How to write lines from a input file to an output file in reversed order in python 3

Question

What I want to do is take a series of lines from one text document, and put them in reverse in a second. For example text document a contains:

hi
there
people

So therefore I would want to write these same lines to text document b, except like this:

people
there
hi

So far I have:

def write_matching_lines(input_filename, output_filename):
 infile = open(input_filename)
 lines = infile.readlines()
 outfile = open(output_filename, 'w')
 for line in reversed(lines):
 outfile.write(line.rstrip())
 infile.close()
 outfile.close()

but this only returns:

peopletherehi 

in one line. any help would be appreciated.

• Abbas: Concatenate a new line when writing to file: outfile.write(line.rstrip() + '\r')
• James Pinson: oh wow how did i miss that.. thankyou very much!

Answer 1

One line will do:

open("out", "wb").writelines(reversed(open("in").readlines()))

Answer 2

You just need to + '\n' since .write does not do that for you, alternatively you can use

print >>f, line.rstrip()

equivalently in Python 3:

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

which will add a new line for you. Or do something like this:

>>> with open('text.txt') as fin, open('out.txt', 'w') as fout:
 fout.writelines(reversed([line.rstrip() + '\n' for line in fin]))

This code assumes that you don't know if the last line has a newline or not, if you know it does you can just use

fout.writelines(reversed(fin.readlines()))

Answer 3

Why do you rstrip() your line before writing it? You're stripping off the newline at the end of each line as you write it. And yet you then notice that you don't have any newlines. Simply remove the rstrip() in your write.

Less is more.

Update

If I couldn't prove/verify that the last line has a terminating newline, I'd personally be inclined to mess with the one line where it mattered, up front. E.g.

....
outfile = open(output_filename, 'w')
lines[-1] = lines[-1].rstrip() + '\n' # make sure last line has a newline
for line in reversed(lines):
 outfile.write(line)
....

• jamylak: The last line in the file might not have a newline
• Travis Griggs: Good catch. Pesky assumptions. :) Personally, I think I'd deal with that situation directly, rather than strip and replace every newline that exists.
• jamylak: I thought about that but I decided it was more practical for me to just have the one liner instead of having to check 1. you have more than 0 lines and 2. the last line has a new line

Answer 4

with open(your_filename) as h:
 print ''.join(reversed(h.readlines()))

or, if you want to write it to other stream:

with open(your_filename_out, 'w') as h_out:
 with open(your_filename_in) as h_in:
 h_out.write(''.join(reversed(h_in.readlines()))