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 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 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?

Solution 1

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

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

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 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())

View more solutions

Comments

[BasedRebel]

Be aware that zip() will pull the full contents of both files into memory (in Python 2.x)

[Martijn Pieters]

Better use from itertools import izip or izip_longest.

[jmtoung]

I get the issue: File "MergeANNOVARResults.py", line 10 with open(refseq) as refseq_fh, open(gencode) as gencode_fh: ^ SyntaxError: invalid syntax

[user58925]

Would this load all of textfile1 and textfile2 to memory?