Reading two text files line by line simultaneously

Question

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

There is a Java version of this 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?

Asked By: alvas

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Source

Answer 1

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

Answered By: Ashwini Chaudhary

Answer 2

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

In Python 2, replace built-in zip with itertools.izip:

    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))

Answered By: Fred Foo

Answer 3

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 "%st%s" % (lineA.rstrip(), lineB.rstrip())

Answered By: bruno desthuilliers

Answer 4

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:

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

Answered By: YU Chang

Reading two text files line by line simultaneously

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