CODEWITHSUNDEEP
pythonmergealternate
maggie
maggie

Reputation: 13

How to merge two text files with alternating lines?

I need to read two text files and write their alternating lines to a third file. For example:

File1:

A1  
A2

File2:

B1  
B2

Output file:

A1  
B1  
A2  
B2

Upvotes: 1

Views: 1684

Answers (2)

shx2
shx2

Reputation: 64318

from itertools import zip_longest
with open(filename1) as f1, open(filename2) as f2, open(outfilename, 'w') as of:
    for lines in zip_longest(f1, f2):
        for line in lines:
            if line is not None: print(line, file=of, end='')

EDIT: to fix the problem in cases where the input files don't end with a newline, you change the print line to this:

print(line.rstrip(), file=of)

Upvotes: 3

Łukasz Rogalski
Łukasz Rogalski

Reputation: 23223

Since files are iterables, they can be ziped together. Since we want to support a case where number of lines in both input files does not match, we use zip_longest instead of built-in zip. This approach does not require loading whole file to memory, so it can be used to merge even a very big files without getting a MemoryError.

import itertools

with open('file1.txt') as src1, open('file2.txt', 'r') as src2, open('output.txt', 'w') as dst:
    for line_from_first, line_from_second in itertools.zip_longest(src1, src2):
        if line_from_first is not None:
            dst.write(line_from_first)
        if line_from_second is not None:
            dst.write(line_from_second)

Upvotes: 0

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