Python Line Processing Pattern

Published: 2020-05-17 by Pradeep Gowda.
Tagged: python

Sometimes you want to write a program that has to modify line(s) in a text file. The “UNIX” way of doing this is of course to use sed (stream editor) or awk. Since I can type faster than think in awk, I write python instead.

The common pattern for such programs is:

    for file in matching_files
        for line in file
            if line matches pattern
                modify the line
                print the line
                write the line

One twist to the above is, you don’t want to overwrite the file before you know the overwriting can complete successfully.

The defensive way to handle this is: put the source files in a git repo, so that you can always revert the changes back. This is also useful in git diffing the changes so that you can verify your program actually does what you wanted.

In addition, you can write the changed contents to a temporary file, and rename that file to the original file if there were no errors. Essentially.

$ ./processing_script original.txt > temp.txt && mv temp.txt original.txt

I learnt about NamedTemporaryFile module recently, and I used it to do the following.

#!/usr/bin/env python
This program will rewrite the following:

    title: AWK programming language
    kind: notebook
    tags: programming-language
    date: 2020-02-12T05:51:45

    title: AWK programming language
    kind: notebook
    tags: programming-language
    date: 2020-02-12
    updated: 2020-02-12T05:51:45
import glob
import shutil
import tempfile

def main():
    files = glob.glob('*.md')
    for mdfile in files:
        temp = tempfile.NamedTemporaryFile('w', delete=False)
        with open(mdfile) as inp:
            for line in inp:
                if line.startswith('date: '):
                    timestamp = line.split('date: ')[1]
                    dt = timestamp[:10]
                    print(f'{mdfile} {dt}')
                    temp.write(f'date: {dt}\n')
                    temp.write(f'updated: {timestamp}')
        shutil.move(, mdfile)

if __name__ == '__main__':

Update: David Glick pointed out that – You can skip “lines = inp.readlines()” and just iterate over inp; it’ll give you one line at a time.

Update-2 (2020/5/20): If you want to know more about awk, which is really the best know “line processing” language, see this video by Ben Porter from Apr 2020.