Reading two text files line by line simultaneously

PythonFileIoReadfile

Python Problem Overview


I have two text files in two different languages and they are aligned line by line. I.e. the first line in textfile1 corresponds to the first line in textfile2, and so on and so forth.

Is there a way to read both file line-by-line simultaneously?

Below is a sample of how the files should look like, imagine the number of lines per file is around 1,000,000.

textfile1:

This is a the first line in English
This is a the 2nd line in English
This is a the third line in English

textfile2:

C'est la première ligne en Français
C'est la deuxième ligne en Français
C'est la troisième ligne en Français

desired output

This is a the first line in English\tC'est la première ligne en Français
This is a the 2nd line in English\tC'est la deuxième ligne en Français
This is a the third line in English\tC'est la troisième ligne en Français

There is a Java version of this https://stackoverflow.com/questions/10831007/read-two-textfile-line-by-line-simultaneously-java, but Python doesn't use bufferedreader that reads line by line. So how would it be done?

Python Solutions


Solution 1 - Python

from itertools import izip

with open("textfile1") as textfile1, open("textfile2") as textfile2: 
    for x, y in izip(textfile1, textfile2):
        x = x.strip()
        y = y.strip()
        print("{0}\t{1}".format(x, y))

In Python 3, replace itertools.izip with the built-in zip.

Solution 2 - Python

with open(file1) as f1, open(fil2) as f2:
  for x, y in zip(f1, f2):
     print("{0}\t{1}".format(x.strip(), y.strip()))

output:

This is a the first line in English	C'est la première ligne en Français
This is a the 2nd line in English	C'est la deuxième ligne en Français
This is a the third line in English	C'est la troisième ligne en Français

Solution 3 - Python

We could use generator for more convenient file opening, and it could easily support to iterator on more files simultaneously.

filenames = ['textfile1', 'textfile2']

def gen_line(filename):
    with open(filename) as f:
        for line in f:
            yield line.strip()

gens = [gen_line(n) for n in filenames]

for file1_line, file2_line in zip(*gens):
    print("\t".join([file1_line, file2_line]))

Note:

  1. This is python 3 code. For python 2, use itertools.izip like other people said.
  2. zip would stop after the shortest file is iterated over, use itertools.zip_longest if it matters.

Solution 4 - Python

Python does let you read line by line, and it's even the default behaviour - you just iterate over the file like would iterate over a list.

wrt/ iterating over two iterables at once, itertools.izip is your friend:

from itertools import izip
fileA = open("/path/to/file1")
fileB = open("/path/to/file2")
for lineA, lineB in izip(fileA, fileB):
    print "%s\t%s" % (lineA.rstrip(), lineB.rstrip())

Attributions

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Content TypeOriginal AuthorOriginal Content on Stackoverflow
QuestionalvasView Question on Stackoverflow
Solution 1 - PythonFred FooView Answer on Stackoverflow
Solution 2 - PythonAshwini ChaudharyView Answer on Stackoverflow
Solution 3 - PythonYU ChangView Answer on Stackoverflow
Solution 4 - Pythonbruno desthuilliersView Answer on Stackoverflow