchStr = "Search Criteria here"

with open('', 'rt') as fInput:
    previous = next(fInput)
    for line in fInput:
        if line.find(searchStr) != -1:
            searchList.append(previous)
            searchList.append(line.lstrip('\n'))


with open('Output.txt','a') as fOutput:
    OutPut.write("\n".join(searchList))

The code above saves to a file like this with spaces in between the first and second line:

mm/dd/yyy  hh:mm:ss.MMM File sent to xyz:

                             Line 2: Search Criteria here

mm/dd/yyy  hh:mm:ss.MMM File sent to xyz:

                             Line 2: Search Criteria here

I want save all 10 lines, exactly as they are in the input file.

Answer 1

First, read the file and find the line numbers that match. Keep track of the line numbers for later.

all_lines = []
match_lines = []

with open('in_file.txt', 'r') as fInput:
    for number, line in enumerate(fInput):
        all_lines.append(line)
        if searchStr in line:
            match_lines.append(number)

Then, loop over the match_lines list and output the lines you care about from all_lines:

num_lines_before = 1
num_lines_after = 10
with open('out_file.txt', 'w') as fOutput:
    for line_number in match_lines:
        # Get a slice containing the lines to write out
        output_lines = all_lines[line_number-num_lines_before:line_number+num_lines_after+1]
        fOutput.writelines(output_lines)    

To test this, I'm going to create a io.StringIO object to read/write a string as a file, and ask for one line before and two after:

import io

strIn = """This is some text
12345
2 searchforthis
34567
45678
5 searchforthis
63r23tf
7pr9e2380
89spver894
949erc8m9
100948rm42"""

all_lines = []
match_lines = []
searchStr = "searchforthis"

# with open('in_file.txt', 'r') as fInput:
with io.StringIO(strIn) as fInput:
    for number, line in enumerate(fInput):
        all_lines.append(line)
        if searchStr in line:
            match_lines.append(number)

num_lines_before = 1
num_lines_after = 2



# with open('out_file.txt', 'w') as fOutput:
with io.StringIO("") as fOutput:
    for line_number in match_lines:
        # Get a slice containing the lines to write out
        output_lines = all_lines[line_number-num_lines_before:line_number+num_lines_after+1]
        fOutput.writelines(output_lines)    
        fOutput.write("----------\n") # Just to distinguish matches when we test
    
    fOutput.seek(0)
    print(fOutput.read())

Gives this output:

12345
2 searchforthis
34567
45678
----------
45678
5 searchforthis
63r23tf
7pr9e2380
----------