How to get all the words between two specific words from a text file and write it in a new text file using python

Question

Lets say I have a text file containing

Section 1
What: random1 
When: random2
Why:  random3 
Where: random4
How: random5
Section 2
What: dog1
When: dog2
Why: dog3
Where: dog4
How: dog5
Section 3
What: me1
When: me2
Why: me3
Where: me4
How: me5

I want to create a function to take the text file and look for two words and copy everything in between and keep collecting data and put it in a new text file.

For example: def my_function(document, start, end): in the interaction window I would put my_function("testing.txt, "when", "why") and it should create a new text file containing the data:

when: random2
when: dog2
when: me2

So the function takes all the data between those two words and those two words occur more than once so it would have to keep looking through the file.

A user in a different thread has posted a solution that may help me but I am not sure how to put it in a function and I don't understand the code used.

This is from a different thread , solution by: falsetru

import itertools

with open('data.txt', 'r') as f, open('result.txt', 'w') as fout:
   while True:
      it = itertools.dropwhile(lambda line: line.strip() != 'Start', f)
      if next(it, None) is None: break
      fout.writelines(itertools.takewhile(lambda line: line.strip() != 'End', it))

Answer 1

def fn(fname, start, end):
    do_print = False
    for line in open(fname).readlines():
        if line.lower().startswith(start.lower()):
            do_print = True
        elif line.lower().startswith(end.lower()):
            do_print = False
        if do_print:
            print(line.strip())

This produces the output:

>>> fn('testing.txt', 'when', 'why')
When: random2
When: dog2
When: me2

It works by just going through the file line by line and setting a flag True whenever the line starts with start and False whenever the line starts with end . When the flag is True, the line is printed.

Since the examples in the post had mixed case, I used the method lower to make the tests case-insensitive.

Answer 2

This will do what you describe. I've added a dest_path input to specify the output file.

def my_function(source_path, dest_path, start_text, stop_text):
    # pre-format start and end to save time in loop (for case insensitive match)
    lower_start = start_text.lower()
    lower_stop = stop_text.lower()
    # safely open input and output files
    with open(source_path, 'r') as source, open(dest_path, 'w') as dest:
        # this variable controls if we're writing to the destination file or not
        writing = False
        # go through each line of the source file
        for line in source:
            # test if it's a starting or ending line
            if line.lower().startswith(lower_start): writing = True
            elif line.lower().startswith(lower_stop): writing = False
            # write line to destination file if needed
            if writing: dest.write(line)

Note that the files are closed automatically when the with block finishes.